Foundation of Ontological Philosophy

Ontological philosophy is a new way of doing philosophy. Implausible though it may sound at this late date, after more than 2 millennia of trying, there is a new way of doing philosophy. And it is one that works.

Furthermore, since ontological philosophy is a form of naturalism that uses the empirical method, it is equally a new way of doing science. In other words, it unites philosophy and science. Not surprisingly, it has profound and far reaching implications.

Philosophy. Philosophy is different from ordinary ways of knowing. It aspires to a kind of knowledge that is prior to everyday reasoning, such as modern science and everyday practical reasoning. Since it takes a special foundation to defend a more fundamental kind of knowledge, foundationalism is the heart of philosophy. Ontological philosophy is a new kind of “first philosophy.”

In the past, philosophers have used epistemological foundations to argue for more fundamental truths. They used reflection on how we know to arrive at a theory about the nature of reason and knowledge such as the intuition of forms, certainty about ideas in the mind, and the language-users’ understanding of language. Such approaches to justifying a more fundamental kind of knowledge have failed, however, to garner general acceptance (mainly because they lead to metaphysical dualism and skepticism). Indeed, the failure of traditional, “epistemological” philosophy is one of the few points on which most contemporary philosophers can agree.

Ontological philosophy. It is not hard, therefore, to see why we might wish there were a new way of doing philosophy. And there is one. For it is possible to use empirical ontology (the acceptance of whichever ontology is the best ontological explanation of what is found in nature), rather than epistemology, as the foundation for justifying a more fundamental kind of knowledge.

By “ontology,” I mean a theory about the basic substances that constitute the world, where “substances” are self-subsistent entities that never come into existence and never go out of existence. That is what we implicitly assume when we take objects in the natural world to exist independently ourselves. They must be made of something that can exist on its own, or else they would depend on us. (And they must be related to one another in some way to exist together as a world.) To be the best ontology, as the empirical method requires, however, such a theory would have to postulate the fewest and simplest basic substances (and basic relationship) that can explain everything in the world.

Suppose there were an ontology that is demonstrably better than any alternative, including those offered by physics. And suppose that it entailed further propositions about the world that were not already recognized as true. Such an ontology would be a foundation for philosophy, for what else it implied would be ontologically necessary. Its implications could be denied only by giving up the best ontological explanation of the world. They would be ontologically necessary truths. Such truths would be more fundamental than and, thus, prior to what is known by ordinary means.

As it happens, there is such an ontology. It is “spatiomaterialism,” the theory that the world is constituted by space as well as matter enduring through time as substances. It is a better ontological explanation of the world than any alternative currently considered by naturalists. And it has many implications about the world that are not currently recognized as true, much less as necessary. It does what philosophy has always aspired to do.

The main reason that naturalists do not already accept spatiomaterialism is that they do not choose which ontology to believe by inferring to the best ontological-cause explanation of the world. Instead, they believe in empirical science, which infers to the best efficient-cause explanations of what happens in the world, and they accept whatever ontology is required for scientific theories to be true. The Einsteinian overthrow of the Newtonian belief in absolute space and time has led naturalists to assume that space cannot be a substance enduring through time. But, as will be shown below (under Change), it is possible to explain the truth of both Einstein’s special and general theories of relativity on the assumption that space endures through time (and, thus, is absolute). That is clearly a better ontological explanation of the world than ontologies derived from realism about theories in physics, because it is simpler and less puzzling than the belief that spacetime is what contains all the matter in the world.

Spatiomaterialism is also better than forms of materialism that take it for granted that bits of matter have spatial relations and that spatial relations can change over time, for it explains why they have spatial relations and how change is possible. Furthermore, since spatiomaterialism can explain the truth of all the other basic laws of physics, science offers no reason to doubt that it is true.

Empirical ontology affords, therefore, a way of doing philosophy that is not currently being considered. And as we shall see, it has many profound consequences.

Epistemological philosophy. Ontological philosophy is different from traditional philosophy, because philosophers have traditionally taken an epistemological approach. They tried to demonstrate more fundamental truths about the world than ordinary ways of knowing by taking as their foundation a theory about the nature of reason which was arrived at in some way by reflecting on how we know. Those truths were called “necessary,” but since the foundation was epistemological, rather than ontological, all that could be accomplished was to show that they are certain. In epistemological philosophy, what distinguishes necessary truths from ordinary knowledge is certainty (rather than being entailed by a deeper explanation of the world). Certainty is epistemological necessity.

To be sure, the systems constructed by the most ambitious epistemological philosophers had ontologies, and the claims they made about substances were supposed to be necessary. But these ontological truths were not ontologically necessary; they were truths about ontology that were supposed to be epistemologically necessary, or certain. That is because ontology is just an afterthought in traditional philosophy. The primary goal is to show the conclusiveness of the certain propositions about the world. But insofar as those necessary truths entailed theories about what exists, epistemological philosophers found themselves committed to some ontology or other. In other words, their ontological theories, or metaphysical systems, as they are called, were just implications of their epistemologically necessary truths, not their foundations.

Furthermore, these implications were unwelcome in the end, for their metaphysical systems inevitably cast doubt on their epistemological argument, leading to skepticism. Since success in epistemological philosophy comes from demonstrating that something beyond the epistemological foundation can be known (or so-called realism), it entails a problematic ontological dualism of some kind. In addition to whatever accounts for the existence of their epistemological foundation, epistemological philosophers find themselves committed to the existence of the other kind of substances whose reality they are demonstrating, and as it happens, it is never easy to explain how such different kinds of substances fit together as parts of a single world. Thus, realism leads by way of some problematic ontological dualism to anti-realism, or skepticism about the reality of what is supposed to be demonstrated, and the failure of epistemological philosophy in inevitable.

The foundation of ontological philosophy. Though ontological philosophy is based on ontology, rather than epistemology, it must also secure its foundation. That requires defending a specific theory about the nature of the world, and as mentioned above, the specific theory that will be defended here is spatiomaterialism. It cannot be justified by reflecting on how we know without reducing to epistemological philosophy. By calling it “empirical ontology,” I mean to suggest that it is justified empirically. Before saying what I mean by the empirical method, however, let me say a bit more about the other two assumptions on which spatiomaterialism will be justified.

Naturalism. The following defense of spatiomaterialism assumes that what is being explained by empirical ontology is the natural world. By the natural world, I mean everything in space and time. This is a form of naturalism, for it is to assume that the world is just the natural world.

This kind of naturalism is implicitly assumed by natural science. Naturalism is implicit in science’s commitment to the empirical method, for science has traditionally limited the evidence that is relevant in choosing among theories to observation, or what can be known by perception. Everything that can be known by perception is located in space and time.

Reflection, by contrast, has been excluded by the empirical method of traditional science. That has enabled science to set aside the reflection-based epistemological theories of traditional, epistemological philosophy. Ontological philosophy also relies mainly on perception.

But there is no principled reason to exclude reflection as a source of information about the natural world. Reflective subjects are, after all, parts of the natural world, and in the end, an ontology of the natural world will have to explain what is known about the world through reflection as well as what is known through perception. What is still excluded from ontological philosophy, however, is the use of reflection as a foundation for proving necessary truths. The foundation of necessary truths in ontological philosophy is the ontological explanation that best explains what is perceived. Only ontologically necessary truths are justified from its ontological foundation. As it turns out, however, spatiomaterialism puts ontological philosophy in a position to explain why it has seemed that some propositions can be known with certainty.

Ontological explanation. What makes it possible for empirical ontology to be used as a philosophical foundation is the recognition that ontology can be a kind of explanation. "Ontology" means, literally, "theory of being." It is a theory about the nature of existence, and ontology can be explanatory, if existence can be reduced to basic substances and how they exist together as a world. That is to assume that substances, as substances, are self-subsistent entities. Since basic substances exist on their own, each distinct from all other substances in the world, it may be possible to explain everything in the world by showing how it is constituted by basic substances of certain kinds with a certain basic relationship to one another.

For naturalists, the world in which everything is to be explained ontologically is the natural world, or what is found in space and time. But to explain everything in such a world is not merely to explain the existence of the objects in space. It is to explain all their properties, their relations to one another, and how properties and relations change as time passes. In other words, the natural world can come down to a few basic kinds of substances related in certain basic ways only if that can explain everything in the natural world and everything about the natural world. The inability to explain the possibility of some aspect of the world would show that the world is not constituted by the basic substances and relationships postulated by the ontology.

Empirical method. When ontology is understood as a kind of explanation, it is possible to use the empirical method to choose which specific ontology to believe. By the empirical method, I mean the method used by science. I assume that that method is basically an inference to the best explanation of what is found in the natural world. Thus, by empirical ontology, I mean the project of inferring to the best ontological explanation of what is found in the natural world.

No attempt will be made to justify the empirical method. Justifying the empirical method is a road traveled by traditional philosophy, and ontological philosophy takes a different road by simply using the empirical method, as science does. This way of judging between conflicting theories is what beings like us do naturally. (Later, when we take up necessary truths about evolution, we will trace that disposition to the function of the brain and how the brain works.)

Our main departure from empirical science is, therefore, to apply the empirical method to ontology, rather than just to theories about efficient causes of what happens. That is, we shall be deciding what to believe about the nature of what exists in the world, rather than only what to believe about the causes of what happens there.

More precisely, we shall infer to the simplest and fewest basic substances (and basic relationship among them) that can explain everything in the world, that is, every kind of object in the natural world and every aspect of the natural world, including those which have to do with how things change over time.

Ontological science. Empirical ontology affords, therefore, a way of doing philosophy that is not currently being considered. It is equally, however, a new way of doing science, because ontological philosophy is tantamount to recognizing ontology as a more basic branch of natural science than physics. That means that the basic substances (and basic relationship) discovered by empirical ontology must be able to explain the truth of all the basic laws of physics, much as physics has often been thought to explain the laws of less basic branches of science, such as chemistry and biology. But that does not mean that science is any less empirical, not as long as ontology also uses the empirical method. Nor is this a trivial or meaningless change in science, for it makes all the explanations of less general sciences reducible to the most basic branch of (ontological) science.

Ontology is not, however, quite like other branches of science, because its uses substances, rather than laws of nature, to explain what is found in the world. That is the difference between ontological-cause explanations and efficient-cause explanations. Efficient-cause explanations depend on laws of nature to connect efficient causes to their effects, and accordingly, to infer to the best efficient-cause explanations is to attempt to discover the simplest and most comprehensive laws describing the regularities found in nature. But the causes in ontological explanation are the basic substances and the basic relationship among them, and since things are explained ontologically by showing how they are constituted by substances, ontological explanations do not depend on laws of nature. Ontological explanations show how basic substances are identical to what is found in the world. And since the laws of nature are explained ontologically (by showing how the basic substances and relationships postulated by the ontology make the laws true), the explanations given by ontological science all cite substances as causes in the end.

It is now widely recognized that laws in less general branches of science are not reducible to the laws of physics. But as we shall see, when empirical ontology is seen as the most basic branch of natural science, it is possible to reduce not only the basic laws of physics, but also the laws of all the less general branches of natural science, including biology, physiology, and the social sciences, to the best explanation in the most basic branch of science.

To be sure, it is an ontological reduction, rather than scientific reduction (or reduction to the laws of physics). But in “ontological science,” all the theories of the less general branches of natural science can be reduced to the most basic branch, accomplishing a great unification of scientific knowledge.

That is how empirical ontology unites philosophy and science. But a difference between them can still be discerned because of their different interests. While philosophy sees empirical ontology as a foundation for defending ontologically necessary truths about the world, science sees it as a way of explaining the truth of theories in physics and other branches of science.

That is, the recognition that ontology is a more basic branch of empirical science than physics introduces the project of discovering the simplest and fewest kinds of basic substances that can explain the truth of the laws of physics. That is spatiomaterialism, and combined with the truth of the laws of physics, it entails the ontological necessary truths by which all the theories in less general branches of science are reduced to a simple ontological theory.

the Wholeness of the World. The reason for calling this philosophical argument “the Wholeness of the World” is that spatiomaterialism explains everything in the world. As an ontological theory, spatiomaterialism must be able to account for (in the sense of explaining the possibility of) everything found in the world, including not only all the objects in space, but all their properties, relations and how they change. But it can lead to new beliefs about the world only by demonstrating ontologically necessary truths about the world. In some cases, what is new is just recognizing the necessity of what is already believed to be true, but in other cases, the beliefs themselves are new. It is the completeness of its ontological explanation in this latter sense that earns this argument the title, "the Wholeness of the World." Once spatiomaterialism is elaborated in a way that can explain why the basic laws of physics are true, its implications hold in every possible spatiomaterial world like our own, and those ontologically necessary truth explain the nature of the world in a most complete way. How complete it is can be suggested by mentioning that it explains all the puzzling phenomena that seem to lie beyond the limits of science and have raised doubts about naturalism, including consciousness, goodness, and even how there can be something worthy of worship, or holiness, in a strictly natural world. What makes this possible are its implications about the nature of evolutionary change, and the completeness of this theory of evolution is evident in how many organisms in our world turn out to have essential natures, including not only plants and animals, but also subjects like us who come to know that the world is whole in this way. But it will not be possible to explain fully what all is meant by “the wholeness of the world” until the conclusion, because its various aspects fit together in a way that makes the world even more whole than can be seen at first.

Insofar as it is a complete explanation the nature of the world, it is not merely an explanation of the world. It is the explanation of the world. That is the sense in which it is the Absolute Truth. This is to deny the conceptual relativism of contemporary kantians, like Hillary Putnam, because there is no other theory that can explain everything in and about the world as simply as one based on spatiomaterialism. Ontological philosophy is the "metaphysical realism," the "One True Theory," and the "God's Eye View" of the world whose possibility is denied by such so-called internal realists.

Naturalism. Naturalism is the first assumption of ontological philosophy. It is the belief that the world is just the natural world. By the "natural world," I mean the world disclosed to us by perception, the world where we find ourselves, each having a body alongside others as parts of a world of objects in space that move and interact over time. That is the world of our daily lives.

It is the world to which we are all referring when we speak to one another, as language-using animals, about ordinary matters. We refer to objects in space, attribute properties and relations to them, and explain what happens to them. But some of the objects in space are also subjects, like ourselves, and we describe them in a special way. To them we attribute intentions, desires, thoughts, beliefs, perceptions and other subjective (or psychological) states. They are known by reflection, rather than perception (though knowing about the subjective states of others usually depends on perception as well). But that does not mean that subjective states are not parts of the natural world. They are parts of the natural world because they are states of beings like us, who exist as animals in the natural world. The natural world includes, therefore, not only what is known by perception, but also what is known by reflection.

What exists. The role of naturalism in ontological philosophy is to identify what needs to be explained, and for that purpose, it is appropriate to understand it in terms of its implications about what exists and what does not exist.

Positively. Positively, naturalism is the belief nothing exists but what is located in space and time. All the objects we perceive are located in space. Indeed, they are all related to one another as parts of a single world, since all the locations in space are connected to one another continuously in three independent dimensions. But objects can also move and interact with one another, and the events involving them are also parts of the same world, because all moments in time are connected continuously in a single dimension.

Though naturalism assumes that whatever exists is located in space and time, that does not mean that whatever has a location in space and time exists. Though events in the past and future have locations in space and time, they may not exist. Whether they do or not depends on how we resolve a profound ontological issue about the relationship between existence and time. We must decide whether to believe that existence itself is in time, so that only the present moment exists (or "presentism"), or to believe that time is just another kind of relation, like space, which holds among the things that exist. (See Ontology: Temporality and Spatiomaterialism: Time.)

Negatively. Space and time are so inclusive that naturalism may seem to be obviously true, but the significance of this assumption comes into better focus when we consider it negatively. For naturalism is also the denial that anything exists outside space or time.

God. God, for example, is supposed to exist outside both space and time. That is, at least, what traditional theists (and deists) must hold, for they believe that God is the creator of the natural world. (Nor is God part of the natural world by virtue of being ubiquitous, for that means existing everywhere in space at once, and if that were how God exists, He would be space.) Belief in a creator-God is a kind of supernaturalism. In fact, that is what was being scorned by those who first called themselves "naturalists" in the eighteenth century. They expected to be able to explain everything in the world without appeal to anything outside nature, and that negative sense of "naturalism" is what is intended here.

Forms. It is not just God, however, that naturalism denies. Neither are there any Platonic Forms. Plato held that there are objects knowable only by reason, such as mathematical objects, justice, and the nature of human beings, and even The Good Itself, which exist independently of the natural world. By that he meant that they existed not only outside space, but also outside time, for he he described it as a Realm of Being, opposite in nature from the Realm of Becoming, or nature.

Plato's main reason for postulating the Forms was to explain the nature of goodness objectively. He held that all the other forms follow from The Good itself, making them, and what participates in them, good. But this motive for believing that something exists outside space and time now generally takes the form of the belief in a supernatural God. Platonism is still defended, however, in the philosophy of mathematics. For example, numbers are supposed to be abstract objects. But since what makes them abstract is that their existence is not supposed to depend on anything located in space and time, naturalism must deny their existence.

Minds. Minds are also denied by naturalism, if they exist outside space, as the tradition of modern philosophy would have it. Though Descartes assumed that minds are in time, he denied that they are in space. (He argued that mind has a unity that precludes its being extended, which he took to be the essential property of objects in the natural world. Thus, he believed that mind is an opposite kind of substance from body, with mind and body existing independently of one another.) Insofar as minds are supposed to exist outside space, naturalism must deny their existence.

Problems. Naturalism holds, therefore, that there is nothing to be explained but the natural world. However, that does not mean that it can simply deny the existence of Cartesian minds, Platonic Forms, a transcendent God, and whatever else is supposed to exist outside either space or time. Naturalism must explain everything in space and time, and in each case, certain natural phenomena have led people to believe in the existence of these supernatural entities. Though those phenomena may depend on reflection, not just perception, they are clearly part of the natural world, for they occur to subjects like us in space and time. Thus, like everything else in space and time, they need to be explained.

Consciousness. What makes the mind seem immaterial is consciousness, that is, the way in which whatever we experience has an appearance to us. When we perceive a green leaf, for example, the color of the leaf has a certain intrinsic quality, and even though that quality seems to be located in the leaf, it has an appearance to us which we could not explain to someone who was blind from birth. The same holds not only for other colors, but also for sounds, odors, tastes, and bodily sensations of all kinds. These peculiar objects of reflection are called "phenomenal properties," "qualia," "raw feels," or the like, and they abound in normal perception. In perceiving the leaf, for example, we see many green qualia as covering its surface along with color qualia of other kinds on its stem and other nearby objects. Other kinds of sensory qualia seem to make us aware of its odor, its coolness, its taste, and the like. Each simple phenomenal property seems of have a certain location in space relative to the others at the time, and in the case of bodily sensations, such as itches and pains, they seem to have a locations in some part of the body which, in turn, is located in some part of the same phenomenal space as other objects of perception. Much the same kinds of appearances occur to us in remembering, imagining, and any kind of thinking about objects in space, though they are fainter, less distinct, and not always as spatially coherent. Indeed, even emotions, abstract thoughts, and other mental events have appearances for the subject to whom they occur.

This fact about experience is what will be meant here by "consciousness." Consciousness can make it seem that the conscious subject is not just an object in space, not merely a body alongside other objects in space, because each subjective state involves the appearance of many different kinds of qualia (or simple phenomenal properties) to the subject at the same time. This is the unity of mind to which Descartes pointed in order to show that mind is a basically different kind of substance from body. It means that mind cannot be cut up or divided into parts like extended objects in space. In other words, consciousness is not located in space, like a material object, but rather seems to contain a space of its own, because each sensory qualia appears to have a spatial location relative to all the others, as in the colors that appear to be on the surface of the leaf or its stem. Descartes called these appearances "ideas" and the subjects to whom they appear "minds," but the natural phenomenon to which he was pointing is the fact that there are such appearances to beings like us: qualia of many kinds all have locations in a phenomenal space, which is distinct from the space in which material objects exist.

The essential difference between mind and body led Descartes to believe that mind is a substance that is not located in space at all. Being indivisible, mind could not be part of extension, and thus, it was supposed to be an immaterial substance. Naturalism must deny that there are any minds in that sense. But to be credible, naturalism must somehow explain consciousness as a natural phenomenon. For we are certainly parts of the natural world, and it is hardly plausible to deny that we are conscious.

Goodness. If naturalism could explain consciousness in beings like us, it might seem that there would be nothing left to explain about Platonic Forms, because the abstract objects that appear to the experiencing subject in reasoning could be explained in the same way as ideas in the mind. (An explanation of abstract entities is, in any case, rightly demanded of naturalists, and brief statement of the one given here can be found in Relations: Ontological theory of mathematical knowledge.) There is, however, another aspect of the phenomena that led Plato to believe in Forms that would remain unexplained. Plato believed in the existence of Forms not merely because they are objects of rational intuition, but also because he believed that they are ideal and that things in nature are striving to be like them. That was his theory about the nature of goodness. Just as we try to be virtuous human beings, natural objects strive to be like their Forms, because the Forms are good.

Not only Platonists believe that there is a real difference between good and bad. It seems obvious to many people that goodness is something about the object, state, or event that makes it so that it ought to exist, whatever we may believe about it. For example, what makes an action morally right or wrong for beings like us is something about the action itself that makes it worth choosing, not just something we may believe or feel about it. Thus, goodness is also an aspect of the world that naturalism must explain.

The first attempt to explain goodness naturalistically was made by Aristotle. He thought that every natural object (as opposed to artifact) changes on its own for the sake of attaining an end, or final state, which is the fullest actualization of its essential form, and he explained this phenomenon by holding that there are "final causes" at work in the natural world along with efficient causes. For example, the acorn grows into an oak tree because the final cause of its natural kind is to be a mature oak tree. Growth and development are due to what is called "final causation." Aristotelian teleology, as it is called, explains how goodness is something objective by postulating a special kind of "force" in nature.

The belief in final causation was decisively rejected by most naturalists with the rise of modern science in the Renaissance. Modern science began with the discovery of laws of nature by which events in nature can be predicted, and explanation by such efficient causes was so obviously explanatory that, by contrast, explanations by final causes had to be rejected as merely descriptions of phenomena which call for explanation by efficient causes. Thus, teleology was rejected by naturalists. Nor could they reconcile the belief in final causes with their new found mechanism by holding that natural objects are designed to work mechanically toward certain ends, because that way of explaining the objectivity of goodness required them to believe in a God who created the natural world.

Many naturalists believe such that a naturalistic explanation of the difference between good and bad has been given by Darwin's theory of evolution. Darwin showed how organisms acquire traits that seem to be directed toward ends as a result of the natural selection of random variations on their heritable traits as the organisms succeed in reproducing. That explains why organisms seem to be changing in the direction of ends which are good for them. Thus, the difference between good and bad does not depend on how we feel about it. And Darwin's explanation involves only efficient causes. Thus, it is sometimes seen as the reduction of teleology to efficient causes.

However, most of those who believe that there is a real difference between good and bad and between right and wrong are not satisfied with the Darwinian explanation because of its accidentalism. As contemporary Darwinists understand it, natural selection is caused by external changes in the environment, which are inherently unpredictable, and that makes what evolves far too accidental to explain the difference between good and bad that is objective in the sense that they mean. (For a discussion of its accidentalism, see Change: Accidentalism.) They will insist that there is more for naturalism to explain about this phenomenon before they will be convinced that the world is just the natural world. Teleology is, therefore, still a problem for naturalism.

Holiness. Again, however, it might seem that if naturalism could give an adequate explanation of the objective difference between good and bad, it would not be necessary to explain the belief in God. God has been the traditional foundation for explaining why good is different from bad, for it is supposed to come down to his inscrutable purpose in creating the natural world. But even if there were a naturalistic explanation of the difference between good and bad, many who believe in God would not be satisfied, because what they believe in is not just that there is an objective difference between good and bad. They also believe that there is something worthy of worship, something so inherently good that we ought to accept it as the highest good, submit our wills to it, and treat it in a uniquely reverential way, that is, as something sacred or holy. The faithful believe that they have experiences of a kind that reveal the actual existence of such a thing to them, and the universality of religion among the cultures of the world makes this a phenomenon that must also be explained by naturalism, even though it denies there is any God existing outside space or time.

 One way for naturalists to explain consciousness, the belief in a real difference between good and bad, and the sense that there is something in the world worthy of worship is to deny the reality of these phenomena. Naturalists can hold, in other words, that their critics are simply mistaken in how they describe these phenomena -- that what is being referred to is something quite different from what they believe.

Consciousness might be dismissed as a belief that results from a linguistic confusion (such as the belief in a "private language" or the acceptance of "folk psychology").

The belief in a real difference between good and bad might be explained away as a mere projection of our subjective feelings onto the world (in much the same way as objects in nature seem to have the colors and other phenomenal properties that are just ideas in the mind).

And the belief in something worthy of worship might be explained as simply what is feels like to submit to a higher authority.

Naturalists have given such explanations in the past. But they have not convinced those who take these phenomena to be real, and thus, naturalism has rightly been treated as just one possible view of the world among others. Though naturalism may be plausible to many people without an adequate explanation of these phenomena, there is good reason to doubt its truth as long as these explanations are not accepted as adequate by those who appeal to these phenomena. Theists, mind-body dualists, and those who believe in objective goodness are rational beings too, and if naturalism is a reasonable view, it should be reasonable to them. Thus, the burden that naturalism must bear is rather large. It must be able to explain everything in the world, including these problematic phenomena, to the satisfaction of every rational being, including those who have been led to believe in entities existing outside space or time — that is, as long as they are willing to give reasons and not just be arbitrary and dogmatic in their assertions about what exists.

Ontology. The second assumption of ontological philosophy is about ontology. Ontology is, literally, the study of the nature of being (or existence), and what we shall assume is that ontology is a kind of explanation.

Ontological philosophy takes ontology to be a kind of explanation in which the causes are basic substances (along with their basic relationships to one another), and the effects are what is found in the world, or all the phenomena. Given the existence of certain kinds of basic substances and basic relationships, it explains the things found in the world by showing how their existence is constituted by such substances and relations among them.

If ontology is a valid kind of explanation, an adequate ontology should explain everything found in the world, for it is a theory about the nature of existence and what we mean by "the world" is everything that exists. To assume that ontology is a valid kind of explanation is to assume, therefore, that everything found in the world can be explained by showing how its existence is constituted by basic substances, given how they exist together as a world — including all the objects in the world, all their properties, all their relations to one another, and every way that they can change. It holds, in other words, that nothing exists, ultimately, but the basic substances.

The other way of doing philosophy is based on epistemology, and for epistemological philosophy, ontology is something quite different. Ontology is simply a thesis about what exists. Epistemologists base their claims about certain truths being necessary relative to our ordinary ways of knowing on a theory about how we know. Thus, they find themselves committed to the existence of entities of all the kinds that are known, including the entities presupposed by their foundation as well as all the additional entities entailed by their conclusions (assuming that they succeed in defending those conclusion). Since it is committed to the reality of additional entities of some kind, its ontology is called "realism." It is the belief in the "reality" of those additional entities. But since, as it turns out, they never fit together intelligibly with the entities constituting the epistemological foundation, realism is a form of ontological (or metaphysical) dualism that engenders skepticism. Hence, realists have always had to do battle with so-called anti-realists, who accept only the entities presupposed by their epistemological foundation. To mark how this view of ontology differs from ontological philosophy, let us call it "ontology as realism."

Ontology as a form of explanation. For ontological philosophy, ontology is explanatory. We assume that a certain kind of explanation is valid, which is to believe that there are causes and effects of certain kinds. In this case, the causes are the basic substances and their basic relationship to one another, and their effects are what they can constitute, which includes, if adequate, everything that can be found in the world, including all the objects, their properties and relations, and how they change over time.

Ontological causes. To see how such effects are produced ontologically, let us consider, first, the nature of the causes, both the substances and their relations, and, then, their effects.

Substances. Substances are one part of every ontological cause, and in order to explain how they help produce effects, we must consider both the nature of substance itself and a relevant difference among the kinds of basic substances that may be postulated by an ontology.

Nature of substance. Substance, we shall assume, has a nature that includes to two basic aspects. For something to be a substance, it must not only have a certain determinate nature, but must also be self-subsistent. That is, a substance must have, as a substance, both an essential aspect and an existential aspect to its nature.

Essential aspect of substance. A substance must have an essential aspect to its nature as substance, because in order to exist at all, it must exist in a determinate way. It is not possible for anything to exist without existing in a determinate way; indeterminate existence would be tantamount to nothing existing. The essential aspect of a substance includes all its kind-differentiating properties that do not change as time passes.

To assume that substance as substance has essential properties is not to assume that properties exist in addition to the substances that have them. We can and shall assume that properties are simply aspects of the substances themselves. Thus, essential properties are simply how substances exist, implying that substances can exist in different ways, as in substances being of different kinds. Beings like us can think about aspects of substances and distinguish their aspects from one another, and when we do, we are thinking about their properties. But ontological philosophy cannot answer questions about how rational beings have the ability to think about the aspects of substances as distinct from the substances themselves until it has explained the nature of what exist and the existence of beings in the world, like us, who can think at all. (See, for example, Change:AbstractObjects, or for a briefer statement of the entire theory about the nature of reason, Relations: Ontological theory of mathematical knowledge.)

We will take up the kinds of basic substances after explaining the nature of substance as substance.

Existential aspect of substance. Substances also have an existential aspect to their nature as substance. They must, because, in an ontological explanation of the world, it is the existence of substances (in certain relations) that explains the existence of what is found in the world. Substances are, in other words, self-subsistent.

Existence is, therefore, a property of substance as substance, just as having an essential aspect is. But in both cases, these aspects of substances have to do with their having aspects. The essential aspect is that they have an aspect of the kind we will call their "essential nature," and the existential aspect is that what has such an essential aspect exists independently of the rational being who know about them. That there are aspects of substances that have to with their having aspects is no more puzzling than that they have aspects at all and is answered in the same way, as we shall see, by the ontological explanation of the nature of reason. (See Stage 9, Rational Spiritual Animals, under Reproductive Global Regularities under Change.)

There are, however, two aspects to the existential aspect of the nature of substance as substance: particularity and temporality.

Particularity. First, substances are self-subsistent in the sense each substance has an existence that cannot be reduced to the existence of any other substance or substances in the world. Each substance exists on its own. That is not to say that substances must be able to exist even if all the other substances were to drop out of existence. (For example, it may not be possible for material substances, given their essential nature, to exist without having spatial relations to other material substances.) It is merely to say that there is something in the world whose existence would not be accounted for if only all the other substances in the world were assumed to exist. In short, each particular substance has an existence that is distinct from every other substance in the world.

We must accept that substances are related to one another in one way, at least, since we are assuming that there is more than one substance in the world. By "the world," we mean everything that exists, and thus, if there is more than one substance in the world, the world is a whole composed of parts. Since every substance is, by virtue of the existential aspect of its nature as a substance, something that exists, each substance is a "particular" substance in the further sense of "being part of the world." Each substance has a relationship to the world as a whole, and since it has an existence that is distinct from every other substance in the world, it also has a relationship to the other substances as a different part of one and the same world with them. In other words, when we postulate basic substances, we assume that they are parts of one and the same world.

There is another relationship that all substances have, namely, being identical to themselves. Relationships, like properties, are not something in addition to what has them, but merely an aspect of the substances that have them. And we continue to put off discussing how beings like us know about relationships until we explain the nature of reason ontologically. Although identity is a relationship, it is a relationship that something has to itself, and thus, it may be considered another aspect of each substance taken separately, like its properties. That is, each substance is identical to itself.

By the way, this is to assign ontological meaning to each of three basic senses of "is." "Is" can be used to say that something exists, and in that sense it refers to the property of existence, or the existential aspect of substance as substance. "Is" can also be used as a copula, to attach a predicate to a grammatical subject. In this case, it is referring to the relationship between a substance and some aspect of it, either a property that characterizes its essential nature or one that characterizes a changeable aspect of it (such as the roundness of a piece of wet clay). Finally, "is" can be used to assert identity. When identity is asserted of two substances, it says that the two substances have the same relation to one another as each has to itself, that is, that they are identical. But when identity is asserted of aspects of substances, that is, of properties, it has a different meaning, because different substances can have the same aspects and be of the same kind under each aspect. For example, all substances have the existential aspect, and "being" is the same property in each case. Likewise, substances of the same kind have the same essential properties. It will be possible to keep track of which properties are identical and which are different, because one thing an ontology provides by explaining everything in the world is an inventory of all the aspects of substances.

Temporality. Second, we assume that substances are self-subsistent in a temporal sense. Substances do not go out of existence over time, nor do they come into existence. Thus, a substance that exists at one moment must have existed at the previous moment. And it will continue to exist the next moment. Thus, if a substance exists at all, it exists at every moment in the history of world. It is permanent. The substances that exist at any one moment are the same substances that exist at every other moment in the history of the world.

This is a strong assumption to make about the nature of substance as substance, and it is not one that has always been made, even by naturalists.

According to Aristotle, for example, substances come into existence and go out of existence over time in a process of generation and corruption, though he did assume that they also had "material causes," or matter, that endures through change.

Other naturalistic ontologists do not postulate substances at all, but only "tropes," or properties considered as particular entities. Though tropes are supposed to explain everything in the world, they are not substances in our since, for they are supposed to come into existence and go out of existence at determinate locations in space from moment to moment. See Williams.

Though ontological philosophy makes this strong assumption about the temporal aspect of the existential aspect of substance as substance, there is an issue about the temporal aspect that we will leave open for the time being. To hold that substances never come into existence nor ever go out of existence over time is to presuppose that they are in time. That is, time is built into the nature of substance, as part of the existential aspect of the nature of substance as substance. But there are two different views about the nature of time and how it is related to existence. One is the "endurance" theory and the other is the "perdurance" theory.

Endurance theory of time. The first view holds that substances endure through time. This theory assumes that existence itself is in time. That is, only the present exists. The past and the future do not exist. Thus, for a substance to exist at all is for it to exist at the present moment. This view is also called "presentism." But since substances never come into existence, every substance must have existed at every past moment in the history of the world. And since they never go out of existence, every substance will still exist at every future moment in the world’s career. In other words, substances are identical through time: each substances that exists now is identical to some substance that existed or will exist at every other moment in the history of the world.

Since endurance theory assumes that the past and the future do not exist, they must explain the sense in which statements about the past and the future are true. It holds that such statements are true of substances that exist now, though the properties being ascribed to them have to do either with what has happened or with what will happen to them. That is, the aspects of substances which exist now include the states they had in the past and the states they will have in the future.

Perdurance theory of time. The other view is that substances perdure across time (or over time). Instead of assuming that existence is in time, this theory holds that time is a relation that holds among parts of substances. On this view, the past and the future exist in the same sense as the present. Though perdurance theorists can agree that substances never come into existence nor go out of existence over time, what they mean is that each substance is made up of a continuous series of moments stretching all the way back and all the way forward in the temporal dimension. Thus, instead of seeing substances as identical through time, they see substances as involving a part-whole relation: each substance is a whole whose parts include its state at every moment in its history. Thus, corresponding to the part of each substance that exists at any one moment, there is another part at every other moment in the history of the world.

The distinction between the endurance and perdurance theories about the existential aspects of substance as substance can be traced to McTaggart, who argued around the turn of the twentieth century that it is self-contradictory to hold that only the present exists. But recently, it has been resurrected by analytic philosophers defending the so-called "tenseless theory of time," as opposed to the "tensed theory of time". (The tenseless theory holds that statements about the past, present and future can all be translated, without any loss of content, into sentences about the relations of moments in time that hold eternally, whereas the tensed theory insists that some content is lost, namely, what they imply about which moment is actually present, that is, not just present relative to some particular time of utterance. See Oaklander and Smith.) And even more recently, the perdurance theory has been defended, albeit without admitting it, as what is called "four-dimensionalism" against "three dimensionalism." (See Sider.) But the reason I leave the issue open here is because a similar view is currently accepted by naturalists who are trying to be realists about the notion of spacetime introduced by Einstein’s special and general theories of relativity. Spacetime taken ontologically entails the perdurance theory.

Since purdurance theory assumes that all moments in the history of the world are ontologically equivalent, it holds that statements about the past and the future are true in exactly the same sense as statements about the present. There is no need to hold that statements about the past and the future are really about substances that exist now.

Whatever the relationship between time and existence, the temporal aspect of the existential aspect of substance involves a relationship between moments in time. Everyone agrees that moments occur in a continuous series, though endurance theorists think of time as flowing from the past into the future, and perdurance theorists think of time as just an order about the moments that all exist. But since endurance theorists take existence itself to be in time, they take time to be as ontologically basic as existence and substance, and thus, they take temporal relations to be a measure of the separation between different moments in the existence of a substance that is identical over time. Perdurance theorists, on the other hand, take all the moments in the history of a substance to exist in the same way, and thus they explain time, in effect, as how these moments exist together as a substance in the world.

The difference between these theories can be seen in what they imply about change. In a world where substance is permanent, what changes are the properties or relations of substances, or aspects of them. Endurance theory holds that change involves properties or relations coming into existence or going out of existence over time, because if the future and the past do not exist, there is no "place" for them to come from or to go to. On the other hand, perdurance theory holds that properties and relations never come into existence and never go out of existence, because if the future exists, the properties and relations already exist before the change takes place. And if the past exists, the properties and relations continue to exist after the change is long over.

Kinds of substances. The substances that an ontology postulates are the causes by which it explains the world. But in order to explain completely what is found in the world, those substances must be the most elementary substances that constitute the existence of things in the world. Let us call such ultimate parts of the world "basic substances."

All substances have, as substances, the same kind of existential aspect, but the essential aspects of their natures may be different. Thus, there may be different kinds of basic substances making up the world. But it is important to recognize at the outset that the essential natures that distinguish kinds of basic substances from one another may be either temporally simple or temporally complex.

We are assuming that the properties that characterize basic substances are simply aspects of them. The properties that characterize the essential nature of a substance are aspects of the essential aspect of their nature as substance, and they distinguish one kind of basic substance from another. Such essential properties do not change over time.

Temporally simple. Now, a substance that exhibits its full nature at each moment is a simple substance. That is, a substance will be said to have a "temporally simple essential nature" insofar as its essential properties are aspects of it that exist complete at each moment in the history of its existence. The contrast to complex substances will make this clear.

Temporally complex. The essential nature of a substance may also be defined by how its properties change over time. Properties that can change over time are contingent (or "accidents"), but if contingent properties always change in the same way, the way in which they change may be an essential property. For example, the properties a substance exhibits at one moment may depend on the properties it had the previous moment (together with its relations to other substances), and since the regularity about how they change would be a property that the substance has at every moment, it would be an essential property of the substance. But its essential nature would be dispositional. Insofar as the essential aspect of the nature of a substance is defined by a regularity about how its contingent properties (or relations) change over time, it will be said to have a "temporally complex essential nature."

Relations. Substances are only one part of every ontological cause. The other part is the relationship that holds among the basic substances. Relations are necessary for ontological explanation, because substances have nontrivial ontological effects only by working together, that is, by combining with one another in some way to constitute the existence of things found in the world. What makes ontological explanation explanatory is that substances can work together in different ways to produce different effects.

We must assume, therefore, that there is more than one substance in the world. Though it is conceivable that the world is made up of a single substance, nothing in such a world could be explained ontologically, in our sense, for everything found in such a world would be the same as what is assumed by the ontology in postulating that single substance.

Spinoza was not, therefore, giving an ontological explanation of the world in our sense, because according to his Ethics, he assumed that a single substance makes up the entire world.

If there is more than one substance in the world, they must have, as we have noted, at least one basic relationship to one another, for they are parts of the same world. Since their combination causes the world to exist, that relationship together with the substances might be said to explain the world. But if having such a relationship did account for everything in the world, it would be trivial, for nothing that is contained in any one of the ontological causes is really explained. It is merely assumed.

Finally, if the substances in the world had no further relationship to one another, beyond being different parts of the same world, they could not combine to constitute anything, except for the world as a whole. Though each substance might be said to cause itself ontologically (because it would still constitute its own existence), that would explain nothing, for its existence is precisely what is assumed in postulating the substance. It too would be trivial.

Nature of relations. We must assume, therefore, that basic substances have relationships of some kind to one another (beyond simply being parts of the same world). That is not to assume that relationships are something that exist in addition to the substances that have them. We can and will assume that the basic relationships are simply how basic substances exist together as a world. For example, bits of matter may be assumed to have spatial relations to one another as how they exist together as a world; or bits of matter may be assumed to exist together with space as a substance by coinciding with some part of space or other; and parts of space may be assumed to exist together as a world by having unchanging geometrical relations to one another. Such basic relationship are like properties, which, as we have assumed, are simply aspects of substances. But instead of being aspects of substances taken separately, the relationships we are assuming are aspects of the world, or how substances exist together as a world.

The basic relationships among substances being postulated as part of the ontological causes to be used in explaining everything in the world should be distinguished from the two relations, already mentioned, which substances have to themselves or among their parts: the identity relation and temporal relations. We are considering the relationships that an ontology must postulate along with substances in order to explain things ontologically, whereas the identity relation and temporal relations are aspects of how each substance exists on its own and do not depend on how they exist together as a world.

Since naturalism is the belief that what exists is just what is in space and time, one kind of basic relationship that any naturalism will require among substances is spatial. It is hard to see how any substance could be in space and time without having spatial relations to other substances. By spatial relations, I mean the distances that can hold between substances in three independent dimensions, and I assume that such distances are continuously variable.

Though spatial relations are found in the natural world, that does not mean that a naturalistic ontology must assume that having spatial relations is how substances exist together as a world. There is another way of existing together that would entail their having spatial relations: if space is a substance, bits of matter could have spatial relations by coinciding with parts of space. The real nature of spatial relations is another issue that we will leave open for the time being, until we are in a better position to decide what to believe. (See Space under Spatiomaterialism.)

Kinds of relations. As in the case of substances, there is an important difference to be recognized between kinds of basic relations that might be assumed to hold among the substances postulated. Though such basic relationships are just how the basic substances exist together as a world, they can, like the essential aspects of substances, be either temporally simple or temporally complex.

Temporally simple. Relations that exhibit their full nature at the moment that the substances exist together in that way are temporally simple. That is, relations are "temporally simple" to the extent that they are how substances exist together at a single moment in the history of the world. In a world constituted by space and matter, for example, the basic relationship between the two basic substances would be simple in this sense, for it would be true at every moment that each bit of matter coincides with some part of space or another.

Temporally complex. The relations that exist fully at any one moment may, however, change the next moment. That is, some relations may go out of existence over time and other relations come into existence. Such relations would be contingent, and the only way to define the basic relations by which substances exist together as a world may be the way in which contingent relations change over time. If change in contingent relations were regular, the way that substances exist together as a world might be defined by how their contingent relations change, for that would be a relationship that does not change over time. That is, the relations among substances might be dispositional. To the extent that the relationship by which substances exist together as a world have a nature that is defined by how contingent relations change over time, it will be said to be a "temporally complex relation."

For example, an ontology may assume that the way that substances exist together as a world is by having spatial relations. Particular spatial relations change over time, for example, as objects move, and the possibility of such change could be built into the the meaning of "having spatial relations." "Having spatial relations" might accordingly be defined as meaning that substances have spatial relations of some kind or other at each moment, but that they can change from one moment to the next as long as they are all geometrically consistent as a whole. "Having spatial relations" would then be a temporally complex relation among substances, and the substances themselves could have a relatively simple, inert nature.

It is possible to hold that spatial relations are temporally simple without postulating space as a substance. The change in spatial relations could be explained by the temporally complex essential natures of the substances, such as material substances defined as substances that move and interact according to the basic laws of physics. That is, everything that happens in the world, including all the spatial relations that come to exist, might be explained as what is required because material objects obey the laws of physics. What must be assumed is that those material objects had certain spatial relations at the beginning, say at the Big Bang or when God created the world. The spatial relations assumed by such an ontology could be temporally simple, for they could all exist fully at a single moment, at the very beginning. (It might be mentioned, however, that this view would not even be possible, given the Heisenberg uncertainty principle of quantum mechanics, unless there is a so-called hidden variable that makes the indeterminism of quantum theory a mere appearance of the incompleteness of its explanation.)

Ontological effects. Ontological explanations use substances as causes to explain things in the world as their effects. Such causes produce their effects by constituting the things being explained. Since there are relations among substances, different effects can be produced when basic substances are combined in different ways.

It should be emphasized, however, that insofar as the phenomenon being explained is the same as the substance that constitutes it, the explanation is trivial and, thus, not a genuine explanation at all. The explanatory power of ontology comes from showing how the substances cited as ontological causes work together so that jointly they constitute what is being explained. Thus, even if the existence of some object is explained by showing how it is constituted by the combination of various particular substances, the object's properties are still not explained if they are simply the essential properties of the basic substances constituting it. For example, it does not explain why something is moving in a certain direction to say that all its parts are moving that way. The "explanation" in ontological explanations comes from showing how ontological causes work together to produce something that may seem different from them. Anything that is entailed by the essential natures of substances taken separately is not explained, but just assumed.

What is explained by ontological causes includes both the objects found in space and how they change over time.

Objects. The existence of particular objects can be explained by the substances constituting them. Substances have, as substances, an existential aspect to their nature, that is, they are self-subsistent, and the relations by which they exist together as a world permit them to work together in constituting objects. How they do so depends on the specific ontology.

Likewise the natures of objects found in the world, or their properties, can be explained by the substances constituting them because of the essential aspects of their natures as substances, that is, their essential properties, and the relations by which they exist together as a world permit substances to be combined in different ways.

Thus, it is possible to explain a diversity of things in the world. Things may be different in kind because they are constituted by different kinds of basic substances combined in the same way, or because they are constituted of the same kinds of basic substances combined in different ways, or because of some combination of both factors.

If only out of respect for the Pre-Socratic philosophers, it should be noted that the attempt to explain the world ontologically was first attempted about 600 BC, before epistemological philosophy began. These first philosophers were naturalists looking for the "first principle" (or arche) by which to explain the natural world, and they assumed that it must be a "stuff" of some kind that constitutes the existence of everything in the world. Thales thought it was water. His student, Anaximander, insisted it was an inchoate stuff ("apeiron") without properties of its own. And Anaximander's student, Anaximines, argued for it being air. Though these so-called "Ionian" Pre-Socratics disagreed about its essential nature, they all agreed that the world is constituted by only one basic kind of material substance. Their ontologies were forms of monistic materialism. Spatial relations were taken for granted.

As the Pre-Socratics soon discovered, however, none of these ontologies offered an adequate explanation of the natural world, for they could explain neither the diversity of the objects in nature nor the change that occurs in them. The only properties postulated by any of them were those that characterize the essential nature of the single kind of material substance making up the world, and that left unexplained all the properties that distinguish one kind of object from other kinds, not to mention how such properties could come or go from existence as time passes.

Parmenides can be read as making this point. What Parmenides was referring to by his famous dictum. "What is, must be, and what is not, must not be," was a basic aspect of the nature of substance (the temporal aspect of its existential aspect). Substance cannot go out of existence, nor can it come into existence. But since Parmenides agreed that the "first principle" for explaining the world is a single kind of substance (with a temporally simple essential nature), he argued that there cannot be any real change or diversity in the world. Thus, he insisted that change and diversity are an illusion.

Heraclitus drew the opposite conclusion from the assumption that there is only one first principle for explaining the natural world. But he took, as the first principle, change and diversity itself. That was, in effect, to deny that there is any such thing as substance underlying change or diversity. Since the essential natures of substances are defined by their properties, to take the change of properties as basic was to deny that properties are aspects of substances, for otherwise substances would have to be coming into and going out of existence as time passes. Though Heraclitus did assume that change and diversity are guided in a regular way by Logos (which is something like laws of nature), this is to read Heraclitus' famous claim that you cannot step in the same river twice as saying that what exists in the natural world is nothing but properties that change over time.

Between them, therefore, Heraclitus and Parmenides posed a dilemma for any explanatory ontology that would postulate only one basic principle to explain the world: either the first principle is a material substance of some kind and there is no change nor diversity, or else change and diversity themselves are the first principle, and there is no substance. The former fails to explain the natural world, and the latter abandons ontological explanation altogether.

Pre-Socratic philosophy was a process of posing hypotheses, criticizing them, and posing new hypotheses, and it discovered two ways of solving this dilemma.

Pluralists held that the world is constituted by more than one kind of material substance. That made it possible to explain diversity and change by the mixture and separation of different kinds of material substances each with a simple essential nature.

Empedocles postulated four basic substances, earth, air, fire, and water, and he explained the diversity and change of things in the world by their mixture and separation (according to the forces of "love" and "strife"). Anaxagoras gave the same kind of explanation, except that he postulated infinitely many different basic substances (or "seeds," as he called them). In both cases, the essential natures of the basic substances were defined in terms of their qualitative properties, such as hot and cold, wet and dry, and their mixture was supposed to account for all the other sensible qualities of objects. (It was probably the limited range of objects that could be explained by only four basic substances that led Anaxagoras to insist on infinitely many "seeds.")

The other solution to this dilemma was offered by the atomists, Leucippus and Democritus. They are said to have explained diversity and change "quantitatively", rather than "qualitatively," because they took spatial relations into account. They assumed that the material substances are atoms whose natures differ from one another only by their size and shape, and they explained the differences in kinds of objects not only by the shapes and sizes of their constituent atoms, but also by the spatial relations that hold among them. That forced the ancient atomists to believe, however, that the sensible qualities that objects seem to have are actually subjective, a view that was not generally accepted until the beginning of the modern era.

Change. In order to explain change, an ontology must not only assume that substances have a temporal aspect to their existential nature, but also that they can be combined in different ways at different times. In that case, as time passes, an object may change because some of the kinds of basic substances constituting it are exchanged, or because the relations by which the same basic substances are related in constituting it change, or because of some combination of such factors. But that is to assume that, in addition to having relations, the relations among basic substances are capable of change over time.

This is clearly what Empedocles was assuming in holding that the objects perceived in nature change because of the mixture and separation of elements, such as earth, air, fire and water. He took it for granted that they can move, explaining one kind of change by assuming the possibility of another, namely, motion.

The atomists, however, believed that it was necessary to explain how motion itself is possible. That is why they postulated the void as well as all the atoms. They are traditionally understood as having argued that bits of matter would not be able to move, if there were no void, because there would always be other bits of matter in the way. But if there were a void as well as the atoms, atoms would be able to move without obstruction, at least, until they collided with other atoms. However, since the void exists only where atoms do not exist, the void can be understood as a very subtle kind of material substances that atoms can displace more easily than other atoms. On that interpretation, atoms move through the void like fish through water, displacing a fluid-like substance which offers no resistance. We will return to their explanation of the possibility of change.

There is, it should be emphasized, no ontological explanation of change, if the change being explained is the same kind of change that the substances undergoing that change are postulated as having as part of their essential nature. Whether we are explaining objects and their properties or change in them, when cause and effect are the same, there is no ontological explanation, but only ontological assumption.

Explanatory ontology is, in sum, the attempt to reduce everything in the world to the various kinds of basic substances constituting them and the relations by which those substances exist together as a world. But that is explanatory only to the extent that the substances and their relations are more elementary than what they explain and produce those effects by how they are combined. But if it were successful, an ontological explanation of the world would be a simple and complete explanation of the world, for it would show how everything in the world is identical to certain basic kinds of substances and certain basic kinds of relations among them. Everything in the world would be explained in the same way.

Ontology as realism. For traditional, epistemological philosophy, ontology is realism (or, more precisely, its ontology is determined by the position it takes on realism). The foundation of epistemological philosophy is a theory about how we know (or a theory about the nature of reason) which is based on reflecting on our mental processes. From this foundation, it attempts to justify certain conclusions about the world, which would be necessary relative to our ordinary ways of knowing about it. Thus, success generally means that it is committed to the existence of certain entities beyond those assumed at the beginning. "Realism" is the name for belief in their reality. But realism is usually a form of dualism. Epistemologists are already committed to the existence of the subject whose way of knowing is the foundation for their epistemological argument, and realism commits them to the existence of entities of a fundamentally different kind. Hence, they wind up defending some form of ontological dualism, and that typically leads to anti-realism, since the two kinds of substances do not fit together intelligibly as a world. This pattern can be found in every era of the history of Western philosophy. I will suggest how, very briefly, in order to make clear what I mean.

Ancient. Reflecting on the difference between the objects of perception and the objects that seem to be present to us in reasoning about kinds of things, Plato argued that, in addition to all the visible objects in the realm of Becoming, there is a realm of Being where such objects of rational intuition exist as unchanging Forms. He called the latter realm "Being" because the Forms were supposed to be permanent and unchanging. It was supposed to be outside space and time, beyond the natural world of changing, visible objects. Thus, his realism committed him to believing in the existence of both Being and Becoming, and since they are so fundamentally different in their natures, his ontology is clearly a kind of dualism.

Plato’s was a very problematic dualism, because it is hard to explain how entities that are not supposed to be in space and time are related to visible objects which are, much less to show how such Forms could cause visible objects to have the natures they seem to have. That makes it easy to be skeptical about the transcendent realm of Being, and naturalists are already inclined to be anti-realists about abstract entities of any kind, because they assume that everything is located in space.

Aristotle tried to avoid these problems by postulating, instead, substances in the natural world that are compounds of two elements, matter and form. This was not, however, to abandon Plato’s epistemological foundation, for Aristotle continued to assume that the "material cause" is an object of perception and that the "formal cause" is an object of rational intuition. Though essential forms were located in space, they had to have a peculiar nature to play their role, because each had to be located in many different particular substances at the same time and yet be one and the same thing. That earned them the name "universals." Though Aristotle could claim to be a naturalist, he was still a realist about essential forms as something beyond what is known by perception. That landed him with his own ontological dualism because, even though neither matter nor form can exist without the other, the existence of one is distinct from and cannot be reduced to the existence of the other. Realism about universals invited a type of skepticism called "nominalism."

Attempts to avoid matter-from dualism characterize Aristotle’s later work on the nature of substance as substance. Though there is much dispute about it, Aristotle seems to argue in Metaphysics, Books VII and VIII, that substances are basically just essential forms. He apparently reduces the material cause to the fact that forms exist only as particular substances despite being entities that exist as many different particular instances of the same form (that is, as universals). That position seems to reduce matter to a principle of individuation. This later notion of essential form and matter is closer to the distinction between essence and existence assumed here (see Substances above). In any case, Aristotle's conception of being as being (that is, substance as substance) poses so many problems that many traditional philosophers have been inclined to avoid ontology altogether.

Medieval. In the Medieval period, realism took the form of belief in the existence of God, rather than a realm of Being, outside space and time. Theists believed that it was possible to prove the existence of God on the basis of what can be observed in the natural world. For example, they argued from the natural belief that every event has a cause to the existence of God as the first cause, or cause of nature as a whole. And they argued from natural teleology to God, both as the designer of the natural order and as the ultimate final cause of natural things. Realism about God, or theism, committed them, therefore, to believing in the existence of God as well as nature. After Augustine, this ontological dualism was modeled on Plato’s, and it was no less problematic. The fundamental difference in their natures makes it difficult to explain how God and the natural world are related as parts of a single world. It was ultimately left as a mystery that could not be fathomed by finite rational minds. Denial of this kind of realism is generally considered atheism, though mere skepticism about it is often distinguished as agnosticism.

Modern. With the rise of modern science, it was recognized that our perceptual experience of the natural world is something distinct from the natural world itself (as the ancient atomists first held), and the foundation of epistemological philosophy shifted from reflection on how we know in which we are living bodies in the natural world to reflection on how we know in which we are minds where ideas have an appearance. Mind is the epistemological foundation from which Descartes tried to prove the existence of the body and the external world of which it is part. The success of Cartesian philosophy would entail realism about the natural world, and thus ontological dualism. But mind and body are substances with such radically different natures that it is, once again, a very problematic ontology, namely, mind-body dualism.

There were, of course, skeptics about its success, notably, the British Empiricists, and they are interesting for their views about substance. Locke argued that realism about material objects involves belief in a substratum, or substance as nothing but a support of the properties that perception reveals objects to have. Since that was to believe that substances have no properties of their own, it was, in effect, to reduce substance as substance to its existential aspect, and thus, Locke could plausibly hold that substratum is an incoherent idea. But even the existential aspect was denied by Berkeley and Hume. They accepted the "bundle theory" of substances, that is, the view that substances are just the bundle of properties that we seem to perceive in them. In any case, since the foundation of modern philosophy was mind, they were implicitly committed to one kind of substance, and the only ontological position open to skeptics was idealism of some kind or other, though only Berkeley embraced it explicitly.

Later attempts to justify science from the epistemological foundation of modern philosophy led to other forms of realism, though they were not called that. Kant tired to avoid the problems of Cartesian philosophy by holding that space and time are merely forms of intuition in the mind. But since he continued to believed that there are things in themselves, he was implicitly committed to entities that are not in space and time. That landed him with the same kind of problematic ontological dualism as Plato, and like Augustine, he simply denied that it is possible to explain the relationship between the natural world and the things in themselves which are outside time and space.

Contemporary. Early in the twentieth century, developments in logic by Russell and Frege offered a new foundation for epistemological philosophy. Reflecting on our use of language, so-called Anglo-American analytic philosophy took as their epistemological foundation what we all know about the meanings and references of the terms and sentences we use. This foundation has been used in various way, leading to different forms of realism.

Analytic philosophy was able to reformulate empiricism as a justification of science at the expense of modern metaphysics. Logical positivists took the observation of objects in the natural world as the epistemological foundation of science, and they tried to show how scientific conclusions were supported by it. Though their original purpose was to show that whatever is not based on observation is meaningless metaphysics, it was soon noticed that even theories in physics mention unobservable entities, such as electrons, quarks, and force fields. Thus, those who believed in their existence came to called "realists about theoretical entities."

More recently, the recognition that such unobservable entities are not very different from the observable objects on which science bases its theories has led to calling the defenders of science "scientific realists." Scientific realism is taken to involve a commitment to the existence of both the observable and unobservable objects recognized by science. Or in the words of Wilfred Sellars, "science is the measure of all things, of what is that it is, and of what is not that it is not" (p. 173). But disputes still rage in the professional literature about the significance of calling it "realism."

Most recently, philosophers of science have tried to avoid problems about realism by simply abandoning traditional epistemology all together. They often call themselves "naturalized epistemologists," for they hold that the only foundation for justifying science is science itself (that is, the conclusions that science draws about how we know). Though they say that they believe that philosophy is continuous with science, to ontological philosophy, they seem to be giving up philosophy altogether in favor of being cheerleaders for science. See Kitcher and Rosenberg.

Giving up epistemological philosophy does not necessarily mean, however, taking up ontological philosophy. The habit of epistemology makes it seem that ontology is purely descriptive. The job of ontology seems to be just to discover the kinds of entities to which one is committed by holding certain beliefs to be true.

With regard to natural science, for example, ontology is just realism about the conclusions of science.

In the philosophy of mathematics, realism is defended by so-called Platonists, who hold that numbers and other mathematical entities exist independently of the subjects who know about them (in opposition to logicists, who argue that mathematics can be reduced to logical truth, and to constructivists, who argue that mathematical objects are simply constructs of the imagination).

Even language is taken as a foundation for descriptive ontology. Quine (1953, 1960) has argued that talk of classes implies the existence of at least that one kind of abstract entity. Some analytic philosophers now argue that to believe in the truth of descriptive statements is to be committed to the existence of properties as well as the substances that have them, or what might be called substance-property dualism.

Scientific realism leads some analytic philosophers of science to take laws of nature to be real, which entails a dualism of laws and the objects that that obey them.

In any case, realism is not explanatory ontology, but just ontology as realism. It does not use the entities it postulates to explain anything beyond the phenomena on which their existence was defended. That leaves plenty of room for philosophical argument, because descriptive ontologists generally take a skeptical attitude and are inclined to deny the existence of any kinds of entities whose existence is not forced on them by their epistemological foundation. But that is a different issue entirely from explanatory ontology.

Method. The final assumption needed to secure a foundation for ontological philosophy is a method for deciding which of the possible ontological explanations to believe. We will assume that we ought to believe the best ontological explanation of the world, and since we are naturalists, that means preferring the best ontological explanation of the natural world. Since the empirical method can be defined as inferring to the best explanation, that makes the foundation of ontological philosophy empirical ontological naturalism.

The empirical method is the same method that science uses, except for applying it to a different kind of explanation. But it is not the only possible method for deciding what to believe. The alternative is the rational method of traditional, epistemological philosophy. Its foundation was a theory about how we know, which was based on reflecting on our processes of knowing. It might also be considered an inference to the best explanation. But since the way we ordinarily explain what is known by reflection is by giving reasons, the method of epistemological philosophy always came down to the claim that certain truths are required by reason itself. Though the actual standard was different in different eras of Western philosophy, they can all be called forms of the rational method.

The empirical method, by contrast, may be considered an inference to the best explanation of what is known by perception. Perception provides relevant evidence in deciding what to believe because it discloses facts about what exists in the world. But for naturalists seeking an ontological explanation, there is no need to limit the evidence to perception.

Given our assumption, as naturalists, that the natural world is the world disclosed to us by perception, the empirical method might also be described as inferring to the best explanation of the natural world. Though science may limit itself to explaining what is known by perception, the latter formulation is preferable, given our ontological purposes, because there is no need to limit the evidence we have about the natural world to what is known by perception. Reflection should also be accepted as providing evidence about the nature of the substances and relations constituting the natural world, because we believe, as naturalists, that the beings in whom reflection occurs are themselves parts of the natural world. That would not be to revert to the rational method of epistemological philosophy, as long as we take reflection and what is known by it to be something found in the natural world that needs explaining, and not as providing a standard for judging what is true. What is known by reflection is no less evidence of what exists in the natural world than what is known by perception, though when we define "naturalism" ontologically, as holding that the world is just what is in space and time, we are taking perception to disclose its basic nature more completely. Thus, since it is the natural world itself, not just what is perceived, that we are trying to explain ontologically, we shall interpret the empirical method broadly as inferring to the best explanation of the natural world, not just what is known by perception.

Having assumed naturalism and the validity of ontological explanation, the third and final assumption of ontological philosophy is the empirical method. That is, if this argument is logically valid, it will not be possible to reject the necessary truths justified by it, unless one denies naturalism, the validity of ontological explanation, or the empirical method.ⁱ

The empirical method. By the "empirical method," I mean an inference to the best explanation of what is found in the natural world (either by perception or perception and reflection). Though this way of deciding what to believe presupposes a kind of explanation, the method can be stated abstractly, because its standard for judging what is best that can be applied to any kind of explanation, or at least, any kind that cites causes in order to explain effects. So let us consider the method abstractly, and then take up the various applications of it.

Inference to the best explanation of the natural world. The standard for the best explanation is simply explaining the most with the least. The best explanation can be identified as the one that requires the least in the way of causes to explain the most in the way of effects. After explaining what this empirical standard requires generally, we will see how it applies to various kinds of explanation, including ontological explanation.

Scope. The explanation with the greater scope is better, other things being equal. That is, if two explanations are equally simple, the empirical method requires us to prefer one over the other, if it explains more of what is found in the world than the other.

The preference for explanations with larger scopes does not always determine which explanation to believe even when other things are equal. When two theories have overlapping scopes, for example, it may be unclear which explains more.

Simplicity. The simpler explanation is better, other things being equal. What does the explaining in an explanation are its causes, for they produce the effects, which are what is explained by the explanation. Thus, if two explanations explain the same range of phenomena, the empirical method requires us to believe one rather than the other, if it requires fewer causes or the causes it requires are simpler.

Nor does the preference for simpler explanations always determine which theory to believe when other things are equal. There may be a trade-off between fewer causes and simpler causes. There is no way to say in general whether to prefer fewer, more complex causes or a larger number of simpler causes. It depends on the kind of explanation involved or, perhaps, the specific case. And even then, there may be no way to decide.

Scope and simplicity are the basic criteria for judging explanations, but there is no reason to deny that there may be other issues about which is the best explanation that arise when specific kinds of explanations are being considered. Appeal can always be made to the basic standard for judging the best among explanations of the same basic kind: explaining the most with the least.

Two sources of error using the empirical method should be noticed.

First, any limitation in the range of theories being considered can lead to errors. Since the empirical method chooses the best among the possible explanations, it works only insofar as all possible theories are being considered.

Second, any limitation in the range of evidence being considered can lead to errors. Since the empirical method chooses the best explanation of what is in the world, it works only insofar as we have found everything relevant in the world. And as mentioned above, naturalists have no reason, in principle, not to include as evidence, along with perception, what is found out about the natural world by reflection, if it is relevant. The subjects and the mental processes on which they reflect are part of the natural world.

Kinds of inferences to the best explanation of the natural world. Since the empirical method is relative to the kind of explanation being sought, we must have the ability to comprehend some kind of explanation in order to use it. Nor can we say in advance which kind of explanation ought be used. We must simply develop whatever ways of explaining we can understand, and then compare them to see how they fit together or, if we must choose among them, which to believe.

Efficient-cause explanations. The empirical method of science is to infer to the best efficient-cause explanation. Explanation by efficient causes is understood as depending on laws of nature, which describe regularities about how causes lead to effects. It is usually represented by the deductive-nomological model (or covering law model, which can be traced to David Hume). This model holds that an event (or regularity) is explained when a description of it can be deduced from true laws of nature and the relevant initial and boundary conditions.

The initial and boundary conditions, or certain salient parts of them, are said to be the cause, and the event (or regularity) entailed by them and the law of nature is the effect.

This model works well for physics, but there has been a long dispute about its adequacy for other branches of science. Those disputes are not relevant here, since we are more concerned with comparing efficient-cause explanations with other forms of explanation than with details about how it is applied in specific cases. (A better account of the kinds of scientific explanations that this model slights will be given when we take up the necessary truths of ontological philosophy. See Change: Epistemological theories of causation)

Scope. The explanation of any specific event (or regularity) is just one of a whole range of explanations that may be based on the same law, and the scope of the explanation includes all the events (and regularities) that can be explained by it. According to the empirical method, therefore, the best efficient-cause explanation, other things being equal, is the one that follows from the most general laws of nature, that is, the natural laws with the largest scope.

Simplicity. The simplicity criterion requires us to prefer the explanation with the fewest causes and the simplest causes, other things being equal.

The explanation with the fewest causes, in the case of efficient-cause explanations, would be the one with the fewest relevant initial and boundary conditions. Since what makes such conditions relevant are the laws of nature, this is usually the requirement of preferring efficient-cause explanations that require the fewest laws. Thus, given any two explanations with the same scope, the empirical method requires us to prefer the one requiring the fewest laws of nature and the fewest relevant initial and boundary conditions. But if two explanations appeal to the same laws, we should prefer the one that requires the fewest and simplest initial and boundary conditions.

The explanation with the simplest causes may also mean, in the case of efficient-cause explanations, the one with the simplest laws of nature. The criterion of simplicity in this case has notorious problems, because natural laws formulated in terms of quantitatively precise mathematical formulas can be simple in different ways. However, even without a generally accepted standard of mathematical simplicity, scientists usually manage to reach agreement on this matter. Those issues need not, in any case, concern us, given the altitude of our comparison of these forms of the empirical method.

Since criteria for explaining the most with the least can be traded off against one another, the empirical method does not necessarily determine which theory to believe in science. But this is how the goal of science is usually formulated. The so-called "holy grail" of contemporary physics is an example. That goal is to find a single, basic natural law that would cover all the forms of motion and interaction among bits of matter that physics recognizes, including not only electromagnetism and the weak and strong (or color) forces, but also gravitation. This goal shows a commitment to finding the simplest explanation with the largest scope, though physicists have encountered intractable problems in their quest to formulate such a law. (The biggest problem is that it does not seem possible to state Einstein's theory of gravitation in the same kind of mathematical formulation as the laws for the other basic forces, that is, as a quantum field theory, without postulating ten or more dimensions of space!)

Efficient-cause explanations are also given in ordinary life, engineering, and less basic branches of science, where the empirical method is applied more loosely. We can understand most causal connections apart from formal deductions for mathematically formulated laws of nature, because we have a form of imagination (spatial imagination) that enables us to think about the relations of objects in space and to how they change as objects move and interact over time. Spatial imagination represents very basic regularities, which are implicit in the laws of physics, but it can also represent what specific laws of nature require against this background understanding. This remarkable capacity is easily overlooked, because it is built into our faculty of perception as our way of understanding what perception discloses about nature. In any case, this way of understanding efficient-cause explanations enables us to use the empirical method, because, despite its non-formal nature, it enables us to see which theory explains the most with the least.

When events that depart from expectations, such as accidents, for example, are explained by efficient causes, the empirical method enjoins us to prefer the explanation that requires the simplest causes (the simplest deviations from normal, which are most likely) and the fewest causes (rather than a combination of independent deviations). But it also requires us to prefer the explanation with the largest scope, and thus, we prefer an explanation that can also account for other details about the accident. Or in the case of regularities generated by a mechanism of some kind, the empirical method would have us prefer the simplest mechanism that can explain the most about the regularity in its behavior. Such judgments depend more on our capacity for spatial imagination than precise formulations of laws of nature, though the latter may be relevant in choosing among them when more precise quantities are relevant.

Rational-cause explanation. Though social science also uses the empirical method of natural science, it has another kind of explanation which it shares with the humanities, distinguishing it from natural science. It is called "rational explanation." Since it explains phenomena by causes, the empirical method can be used in inferring to the best rational explanation. But the nature of rational explanation is such that the empirical method does not, in general, lead to agreement about what to believe about the world. What follows is not meant to defend rational explanation in science, but merely to show how rational explanation can be seen as another instance of the empirical method.

It is possible to explain what rational beings like us do and believe by the reasons that lead them to choose to do it or to believe it. For example, actions can be explained by the beliefs and desires that are responsible for them, and beliefs can often be explained by the perceptions and established beliefs that are responsible for them. When we are explaining the actions or beliefs of other subjects, what is explained are ultimately objects of perception, just as in natural science, for we know about their intentions and beliefs of others only by perceiving their behavior. Some of that behavior is, of course, verbal behavior, which is especially revealing, but this kind of explanation can also be given of other animals, notably, mammals. What makes human beings basically different is that they are reflective subjects. That is, in them, beliefs, desires and perceptions are not mere causes of actions and belief, but causes that have effects on other beliefs or behavior by way of the subject’s reflecting on them. These causes are so special that they are called "reasons." Furthermore, what enables us to identify these causes and see their roles in causing action and belief is reflection.

Reflection plays a role in rational-cause explanation that is analogous to the role of spatial imagination in ordinary efficient-cause explanations and the laws of nature cited in more formal scientific efficient-cause explanations. What enables us to connect cause with effect in the case of rational explanations is reflection on our own capacity for reasoning. When we explain another person’s action by citing certain beliefs and desires, our ability to tell the relevance of those beliefs and desires as causes of the action in question comes from reflecting on what we would do if we had certain desires and we believed that we were in the relevant situation. Likewise in seeing the relevance of reasons as causes explaining certain beliefs, we reconstruct the argument in our own brains.

Rational explanation works well enough in the case of the actions and beliefs that occur in the ordinary practice of carrying out our lives. Insofar as the actions and beliefs to be explained have to do with moving bodies around in a world of objects in space in order to satisfy desires, we can understand the causes of the other’s behavior by reflecting on what our own spatial imagination would lead us to do in the situation. That is the kind of behavior that can be explained rationally in other animals. But we can usually reach agreement about ordinary social interactions of human beings as well, because members of a society share expectations about one another’s actions and beliefs. To explain a particular action or belief is usually just a matter of identifying which of the familiar reasons happened to be responsible for it in that case.

Agreement about which is the best rational explanation is reached easily in such ordinary causes, and it can be seen as an application of the empirical method. Familiar reasons are the simplest in the sense that they fit into the background of beliefs and desires that people share, and we usually prefer explanations that require the fewest familiar reasons to explain any particular action or belief. In short, we assimilate their behavior to what is normally expected. Furthermore, the scope of such explanations is maximally large, because the rational explanation is confirmed by how normal expectations also explain other aspects of the person’s behavior.

Actions or beliefs that are unusual, however, cannot be assimilated to the normal pattern. They call for rational explanation in a way that can also be seen as an application of the empirical method. We start, as always, from the neutral background of ordinary behavior and beliefs with generally accepted reasons in the society and we try to identify the special reasons that are responsible for the unusual beliefs or behavior. These are desires, beliefs or perceptions that stand out as different from that neutral background, and since the empirical method requires us to explain the most with the least, we look for the explanation that requires the fewest deviations from the background and the simplest (or most plausible) ways in which they might deviate. And we look for the combination of such deviations with the largest scope. The same beliefs and desires can cause many different actions and beliefs, and thus, we prefer the rational explanation of the action (or belief) in question that can also explain other actions (or beliefs). The more of a person’s behavior that a rational explanation can explain, the better the explanation, other things being equal.

Though each of us may use the empirical method to decide what to believe about the reasons for a person’s behavior or beliefs, this may not lead us to agree on which the explanation. The problem is that rational explanation depends on reflection, rather than just perception. Each of us must use our own processes of reasoning to judge which possible reasons explain the most with the least. Those reasoning processes involve our own beliefs about the world, the perceptions that we have had, our own desires, values and what we have already decided to do or believe on the basis of them. And the further what is being explained is from the familiar, everyday actions and beliefs that we have all made part of our way of viewing the world, the more differences tend to show up in how we think. People have vastly different views about the most general and basic issues, such as the nature of the world, what is possible, where beings like us come from, what is the purpose of life, what is good and bad, what to strive for, what is worth worshiping, and the like. And such differences extend into everyday actions and beliefs when those giving the explanations come from different cultures. Since what is the best rational explanation depends, in part, on which set of background beliefs and goals the explainers themselves accept, the empirical method does not, in general, make it possible to reach agreement.

It is widely recognized that the social sciences and humanities are not as objective as the natural sciences. But that is not an indication of any inherent weakness in the empirical method. It is, rather, an indication of the difference between the forms of understanding that are required for the explanations involved. Spatial imagination is more uniform than rational imagination, and that makes it easier for people to agree about which theory explains more with less. What the relativism of the social sciences and humanities shows is not the weakness of the empirical method, but the weakness of rational explanation (at least, as long as we come from different cultures and have different basic beliefs and values).

Ontological-cause explanations. The empirical method can also be used in philosophy (and science) by inferring to the best ontological-cause explanation of the world. The nature of ontological explanation has already been explained: it explains the existence of everything found in the world by showing how it is constituted by basic substances and the basic relationship by which they exist together as a world.

This kind of explanation is intelligible to us because of our spatial imagination (that is, the capacity to think coherently about spatial relation and how they change as a result of motion). That is the same capacity on which efficient-cause explanation depends. The difference is that what is being explained by ontological explanations includes the existence and basic traits of the objects found in the world, such the fact that objects have spatial relations and that change is possible, not just what happens to them. But an adequate ontology must also be able to explain why (true) efficient-cause explanations are true. The relationship between an efficient cause and its effect is a kind of regular change, and an ontology must show how the regularities described by the basic laws of physics can be just aspects of basic substances enduring through time with the basic relationship that makes them parts of the same world. That is how ontological-cause explanations are more basic than efficient-cause explanations -- they explain the premises of efficient-cause explanations, both the laws of nature and the initial and/or boundary conditions. .

Ontological explanations differ from one another in the kinds of basic substances they postulate and what they assume about how substance exist together as a world, and empirical ontology decides which is true by which offers the best ontological explanation of the world, that is, which explains the most with the least.

Scope. It might seem that ontological theories are all alike in scope, because they all claim to explain the possibility of everything found in the world. The failure to account for any aspect might be said to show that it is not an ontological explanation at all, must less an adequate one. This is not quite true, however, for two reasons.

First, because there is a difference between explaining and merely assuming. The causes by which an ontology explains the world are the substances it postulates and the basic relationship it takes them to have, and thus, to the extent that what is being explained about the world is the same as what is assumed by the ontology, it is not really explained, but merely assumed. To some extent, that may be true of every possible ontology, but the best one will be, other things being equal, the one in which more is explained and less is merely assumed. That one has the greater scope.

The second reason is that, in an ontological explanation, there is a difference between explaining the possibilities of aspects of the world and explaining their necessity, and the more aspects of the world that are shown to be necessary, the better the ontological explanation.

What an ontology entails about the world holds necessarily. Though that determines the range of what is possible, contingent aspects of the world are left to be known though experience of what is actual. An ontology does not itself explain why certain contingent conditions are actual and others not; that requires an efficient-cause explanations. However, since it must explain the possibility of what is contingent, it may be said to "account for" whatever falls within the range of the possible.

Thus, the minimum requirement of an ontological explanation is that it, at least, "accounts for" everything in the world (in the sense of showing that it is possible). And if anything is found in the world that could not exist, if the ontology were true, then the ontology must be false. But ontologies that are not falsified may differ in the range of what they show to be necessary and what they imply is merely contingent. The principle of explaining the most by the least would require those committed to the empirical method to prefer ontological explanations in which more about the world is shown to be necessary and less turns out to be merely contingent. Thus, there is another possible difference in scope among ontological theories

Simplicity. The simplicity criterion requires us to prefer the explanation with the simplest and fewest causes, other things being equal. In the case of ontological causes, the explanation with the simplest and fewest causes would be the one that postulates the simplest and fewest kinds of basic substances and simplest basic relationship among them. Thus, given two ontological explanations with the same scope, the empirical method requires us to prefer the one that postulates the simpler basic substances, the fewer kinds of basic substances, and the simpler basic relationship among them.

Though it is generally clear which theory has the fewer basic substances, it may not be clear which kinds of basic substances and which basic relationships are simpler. From what we have assumed about the essential natures of basic substances and relationships, however, there is one clear criterion. We have seen that the essential natures of substances may be temporally simple or temporally complex, depending on whether their essential properties exist fully at each moment or they are dispositional and have to do with regularities about how contingent properties change over time. And we have seen that there are also such differences in the simplicity of the basic relationship by which an ontology describes how they are parts of the same world. Thus, given two ontological explanations with the same scope and same number of kinds of basic substances, the empirical method requires us to prefer the ontological explanation whose substances have the simplest essential natures and the simplest basic relationship to one another.

When all these criteria weigh in for the same alternative, the empirical method is decisive. But trade-offs among them can keep the empirical method from telling us which ontological theory to believe. That does not necessarily mean, however, that limitations in the mechanical application of these criteria can be used to argue that no choice can be made among theories in which there are trade-offs. It may still be obvious, when specific trade-offs are considered, which one explains the most with the least.

The rational method. For epistemological philosophy, by contrast, the method of choosing what to believe is not the empirical method, but the rational method. This is not quite the same as an inference to the best rational-causal explanation, because what epistemological philosophy needs in order to be a kind of philosophy is a foundation from which to prove necessary truths about the world. What makes epistemological philosophy different from ontological philosophy is that it uses as its foundation a theory about the nature of reason rather than a theory about the nature of the substances constituting the world. And the necessity of its implications comes down to their certainty, given the certainty of the epistemological foundation. Its reliance on a theory about how we know about the world is what earns it the name "epistemological" philosophy (epistemology being, literally, the explanation of knowing). Moreover, such a foundation is secured by reflecting on how we know. As we have seen, reflection is what enables us to give rational-case explanations of the beliefs and behavior of other beings like use. But epistemological philosophy uses reflection to explain how reason works in general. That is, it uses reason's own power to reflect on how it works to defend a theory about how reason works, rather than merely to say which reasons are responsible for particular conclusions about what to believe or do.

Its theory of how we know is supposed to show that certain truths must hold of the world, and its success in using its foundation to prove necessary truths about the world is called realism. Since it would show that something exists beyond its epistemological foundation, it typically leads to metaphysical dualism of one kind or another (as we have seen in Ontology: As realism).

The theories about the nature of reason used by epistemological philosophy are all based in one way of another on a faculty of intuition, which is taken for granted. (The reason for this reliance on intuition is explained in Change: Evolutionary stage 10: The career of epistemological philosophy.)

There is, however, so little agreement in the history of philosophy about the nature of reason that the best way to explain the rational method of epistemological philosophy is to survey the main kinds of theories about the nature of reason that have developed in the history of philosophy.

Ancient Philosophy. Plato assumed that we know by a kind of intuition in which the objects of knowledge are present to the subject. In the case of perception, they are visible objects in space which can move and interact with other objects, and these he assumed were parts of what he called the "realm of Becoming." We also have a capacity to reason about things, in which we understand their natures, and the objects that are present to us in this way of knowing are what Plato called the Forms, which he believed exist in a realm of Being outside space and time. His "doctrine of recollection" is a myth that explained this rational intuition as resulting from our immortal souls having existed in the presence of the Forms prior to our acquiring bodies in the realm of visible objects. Since the objects of rational intuition are the natures that we recognize in visible objects, he thought that the Forms were responsible for visible objects having whatever natures they seemed to have. Thus, by intuiting the Forms directly, we could know truths about them that are necessary relative to perception, that is, our ordinary way of knowing. That included knowing what is good about visible objects, since the Forms were supposed to follow from The Good Itself and visible objects were supposed to be striving to be like their Forms.

Aristotle also understood perception and reason as forms of intuition that make their objects present to us, though he explained them differently. Perception was supposed to be the result of our sensitive soul taking on the same kinds of sensible forms that exist in the particular substances, and reason was supposed to be the result of our rational souls taking on the essential forms of the objects as a result of "induction" from our perceptual experience of many instances of their kinds. Knowing the essential form of an object gives us knowledge of what holds necessarily, because according to Aristotle, there are final causes at work in nature that make natural substances change in the direction of an end state which is the fullest actualization of their essential form. Not only does that explain certain changes that they undergo, but it also tells us what is good for them. This knowledge, Aristotle argued, was prior to the received, ordinary ways of knowing the true and the good.

Medieval philosophy. Medieval philosophy is basically a continuation of Platonic dualism, except that The Good Itself, or a Form, is replaced by God, or a person. Thus, it retains the theory about the nature of reason on which ancient epistemological philosophy was based. If anything was new in the Medieval period, it was how the new view about the nature of the transcendent being was used to argue for its existence. And the main reason that these argument for the existence of God were not compelling in the end is that they are based on the assumption that principles recognized to be valid within the natural world can be applied to the natural world as a whole.

The belief that every event has an efficient cause can be used, for example, to show that there must be a first cause, when it is assumed that the world as a whole is an event to be explained. Final causation affords a similar proof of the existence of God. Given that every natural change within space and time has a final cause, it could be argued that there must be a final cause of the natural world as a whole, as long as it was assumed that the world as a whole is a kind of natural change and can be explained by the same principle. The argument from design works in the same way. Given that artifacts can change for the sake of an end that is good for them only because they are designed to do so by their creator, the fact that nature itself involves change for the sake of ends that are good could be used to show that there is a creator who designed the natural world to bring about such ends. Even the argument from the recognition of a difference between better and worse to the existence of something that is best can be used to show the existence of God when it is assumed that the world as a whole is not the best.

The ontological argument was the most original use of the rational method in the medieval period to prove the existence of God, and given our assumptions about the nature of existence, we can see the fallacy involved in it. As Anselm put it, since we can think of being "than which none greater can be conceived," God exists. For if the being we are thinking of did not exist, there would be a greater being, namely, one with all the same perfections we were thinking of plus existing. The premise of this argument is that absolute perfection entails existence. But if existence and essence are the two basic aspects of the nature of substance as substance, existence is not entailed by perfection, for perfection characterizes a things essential nature and that is a different aspect of any substance from its existential nature. The perfect being would exist only if he is a substance, and not just a conceivable essence.

This is not quite Kant’s critique of the ontological argument, for he argued that existence is not a property at all. On our theory about the nature of substance, existence is a property, albeit a very basic property — as basic as having an essence is. Having both properties is what makes something a substance.

Modern philosophy. Modern philosophers had a fundamentally different theory about how we know, for they had given up naïve realism about perception and recognized that the appearance of the natural world in perception is part of the subject, which they understood as ideas in an immaterial mind. That was also to give up the belief that reason is a direct intuition of Forms existing independently of the mind. But on reflection, they found certain ideas in the mind whose truth they could not doubt, and such so-called clear and distinct ideas were taken to be truths that hold necessarily. Descartes believed that clear and distinct ideas enabled him to prove (by way of proving the existence of God) that a natural world exists independently of the mind and is the cause of our perceptions. He also believed that this showed that the natural world has the essential nature of extension, and thus, he claimed that philosophy provided knowledge about the natural world that is necessary, relative to what is known by perception. Since rational knowledge is prior to what is known by experience, Descartes believed that he had justified the method of modern science as a way of learning the details of natural mechanisms. Other rationalists, such as Spinoza and Leibniz, argued from similar theories about the nature of reason to necessary truths about the natural world.

Kant defended necessary truths about the natural world on a theory about how we know that sees the mind as constituting in part what is known, including the natural world investigated by science. Thus, Kant could argue that the part of what is known that depends on the mind’s contribution is a priori knowledge about the natural world, holding universally and necessarily relative to what perception discloses about what is actual in the world, or what he called synthetic a priori.

Kant’s theory of knowledge forced him to deny that we could know the real nature of things in themselves, that is, what really exists independently of mind, but Hegel adapted Kant’s theory of knowledge in a way that enabled him to claim for philosophy the power to know the real nature of the world. He assumed that that the object of knowledge was entirely constituted by a mental substance through what he called dialectical reason, and thus, by reflecting on the nature of dialectical reason, Hegel also thought that it was possible to show what holds necessarily about the world, relative to what is known by science or other ordinary ways of knowing.

Contemporary philosophy. Even contemporary analytic philosophy had a rational method of knowing what is necessary about the world. They assumed that as users of language, we know the meanings and reference of the terms and sentences we use. Though we can use language to describe what we observe in the world and, thereby, follow the empirical method in science, they argued that there are certain truths that hold necessarily about the world because they are entailed by the meanings of the terms we use. Thus, analytic philosophy had a rational method for justifying necessary truths, though it was much less ambitious than earlier kinds of epistemology, because what is necessary was limited to analytic truths.

In each era, there have also been skeptics about the rational method, especially when they entailed kinds of ontological dualism, such as form and matter and mind and body, in which it was hard to explain how the two different kinds of substances could be related as parts of the same world. The inability to answer those skeptics led to doubts about the rational method itself and ultimately to the demise of epistemological philosophy.

Spatiomaterialism. Given these three assumptions of ontological philosophy, the final step in securing its foundation for necessary truths is to use them to decide what to believe about the basic nature of existence. As it turns out, the empirical method is decisive. There is one ontology that we must choose over the others, if we follow the empirical method, and it is different from the currently accepted ontologies. The two received views are both ontologies of science. They come from realism about contemporary physics. One is materialism, the view that matter is the only kind of substance constituting the world, whereas the other maintains that an opposite kind of basic substance helps matter constitute the world, namely, spacetime. But as we shall see, naturalists who take ontology to be explanatory and follow the empirical method in deciding what to believe ought to reject both in favor of the view that the world is constituted by space and matter, both existing as substances in time, or what I will call "spatiomaterialism."

We can see that spatiomaterialism is the best ontological explanation of the natural world by considering the various possible theories on each of the basic issues about what exists in the natural world: time, space, and matter. In each case we will decide what to believe by which theory offers the best ontological explanation of what is found in the natural world -- the one that explains the most with the least.

Our conclusion will be, however, that we ought to accept these ontological position if they are otherwise possible. There are ways they may be falsified by certain unobvious phenomena which we are not currently taking into account. I mean the observations used as evidence for Einsteinian relativity, as well as the fact of consciousness, the real difference between good and bad, and the validity of the belief that there is something worthy of worship. We will not be in a position to show how those phenomena can also be explained until we take up the necessary truths of ontological philosophy.

Time. We have already assumed that the world is in time by assuming that substance as substance has a temporal aspect to its nature, but as we have also seen, there is a further issue to be decided.

Possible explanations. We know from our experience of the world that objects are in time as well as in space, but as we saw in Ontology: Temporality , there are two possible theories about the nature of time. We are looking for an explanation of the world by substances, but we can believe either that substances endure or that they perdure over time.

Endurance theory of time. To hold that substances endure through time is to hold that they exist only at the present moment. Existence itself is in time. The past and the future do not exist. This view is sometimes called "presentism," but we are also assuming that what exists are substances. Thus, since substances never come into

existence nor ever go out of existence as time passes, the substances that exist now did exist in the past and will exist in the future. In other words, substances are identical across time. Each substance that exists at one moment is identical to some substance that existed or will exist at every other moment in the history of the world.

Perdurance theory of time. To hold that substances perdure over time is to hold that all the moments of their histories exist in the same way. Time is just a relation that holds among those moments. The past and the future exist in the same sense as the present, for "past’ and "future" are just ways of referring to other moments relative to some moment taken as present. Though the perdurance theory of the temporal existential nature of substances can agree that substances never come into existence nor go out of existence over time, what they mean is that substances are wholes made up of parts, with each substance having a momentary part for each moment in the history of the world.

The best ontological explanation of time. Between these two theories, the empirical method requires us to prefer the one that explains more with less, that is, the one that uses fewer and simpler ontological causes to explain more phenomena as effects. According to each criterion, the endurance theory is clearly superior. Consider, first, simplicity.

Simplicity. The perdurance theory must postulate many more substances as ontological causes than the endurance theory, because it holds that every moment in the history of each permanent substance has a distinct and equal existence.

In fact, each moment is like a substance, according to the perdurance theory, for it is a distinct ontological cause that must be postulated separately in order to explain the world ontologically. But if such moments are substances, they are rather unusual substances, because they lack the temporal aspect of the existential aspect of the nature of substance as substance. (Though they are as eternal as the world, they do not exist at every moment in the history of the world, for they are only one moment in the history of a permanent substance.) Still, they have particularity. Each moment is a particular substance with an existence that is distinct from every other substance (including all the other moments in the history of the same permanent substance). Thus, each has both an existential and essential aspect to its nature (its essential nature being whatever properties hold of the permanent substance at the relevant moment in its history). So let us grant that they are substances of a kind. I will call them "momentary substances," since they do not endure through time but exist non-temporally (if not eternally) as one moment in the history of a permanent substance.

Since every momentary substance must be postulated separately, the perdurance theory requires many more ontological causes to explain each permanent substance postulated by the ontology. Indeed, the perdurance theory must postulate (indenumerably) infinitely many momentary substances for each permanent substance, since time is continuous (as evident in its infinite divisibility), and may well be eternal (that is, infinite in extent). Judging simplicity by the number of ontological causes required, therefore, the empirical method requires us to prefer the endurance theory. The endurance theory needs to postulate only one enduring substance to account for each permanent substance in the world.

It may seem, however, that there is a defense for the simplicity of the perdurance theory. Though its "momentary substances" are greater in number, each is simpler in its nature than enduring substances, and thus, its ontological causes are simpler.

What makes momentary substances seem simpler than enduring substances is that momentary substances do not have to endure through time, but can simply exist eternally as one moment in the history of a permanent substance. But why is that simpler?

Perhaps, the simplicity comes from having a temporally simpler nature. Momentary substances cannot have temporally complex properties, because they are what exists at only one moment in the history of a substance that never comes into existence nor goes out of existence. But that does necessarily make them simpler than enduring substances, for enduring substances can also have essential properties that exist completely at each moment of the existence of the substance. On both views, therefore, the essential properties of substances can exist completely at each moment of the existence of the substance. Thus, the only difference between them is that enduring substances exist at many more moments than momentary substances. But that is just the difference between them. To take that as showing the greater simplicity of the perdurance theory would be to beg the question.

On the other hand, perhaps the greater simplicity is supposed to come from its theory about the nature of existence. The endurance theory holds that existence itself is in time, and since that means time is an aspect of existence, a permanent substance must endure through time in order to exist as a substance. Thus, it might be argued that the perdurance theory is simpler, because it takes existence to be just the self-subsistence of the momentary substances making up the histories of permanent substances. Existence is non-temporal, rather than being in time. And this greater simplicity about the perdurance theory enables it to explain ontologically why permanent substances exist at every moment in the history of the world: each permanent substance is a whole made up of many momentary substances as its parts.

However, that supposed ontological explanation brings out the cost of not assuming that existence is in time. Not only must the perdurance theory postulate infinitely many momentary substances to account for each permanent substance, but it must also assume a basic relationship that gives those momentary substances infinitely many relations to one another. The events in the history of a permanent substance occur in a certain order, and so the momentary substances that must be related in a certain way in order to constitute it. Though those relations may be simply how the momentary substances exist together as a world, they must all be assumed in order to deny presentism.

Thus, not only are momentary substances not simpler than enduring substances in virtue of having temporally simple essential natures, but the perdurance theory must also postulate infinitely many momentary substances with infinitely many relations among them to account for each permanent substances.

As far as simplicity is concerned, therefore, endurance theory is clearly superior. It postulates one enduring substance to account for each permanent substance, whereas the perdurance theory must postulate infinitely many momentary substances with infinitely many relations among them in order to explain each permanent substance. But this ontological extravagance might be justified, if the perdurance theory can explain why permanent substances are in time, and so let us turn to the criterion of greater scope.

Scope. The criterion of greater scope requires us to prefer the theory about the nature of time that explains more to the one that explains less.

It may seem that the perdurance theory does have a greater scope, because it explains at least one phenomenon ontologically that the endurance theory simply assumes. It explains ontologically why permanent substances are in time by showing how they are constituted by momentary substances and relations among them. But this claim to have an ontological explanation of substances being in time does not stand up, for two reasons.

First, it is ad hoc. Nothing is explained by the assumption that permanent substances are constituted by momentary substances and relations among them except their being substances that exist at every moment in the history of the world. To postulate infinitely many ontological causes to explain a single aspect of the world is to explain the least with the most, the opposite of the empirical criterion. To be sure, the endurance theory does not explain this aspect any better. But there is nothing to prefer over the assumption that existence itself is in time.

Second, there is an aspect of this phenomenon whose possibility the perdurance cannot explain. That aspect is how the present moment is different from all the other moments in the history of the world, both past and future. It is something for which the endurance theory can account. And the failure even to account for it (that is, the failure to explain its possibility) means that the perdurance theory is falsified by it.

Endurance theory can account for the fact that one moment in the history of the world stands out as different from all the others, because it holds that only the present moment exists. The present is different from the past and the future in the most basic way, as far as ontology is concerned, because the present exists, while the past and future do not. That is what it means to hold that existence itself is in time. (This is not to explain the phenomenon of the present ontologically, because it is simply what the endurance theory assumes about the nature of existence. But the endurance theory does not have to deny that present is different from the past and future.)

The perdurance theory, on the other hand, cannot even account for the fact that the present stands out as different from all the other moments in the history of the world. It holds that all the moments in the history of a permanent substance exist in exactly the same sense, and so there is nothing ontological that can distinguish any one moment from all the rest that help make up its history. To be sure, the perdurance theory can say how any moment in the history of a permanent substance that is taken as the present differs from those that occur earlier and those that occur latter, for its momentary substances are all related to one another in a certain order. But it has no way to single out any moment in the history of a permanent substance as "now."

This blindness to the present is implicit in what the perdurance theory says about the nature of existence and time. Instead of taking time to be an aspect of the nature of existence, it takes time to be part of the structure of what exists, that is, a certain kind of relationship that exists among its momentary substances. It sees time as a dimension of what exists, much like spatial dimensions, and thus, time contains different moments in the same way that space contains different point, which means that all moment are contained in the same way.

Thus, far from explaining why permanent substances are in time, the perdurance theory cannot even explain the possibility of the most basic aspect of it. Indeed, the phenomenon of the present being different from the past and future would seem to show that the perdurance theory is false.

What the perdurance theorists can do, however, is explain away the phenomenon. That is, it can explain why we experience the present as being different from all the other moments by holding that it is just an appearance that holds for each and every moment in the history of beings like us. We are rational beings, capable of reflection, and it is by reflecting on our experience that we come to believe that the present moment is different from the past and the future. But if the perdurance theory is correct, each of us is just a set of momentary substances that makes up a personal history. Thus, it is possible to hold that the essential nature of every momentary substance constituting a being like us includes the appearance that that moment in one's history is the present and, thereby, different from all the moments in the past that may be remembered and all the moments that may be anticipated. That is, each moment in the life of a reflective subject includes the subjective appearance that it is present, even though it is just another momentary substance that exists non-temporally.

But this is not to explain the present. It is to claim that our sense of the present is an illusion. That is surely an alternative that we want to avoid, if possible, for it is ad hoc. Anything found in the world could be explained away the same way, that is, explained as a mere appearance to the subject by holding that it is actually part of his essential nature as a substance. If it is possible to explain our sense of the present being different from the past and the future in a way that makes it true, we must prefer the theory that does so. Hence, the empirical method requires us to prefer the endurance theory on the grounds that it explains more than the perdurance theory.ⁱⁱ

This point can be seen more clearly if we consider how the present being different from the past and the future is something found in the world by perception, not just by reflection on how it seems to us. We perceive change in the natural world, and if we articulate the beliefs implicit in such perceptions, we find that what we believe is that certain properties go out of existence and other properties come into existence as time passes.

Consider, for example, a bus passing by us on the street. The property of approaching us goes out of existence as the property of being in front of us comes into existence, and then the property of being in front of us goes out of existence as the property of moving away from us comes into existence. That is how we perceive change in the natural world, and it implies that the properties that the bus had in the past do not exist any longer, and that the properties that it will have in the future do not exist yet. That is what we mean by their coming into existence and going out of existence as time passes.

To be more precise, reflecting on our observation, we find that the experience involves past, present and future. At the moment we see the bus is in front of us, we remember seeing it approach and anticipate its moving away. Were the immediate past and future not part of our experience, we could not observe that the bus is moving. But while the present is seen as existing, the past and future are seen as not existing, albeit for opposite reasons. The past event is seen as not existing because it is over, while the future event is seen as not existing because it has yet to happen.

That only the present exists may be only implicit in the observation. But that does not mean that it is not part of what we observe, only that we have not formulated that aspect as a sentence. The belief that the bus’s past and future do not exist now is as much part of the observation of the bus’s motion as the belief that that the bus is a distinct object in space is a part of the observation of the bus itself. It is not surprising, therefore, that this is called the ordinary view of the nature of time. It is what the “man in the street” would say about the past and future if asked about their existence (see, for example, Putnam [1967], p. 240).

The perception of change as "real" in this sense discloses something about the world that cannot be explained by the perdurance theory, because it must deny that any properties come into existence or go out of existence over time. The perdurance theory holds that every moment in the history of every substance exists in exactly the same sense, and so the properties that hold at earlier moments still exist in the same sense as the present, and the properties that hold at latter moments already exist in that sense.

Again, the only way that the perdurance theory can account for this perceived fact about the world is to deny that it is a fact and to hold that what we think is perception of an independently existing world is just an illusion. That is, its defenders can hold that each of the momentary substances making up the histories of beings like us involves, as part of its essential nature, the appearance that change really takes place as time passes. That would mean that, relative to any given moment, we perceive the past and future states of the world as not existing, even though, in fact, they do.

But this is, once again, to explain away the phenomenon, not to explain it ontologically. It could be used to explain anything found in the world, and thus, it should only be invoked, if it is not possible to explain phenomena as what really exists. The perdurance theory has no alternative, because if change is real in this sense, it is false. But we have an alternative, because the perception can be accounted for by the endurance theory.

The perdurance theory does not, therefore, have greater scope than the endurance theory. Its explanation is ad hoc, and what is worse, it is falsified by the phenomenon that it claims it alone can explain, unless we accept further ad hoc assumptions that make the phenomenon illusory.

Nor do any of the arguments for the perdurance theory offered by defenders of the so-called tenseless theory of time give us any reason to accept it.ⁱⁱⁱ

The empirical method requires us, therefore, to prefer the endurance theory over the perdurance theory. It is simpler in both relevant ways (the fewest and simplest ontological causes), and it has a larger scope (in the sense that it can, at least, account for our sense of the present and our perception of change as really occurring in time). It clearly explains more with less.

Nor are the basic aspects of the world that only the endurance theory can explain trivial. The ability to explain change by the endurance of substances through time is the foundation for explaining regularities about change ontologically. If ontological philosophy had to accept the perdurance theory, it would not be able to show the ontological necessity of the connection between cause and effect in efficient cause explanations. Nor would it be able to demonstrate the ontological necessity of global regularities, on which most of the new ontologically necessary truths depend. Without the endurance theory, ontological philosophy would not be a new way of doing philosophy.

This does not necessarily mean that it is true. It is only to say that we must prefer it, if it is possible, for it may turn out that there are other things found in the world contradict the endurance theory. That is what contemporary Einsteinian believe, as we shall see when we take up spatiotemporalism, and thus, they will have to be answered before we can be confident about the truth of the endurance theory.

Space. Naturalists believe that the world is just what is in space and time, and having seen that we should, if possible, believe that substances endure through time, the next question is what we should believe about the nature of space.

Possible explanations. There are basically three alternatives: spatiomaterialism (the belief that space and matter are both substances), materialism (and the belief that space is just spatial relations), and spacetime substantivalism (the belief that the substance that exists in addition to matter is not space, but spacetime). Though the following argument would have to be reconsidered, of course, if a fourth alternative turns up that is simpler than all of these, that does not seem likely. After describing each of these alternatives, I will consider which offers the best ontological explanation of the natural world.

Spatiomaterialism. By "spatiomaterialism," I mean the belief that the substances constituting the world include space as well as matter. It postulates matter, because it assumes that there are substances in space that obey the basic laws of physics. But it also postulates space as a substance. ("Substantivalism" is the traditional name for the view that space is a substance, though as we shall see, substantivalism about space should be distinguished from substantivalism about spacetime, the kind of substantivalism that is taken seriously today.) Finally, spatiomaterialism assumes that the bits of matter are all contained by space in the sense that each of them coincides with some part(s) of space or other. That is how these two substances exist together as a world, and thus, it is the basic relationship that spatiomaterialism assumes as the other part of every ontological cause.

Spatiomaterialism assumes that space is a substance by our definition, for it assumes that each part of space has both the essential and the existential aspects of the nature of substance as substance. The parts of space are all the locations in a single, three dimensional space.

Each part of space has both aspects of the existential nature of a substance, temporality and particularity, because in addition to never coming into existence and never going out of existence, each location in space has an existence that is distinct from from all the other locations in space (not to mention from any bits of matter that may coincide with it). Though spatiomaterialism is compatible with both theories about the nature of time, we shall take it to include the endurance theory, for as we have just seen, the endurance theory is the best ontological explanation of the temporal aspect of substances. (Only the endurance theory is compatible with the present being different from the past and the future, and the perdurance theory even denies that change involves properties coming into existence and going out of existence.)

Though parts of space have the same kind of existential nature as bits of matter, parts of space have the opposite essential nature. Whereas bits of matter exist independently of one another in the sense that each could still exist, even if the other bits of matter did not exist, parts of space depend on one another in the sense that no part of space can exist without all the other parts of space. The essential nature of each part of space includes having geometrically coherent relations to every other part of space. That is, the essential nature of each part of space is defined by its location relative to all the other parts. Thus, parts of space exist only if space exists as a whole. (Indeed, it is the wholeness of space and what it contributes to the natural world that is the key to almost all the new necessary truths based on spatiomaterialism.)

In other words, space has an opposite essential nature from matter because its parts are not prior to the whole. Since each bit of matter is capable of existing independently of all the other bits of matter in the world, there is a sense in which the parts of matter are prior to the totality. But that is not true of space, because no part of space can exist without all the other parts of space. That does not mean, however, that, in the case of space, the whole is prior to the parts, because the whole of space cannot exist without all its parts. Space is whole in a unique way, as we shall see, and one indication of its uniqueness is the way that the parts of space and the whole are equally basic.

Spatiomaterialism assumes that space has a three dimensional Euclidean structure. Though non-Euclidean geometries can be described coherently, they are not as simple as Euclidean geometry. Euclidean geometry is assumed here, because, as it turns out, there is no reason to doubt that the simplest alternative is true.

To be sure Einstein's general theory of relativity implies that spacetime can be curved and can, therefore, be represented only by a non-Euclidean geometry. But what it implies is curved is not just space, but spacetime, and as we shall see (in Change: General theory of relativity), curved spacetime can be explained as an aspect of space as a substance with a Euclidean geometrical structure (basically by variations across space in the velocity of light relative to space).

The version of spatiomaterialism considered here will also assume that space is infinite.

The infinity of space will be assumed, because that is the simplest nature space can have. Even though the parts of space cannot exist without one another, they are distinct substances, and the essential nature of each particular spatial substance is necessarily unique in the sense that it involves a unique relationship to every other particular spatial substance. But the simplest assumption is that all the parts of space have the same kind of essential nature, that is, the same kind of relation to other parts of space as every other part of space. However, if the parts of space all have the same kind of essential nature, a Euclidean spatial substance must be infinite as a whole. For if there were an end to space, no two parts of space could have the same kind of essential nature. Each part would have a different relation to the edge of space (if makes sense at all to talk about an end to space). Thus, the simplest form of spatiomaterialism would hold that space is infinite.

To be sure, most astronomers and astrophysicists currently assume that space is finite, because its finitude is entailed by the use of the general theory of relativity to represent the big bang and the subsequent expansion of the universe. And it would be possible, if necessary, to formulate a version of spatiomaterialism in which space is finite. But it would be a more complex ontological cause than is assumed here, for its parts would have to have systematically different kinds of essential natures. And it may not turn out that the big bang theory is true, as argued in Change: Cosmology.

Notice, however, that the infinity of space is twofold. There are finite distances in space, for that is entailed by its having a geometrical structure, and there are opposite way ways in which it is possible to generate an infinite series. It is possible, in one way, to keep doubling its size, step by step, forever in the same direction, and it is also possible to keep dividing it in half, step by step, for ever. The former means that space is infinite in extent, whereas the latter means that space is continuous (or that parts of space are connected continuously). Both kinds of infinity are assumed here as simply part of the essential nature of space as a substance.

The continuousness of space means that the number of points on a finite line is greater than the number of whole numbers, which is already infinite. Thus, the points on a line are said to be nondenumerable. (It can be shown, furthermore, that there are just as many points on a finite line as there are points on a finite plane and as there are points in a finite volume.) This can seem puzzling, if it is assumed that lines are made up of points, because more than an infinite number of points would be required to make up a line. This may be problematic for mathematics, but not for ontological philosophy, because we do not assume that space is made up by combining points at all. What is essential about points in space is their distances (and directions) from one another (or the metric of their geometrical relations), not how many points there are in any finite distance. In other words, space is not made up of points in the way that ordinary material objects are made up of simpler bits of matter, that is, by assembling them alongside one another; indeed, according to spatiomaterialism, that way of putting bits of matter together as a whole is something that is possible only because the bits of matter all coincide with parts of space. Rather space is made up of points in the sense that the points all have as their essential natures determinate distances from one another in all three dimensions as parts of a single whole space. Indeed, points can be picked out at all only because the parts of space have such spatial relations to one another. That is, once again, the wholeness of space.

Time has a twofold infinite nature like space. Given any finite period of time, it may be doubled forever or divided forever. Thus, not only does time go on eternally, but it also flows continuously (that is, the moments in time are continuous with one another). And that is likewise simply the nature of time. (Two moments in time are related by the amount of time that passes between them, not the number of moments between them; the temporal metric is what makes it possible to pick out moments in time.) The direction of time, however, introduces an asymmetry that is not found in space, separating the issue about whether time extends infinitely toward the past from the issue about whether it extends infinitely toward the future. Though the big bang theory denies the former, it leaves open the possibility that the future may be infinite. We will, however, proceed on the assumption that time is infinite in both directions, postponing discussion of the big bang until Change: Cosmology

The belief that space is a substance may have been what the ancient atomists, Leucippus and Democritus, meant by insisting that the arche, or "first principle," includes two elements, both atoms and the void. In other words, the usual interpretation of atomism, mentioned in Ontology, may be mistaken. What they meant by the void may not have been merely a kind of stuff between atoms that is so subtle that, unlike other atoms, atoms can move through it without resistance. They may have meant that the void is something that exists not only in between atoms, but also underlies each and every atom. This way of thinking about the nature of space may have been obscured by the lack of any better way than "the void" to refer to what they meant. That is, arguably, one of interpreting the ancient atomists, which would make them the discoverers of the view that is assumed here.

Notice that it is not inconsistent to hold that space is contained by space or exists in space, though it holds for a different reason from matter. It is not inconsistent, because the parts of space are substances and each part of space is, indeed, is located in space or is contained by space in the sense that it has a location relative to all the other parts of space. Indeed, that is part of the essential nature of each part of space. Bits of matter are also contained by space or in space in the sense of having a location relative to other bits of matter. But according to spatiomaterialism, that is not part of the essential nature of matter. Nor is it simply how bits of matter exist together as a world. It is, rather, a result of each bit of matter coinciding with some part(s) of space. It is space that gives bits of matter their spatial relations to one another. Given that space and matter are both ontological causes, the ontological cause of bits of matter all having spatial relations to one another is the basic relationship by which the two opposite substances exist together as a world. It is because the parts of space are contained by space that the bits of matter coinciding with any part are contained by space.

Spatial relationism. By "spatial relationism," I mean the belief that matter is the only kind of basic substance in the world and that space is nothing but the relations that hold among bits of matter. Matter is assumed to have the essential nature described by the basic laws of physics, and spatial relations can be explained in one way or another as how bits of matter exist together as a world. And we will take spatial relationism to include the endurance theory of time (as we did spatiomaterialism), since that is the best explanation and spatial relationism is compatible with it.

Spatial relationism is basically a negative thesis. It is the denial that space is a substance distinct from the material substances in the world. It denies that space exists independently of matter by holding that spatial relations have the same status as properties of matter.

It is not necessary to postulate any substances in addition to matter in order to account for spatial relations, any more than it is necessary to postulate additional substances to account for the properties of material substance. They can be understood as nothing but aspects of the material substances postulated. Just as (monadic) properties are aspects of the substances themselves taken separately, the relations among them are aspects of their existence together as a world.

There are subtly different versions of spatial relationism, as mentioned in Ontology: Nature of relations, depending on this ontology explains the possibility change in spatial relations. How the substances postulated exist together as a world is the most basic relationship that an ontology must assume in addition to the substances to explain how they exist together as a world. That basic relationship determines how substances can be combined and recombined as time passes in order to explain ontologically the diversity and change in nature. Thus, if spatial relations are nothing but how material substances exist together as a world, their basic relationship involves change. That is possible, because the basic relationship among the material substances can be simply having spatial relations of some kind or other, not having any particular spatial relations. That basic relationship does not change even when the particular spatial relations among material substances are changing. But since particular spatial relations do change, there must be some way to explain the possibility of such change. (And since spatial relations change in regular ways, it must also be able to account for those regularities.)

It is possible to hold, on the one hand, that the basic relationship by which material objects exist together as a world has a temporally complex nature. "Having spatial relations" would be defined by describing the regularities in how the spatial relations between material objects change, for example, that they change only continuously over time, that is, by motion. (Material objects do not flit about discontinuously form one place to another.) This would be to define the essential nature of the basic relationship among material substances dispositionally, much as the essential natures of material substances are defined dispositionally when they are assumed to obey the basic laws of physics, that is, in terms of the regularities in how they move and interact.

On the other hand, it may be possible to hold that the basic relationship is temporally simple. "Having spatial relations" would be understood merely as a kind of condition that holds among material objects at each moment as it is present, and the change that occurs in spatial relations would be a result of the essential natures of the material substances. That is, the essential natures of the material substances would be temporally complex, as when they are defined in terms of the basic laws of physics, and the ways in which spatial relations change over time would simply be a consequence of how the basic laws of physics work out.

There are problems with this view, however. One is that the laws of contemporary physics include quantum mechanics, and since they do not describe a fully deterministic process, spatial relations cannot be fully determined by the basic laws of physics (unless there is a so-called hidden variable involved).

But even if the laws of physics were deterministic, there is another problem, for the laws of physics can determine the spatial relations that hold of bits of matter at any one moment only if their spatial relations at some other moment is given. Since the past determines the future, the particular spatial relations may have to be fixed for some earlier point in the history of the universe, presumably at the beginning of the world, such as the Big Bang or when God created it. In any case, the basic relationship would not be temporally simple after all, for having spatial relations would not be a condition that holds only at the present moment, but must include all the particular spatial relations that hold at some other moment in the history of the world.

In either case, however, spatial relationism holds that space is nothing but spatial relations, where those relations are, ontologically, just the basic relationship by which material objects exist together as a world, that is, ultimately, an aspect of the material substances themselves.

Leibniz denied that space is a substance. (And he debated the issue with the Newtonian, Samuel Clarke. See Earman.) But spatial relationism as defined here should be distinguished from the kind of spatial relationism entailed by Leibniz's ontology. Leibniz did not take spatial relations to be how the basic substances exist together as a natural world. The substances Leibniz postulated to explain the natural world were monads, or minds of various kinds, and he explained spatial relations as how monads appear to one another, that is, as ideas in those minds. The way that monads existed together as a natural world was by each being created by God, though Leibniz did hold that the appearances of spatial relations in all the different monads fit together coherently as a natural world.

In any case, defenders of Newton were never able to refute spatial spatial relationism, because they assumed that the only way to prove that space is a substance is by empirical science, that is, by confirming some crucial prediction. Even Newton's theory did not provide any way to measure absolute rest or motion, and all the same phenomena (including the famous rotating bucket; see Earman, pp. 61-90) could also be explained on the assumption that space is nothing but spatial relations among material substances (by taking into account spatial relations to distant stars).

According to Ryansiewicz, the classical debate between spatial relationism and substantivalism about space is no longer meaningful in the context of contemporary physics. But that position is compellingly refuted by Hoefer98.

Spatiotemporalism. Spatiotemporalism agrees with spatiomaterialism that the spatial relations among bits of matter depend on another substance, in addition to matter. What makes it different from spatiomaterialism is that it takes that substance to be spacetime, rather than space. In fact, that makes it fundamentally different from both other theories. Spatial relationism (or materialism) and spatiomaterialism can both accept the endurance theory of time (and both do, as we they have been defined here). But the belief in spacetime as an ontological explanation of the world entails the perdurance theory of time. That is what it means, when speaking ontologically, to deny that space and time are absolute. Though this view is usually called "substantivalism about spacetime," I will call it "spatiotemporalism" in order to bring out the contrast with spatiomaterialism and spatial relationism.

This preference for spatiotemporalism over spatiomaterialism is justified by Einstein’s relativity theories. Minkowski introduced the notion of spacetime in 1908 as a way of summing up what Einstein’s 1905 special theory of relativity implied about the world, and he predicted that it was the beginning of the recognition that space and time are not independent of one another. Einstein then took spacetime as basic in constructing his general theory of relativity, that is, in his explanation of gravitation as a result of a curvature imposed on spacetime by large accumulations of matter in it. And since spacetime must be a substance in order for it to interact with matter in that way, it is now common for philosophers of spacetime to hold that spacetime is the opposite kind of substance that exists in addition to matter and explains why bits of matter have spatial relations. (See Friedman and Earman, for example.)

The basic nature of spacetime is determined by Einstein's special theory of relativity. Einstein called it a theory of "relativity," because it holds that the places and times at which events occur depend on the inertial frame of reference from which they are observed. Different inertial frames have different velocities, and according to Einstein's special theory, they assign different spatial and temporal coordinates to events in the universe. For example, observers on two different inertial frames that happen to be located at the same point at the same time will have different views about which events in the histories of distant objects are occurring at the same time their paths cross. Einstein's special theory holds that their views are equally true, and that implies that the distant objects actually exist now at both moments in their histories. (With additional inertial observers, this means that the distant objects must exist equally at all the moments in their histories that can be said to be simultaneous with different inertial observers here and now). This loss of agreement about the simultaneity of events at a distance might seem to be just a theoretical problem about the nature of objects at a distance, but since Einstein's special theory holds that all possible inertial frames are equivalent, it has the same implications for inertial observers here and now. That is, observers on different inertial frames observing us from a distance would similarly disagree about which moment in our history is simultaneous with their present moment, and thus, in order for all their views to be true, we must actually exist equally at different moments in our history, indeed, equally at all of them. This is to deny presentism, because it forces us to believe that our past and our future exist in the same way as the present.

It is the loss of agreement about simultaneity at a distance that makes the belief in spacetime so problematic. To be sure, no problems arise for physics, because it is always possible to predict from one inertial frame what observers on all the others will say. But when spacetime is understood ontologically, that is, as describing the basic nature of space and time, the denial of simultaneity at a distance entails the perdurance theory of time. What really exists are not substances with spatial relations enduring through time, but a kind of eternal, unchanging four-dimensional world whose parts are spread out not only in the spatial dimension, but also in the temporal dimension. Since the world is constituted by all its parts, different moments in the history of each permanent substance are different parts of the world in exactly the same sense that different permanent substances (including different locations in space) are different parts of the world.

The substantival nature of spacetime is entailed, at least for scientific realists, by the interaction between its curvature and matter that explains gravitation according to Einstein's general theory of relativity. Spacetime could not be what causes material objects to exhibit gravitational acceleration unless it were something that exists in addition to those material objects. This ontological interpretation of spacetime, or substantivalism about spacetime, is what I mean by "spatiotemporalism." It holds that time is ontologically on a par with space (by contrast to spatiomaterialism, which holds that matter is ontologically on a par with space). That is the perdurance theory of time. To hold that time is just another dimension relating parts of substances geometrically is to hold that just as different places in space all exist in the same way, so different moments in time all exist in the same way. This implies that ordinary, permanent substances (that is, substances with a temporal aspect to their existential aspect as substances) do not endure through time, but perdure over time.

The best ontological explanation of space. If we follow the empirical method, we will believe the theory about the nature of space that provides the best ontological explanation of the natural world. That is clearly spatiomaterialism, if it is possible (in particular, not falsified by the any phenomena covered by contemporary physics), because it is better than spatial relationism and better than spatiotemporalism. And since, as I will show, physics does not make it impossible, naturalists who follow the empirical method in deciding which ontology to believe will accept spatiomaterialism.

Spatiomaterialism is better than spatial relationism. It may seem at first that spatial relationism is a better explanation of space than spatiomaterialism, because it postulates only one kind of basic substance, rather than two. Spatial relationism is basically just a kind of materialism that has made its beliefs about space explicit, whereas spatiomaterialism holds that space is a substance existing independently of matter which contains all the bits of matter in the world. On grounds of simplicity, therefore, it might seem that we should prefer spatial relationism to spatiomaterialism.

Simplicity is not, however, the only empirical grounds for preferring one theory over another. There is also the criterion of scope, and by it, spatiomaterialism is clearly superior. Thus, if spatial relationism were simpler than spatiomaterialism, there would be a trade-off between them which keeps the empirical method from choosing between them.

But that standoff is not the final word, because when we look closer at the criterion of simplicity, we will find that spatiomaterialism is at least as simple as spatial relationism, if not simpler. Simplicity is not a simple criterion, for there are two ways in which ontological explanations can be simpler (not only by the number of ontological causes, but also their natures), and by one of them, spatiomaterialism is clearly simpler than spatial relationism. There is, therefore, a standoff on grounds of simplicity, and that makes the criterion of greater scope decisive. Empirical ontologists should prefer spatiomaterialism.

But a decision in favor of spatiomaterialism is even more clear-cut than this may make it seem, for the way in which spatiomaterialism is simpler also reveals another way in which it has a greater scope. In the end, there is no doubt that spatiomaterialism explains more with less. The empirical method will require ontologists, therefore, to prefer spatiomaterialism over spatial relationism. Let us begin by considering the issue about the scope of explanation.

Scope. Spatiomaterialism explains more about the natural world than spatial relationism, because it explains why bits of matter have spatial relations, whereas spatial relationism merely assumes that they do.

Space is a substance with an opposite essential nature from matter, and so if it contains all the bits of matter in the sense that each bit of matter coincides with some part of space or other, the bits of matter acquire their spatial relations from the spatial relations that already hold among the parts of space with which they coincide. That is, space and matter work together as ontological causes to produce spatial relations. Having spatial relations is not just an ontological assumption about bits of matter, since three different ontological assumptions are needed to explain it.

By contrast, spatial relationism does simply assume that bits of matter have spatial relations. To be sure, materialism can "account for" spatial relations; the fact that bits of matter have spatial relations does not show that materialism is false (as presentism and the fact of real change falsify the perdurance theory of time). But that is not to explain why bits of matter have spatial relations. Thus, since we are seeking the best explanation, we must prefer the theory which explains more.

Materialists may demur by insisting that postulating space to explain spatial relations is ad hoc and, thus, not an explanation at all. Though it may appear to be an explanation, they will hold that substantivalism about space is equivalent to assuming that bits of matter have spatial relations. Indeed, it is the same assumption that spatial relationists make, except for being disguised as a substance. Substantivalism about space merely reifies spatial relations.

This objection will not stand, however, because spatial relations are not all that substantivalism about space explains ontologically. It also explains, together with matter, the possibility of change (as well as certain ontologically necessary truths about how bits of matter change, as we shall see later).

The assumption that all the bits of matter are contained by space as a substance implies only that each bit of matter coincides with some part of space or other. But that leaves open which place it is. Moreover, parts of space are connected with one another continuously, so that there are no gaps, so to speak. That is just the essential nature that spatiomaterialism takes space to have. Thus, as space and matter endure through time, it is possible for spatial relations among bits of matter to change, because bits of matter can move across space over time without giving up any of spatiomaterialism's assumptions. The continuousness of time works together with the continuousness of space to make motion possible. Neither space nor matter changes their essential natures, and the bits of matter are always contained by space, always deriving their spatial relations from space. (This way of explaining motion ontologically also implies that motion is the only way that bits of matter can change their spatial relations as time passes. See in Local Regularities under Change.)

Since substantivalism about space explains something more than just why bits of matter have spatial relations, it is not ad hoc, but genuinely explanatory. Spatial relations are only one of several basic phenomena explained by spatiomaterialism.

This is to explain the possibility of change by motion, but it is also possible for spatiomaterialism to explain the possibility of change in another way: by interaction. That is how motion changes, as we shall see. Being in space, bits of matter can move to the same location, and when they do, they are in a position to act on one another, because they not separated from one another by space. (The impossibility of action at a distance -- that is, with spatial substances separating the bits of matter -- is also shown in Local Regularities under Change.)

Thus, since spatiomaterialism can explain the possibility of both motion and interaction, that is, both kinds of change described by the laws of physics, its explanation of spatial relations is not ad hoc. In other words, the greater scope of spatiomaterialism is shown by its ontological explanation of at least three basic facts about the natural world that are just assumptions of spatial relationism -- that bits of matter have spatial relations, that they can change by motion, and that their motion (and other properties) can change by interaction.

This may not be an original argument for substantivalism about space. The way in which space makes motion and interaction possible may have been what Leucippus and Democritus were getting at when they insisted on postulating the void as an element along with the atoms, though that is still a controversial interpretation of ancient atomism.

Simplicity. Since empirically minded ontologists must prefer ontologies that explain more, they have a good reason to prefer spatiomaterialism over spatial relationism. But materialists might hope for a standoff between these two ontologies at this point. The empirical method might fail to choose between them. Although the criterion of greater scope favors spatiomaterialism, the criterion of simplicity may favor spatial relationism, because spatial relationism postulates only one kind of basic substance, not two. Simplicity is not, however, a simple criterion, and when we consider both ways in which explanations can be simple, we will see that there is also a way in which spatiomaterialism is simpler than spatial relationism.

Such a standoff on grounds of simplicity would force ontologists to prefer spatiomaterialism, because they are equal except for the greater scope of the latter. But the empirical method is even more decisive than this suggests, because the way in which spatiomaterialism is simpler is another way in which spatiomaterialism explains more than spatial relationism. Thus, it will be clear in the end that, even though spatiomaterialism postulates two basic substances, rather than one, it explains more with less.

The reason that materialism is not necessarily simpler than spatiomaterialism is that simplicity is judged not only by the number of basic ontological causes, but also by the simplicity of their natures. That is, materialism may not be simpler than spatiomaterialism, even though it postulates only one kind of basic substance, because it may require either matter or the basic relationship among them to have a more complex essential nature than spatiomaterialism. This is how it will turn out, and in order to see why, let us look more closely at the basic relationship assumed by materialist ontology.

The basic relationship among bits of matter, according to spatial relationism, is that bits of matter all have spatial relations with one another. But we are assuming that they are all geometrically coherent, that is, that their spatial relations all fit together as parts of a three dimensional world. That assumption about their basic relationship can be understood in two different ways, and together they pose a dilemma for spatial relationism, for both have consequences that make spatial relationism more complex than spatiomaterialism.

"Having coherent spatial relations" may be taken as an aspect of the spatial relations that all the particular bits of matter have at the present moment, or it may be taken as an aspect of their particular spatial relations at every moment in the history of the world. In the first case, materialism must explain why bits of matter have coherent spatial relations at every moment, and the only way of doing so undermines the way that physical explanations are ordinarily understood. In the second case, the basic relationship of materialism must have a temporally complex nature, for its essential nature must include a fact about the world that spatiomaterialism explains by ontological causes that are temporally simple. Let us consider each horn of this dilemma in turn.

Notice, by the way, that in both ontologies, the basic relationship is not the particular spatial relations that bits of matter actually have, but an aspect of those particular spatial relations. For materialism, it is the geometrical coherence of those spatial relations, whereas for spatiomaterialism, it is that those spatial relations come from bits of matter coinciding with parts of space. The particular spatial relations that bits of matter actually have are taken by both theories to be something that can be known only by experience of the world.

Temporally simple basic relationship. The basic relationship assumed by spatiomaterialism is temporally simple. It assumes that the way that space and matter exist together as a world right now is by each bit of matter coinciding with some part of space or other, and that is temporally simple, for its two basic substances can have that relationship completely at one moment in the existence of the world. And this assumption needs to be made only about the present moment, because if all the bits of matter are contained by space at the present moment, then the fact that substances endure through time, never coming into existence and never going out of existence as time passes, entails that they have the basic relationship at every other moment. There is no way for a bit of matter to escape from space altogether, for it exists now as part of the same world by coinciding with some part of space or other and space endures through time along with matter. For the same reason, it could not get into space, if the bit of matter did not already coincide with some part of space or other.

The basic relationship assumed by spatial relationism can also be temporally simple. It is the assumption that bits of matter have coherent spatial relations, and that relationship can hold completely at any moment in the history of the world. But unlike spatiomaterialism, if that basic relationship is assumed to hold at the present moment, it does not necessarily hold at all other moments in the history of the world. It is conceivable that bits of matter would move and interact in ways that would give them spatial relations that are not geometrically coherent. (Similarly, it is conceivable that the present spatial relations are a result of the motion and interaction of bits of matter from earlier states in which their spatial relations were geometrically incoherent.)

It is conceivable, for example, that when matter of a certain kind is given the shape of a cave, the cave from inside is larger than the cave from the outside. That is, when measuring rods are taken inside the cave and used to measure how large the cave is, the internal distances measured turn out to be greater than the size of the cave when it is measured from the outside.

It might be argued that such spatial relations are not geometrically incoherent, but merely show a curvature about space. They are only incoherent according to Euclidean geometry. But postulating non-Euclidean geometry will not always preserve geometrical coherence. For example, suppose that when matter of a certain kind was shaped into a cave and extended into a tunnel, another entrance cut in the distant wall of the cave would open up in some far distant location in three dimensional space.

Topology explores many such possible connections among regions of spatial relations, and we need only think of them as being the effect of shaping matter in certain ways in order to conceive how the motion and interaction of bits of matter could lead to their having incoherent spatial relations.

In fact, spatial relations do not become geometrically incoherent in such ways, and so having geometrically coherent spatial relations at every moment is a basic aspect of the world than an ontology needs to explain. Now, spatial relationists may insist that they can explain this aspect of the world by the essential nature of matter. They assume that the essential nature of matter is defined by the basic laws of physics, and so they can argue that, if spatial relations are geometrically coherent at the present moment, they will be geometrically coherent at all moments in the future (and in the past), because those future (and past) spatial relations are determined by those bits of matter moving and interacting according to the basic laws of physics. Their geometrical coherence is, in other words, a consequence of the nature of matter. The reason that spatial relations will be geometrically coherent at other times, if they are geometrically coherent now, is that it is physically necessary.

This possibility is suggested by the structure of explanations in physics. As explicated by the deductive-nomological model, the basic laws of physics together with initial and boundary conditions make it possible to predict (or retrodict) any future (or past) state. Thus, if we take momentum to be a property of the bits of matter, the particular spatial relations among bits of matter at any one moment will determine their spatial relations at any other moment. Hence, they will be coherent at every moment, if spatial relations are coherent now.

Notice that this way of explaining why spatial relations are geometrically coherent depends on complete determinism, such as was assumed in Newtonian physics. (It was Laplace who first argued that the laws of Newtonian physics made this possible.) It is not, however, compatible with quantum mechanics, if Heisenberg's principle is taken to represent an indeterminism about what happens in the world as bits of matter move and interact, for such indeterminism would leave plenty of room for bits of matter to acquire incoherent spatial relations. It is, of course, possible that Heisenberg's principle represents merely an inevitable incompleteness about physical theory. There could be a hidden variable that makes physical processes deterministic, though it cannot be measured. But most naturalists would be surprised to find that they are committed ontologically to such an interpretation of quantum mechanics by their acceptance of materialism (that is, reducing space to spatial relations).

Even if physical laws are deterministic enough to explain why bits of matter always have coherent spatial relations, there is an intolerable cost to be paid in our understanding of how physical processes take place. Physicists ordinarily think of what happens in nature as a result of how bits of matter move and interact in space, where their spatial relations are one factor that combines with their motion and the forces they exert as a different kind of factor to determine what happens to them. But that is not possible, if the regularities described by laws of physics are what make spatial relations coherent in the first place, for then there is no way to explain how spatial relations work together with motion and forces as different kinds of efficient causes. Both kinds of factors are simply contingent aspects of bits of matter, and ontologically, they have the same status.

When material objects exert forces on one another, for example, when a planet exerts a gravitational force on a material object near its surface, we naturally think of the acceleration of the object as having two kinds of efficient causes: its distance from the planet and the force exerted by the planet. If object were released farther away, the same force would give it more momentum before colliding with the planet. And if the force were greater where it was released, the object would also have more momentum before colliding. We think of spatial relations and forces as two different kinds of efficient causes determining the later state. But if the basic laws of physics are supposed to explain why bits of matter have coherent spatial relations in the future, there is no way to distinguish the effect of forces from the effect of spatial relations. Instead, laws of physics must be seen as operating on the spatial relations, velocities, and forces that exist at one time to determine new spatial relations, velocities, and forces at a later (or earlier) time. Though what happens is predictable, the cause is not the planet's gravitation force acting on the object across space, because there is no way to distinguish the effect of the space from the effect of the force. Both depend on the regularities described by the laws of physics in the same way.

Likewise for motion. When a material object has a certain velocity, we think of its future spatial relations to stationary material objects as a result of its motion through a space that is already there. Its momentum is something that the object has, and we ordinarily see its future locations as being determined by its momentum together with its location in a space that is somehow independent of it. But that way of conceiving physical causes must be given up, if the conservation of momentum helps cause bits of matter to have coherent spatial relations. The future spatial relations are not caused by moving through space. They are caused by its motion and its past spatial relations, changing according to a basic law of physics which is taken as defining the nature of matter.

This form of spatial relationism makes almost everything that happens in the world depend on the nature of matter, rather than on what it assumes about the nature of spatial relations (namely, that they are geometrically coherent at the present moment). . This can also be seen how materialism explains other aspects of motion and interaction that spatiomaterialism explains by substantival space. Whether or not it is ontologically necessary, it is true that bits of matter do not change spatial relations by flitting about from place to place discontinuously, but only by moving across space as time passes. Spatial relationists would deny that this depends in any way on the nature of spatial relations. They would explain this regularity as just another aspect of the regularities described by the basic laws of physics, which define the nature of matter. The same holds for the materialists' explanation of why bits of matter do not act one one another at a distance.

In other words, to assume that having coherent spatial relations is a basic relationship that holds only at the present moment is to assume, in effect, that matter has an essential nature that is more complex temporally than the matter assumed by spatiomaterialism. Materialists have to attribute more aspects of what happens in the world to the nature of material substance as an ontological cause than do spatiomaterialists. The greater complexity of the essential nature of matter is what contradicts the claim that materialism is simpler than spatiomaterialism.

Temporally complex basic relationship. Instead of making the coherence of spatial relations a consequence of the basic laws of physics, materialists can take the basic relationship by which bits of matter exist together as a world to be having geometrically coherent spatial relations at every moment. This would be to postulate a basic relationship with a temporally complex nature, for the basic relationship would have to work together with the forces described by the laws of physics as another efficient cause determining what happens. And the basic relationship would have to work together with velocity as a second efficient cause determining its future spatial relations. (This form of materialism could also use its basic relationship to explain why bits of matter change spatial relations only by motion and why they do not interact at a distance.)

Though this would allow materialists to interpret the laws of physics as descriptions of how bits of matter move and interact in space, it would be to assume that the basic relationship does for materialism what substantival space does for spatiomaterialism. The materialists' basic relationship would not be simply how bits of matter exist together at the present moment, but a way of existing together at the present moment that also constrains how they can exist together at future (or past) moments in a way that is independent of the constraints imposed by their forces and velocities. Since that is to assume that the basic relationship entails that particular spatial relations can change only in certain ways, it would be to assume that the basic relationship has a temporally complex nature.

But that makes spatial relationism more complex than spatiomaterialism. The materialists' basic relationship would be replacing both space and the basic relationship of spatiomaterialism. Though one assumption is replacing two assumptions, materialism is arguably more complex than spatiomaterialism, because its one assumption has a temporally complex nature, whereas both of the spatiomaterialists' assumptions are temporally simple. That is, aspects of regularities about change that are merely assumed by spatial relationism are explained ontologically by spatiomaterialism, including not only that bits of matter have geometrically coherent spatial relations at every moment, but that they change spatial relations only by motion and that they do not interact at a distance.

Furthermore, it might be argued that, if materialism builds these regularities about change into its basic assumption about how bits of matter exist together as a world, it is violating the spirit of ontological explanation. Ontology tries to explain basic aspects of the world by showing how they are constituted by substances that endure through time with an unchanging nature. Since the basic relationship does not endure through time on its own like a substance, but is merely how the substances exist together as a world, it cannot be a source of regularities about change in the same way as substances can. Thus, not only does spatial relationism fail to explain ontologically why bits of matter always have coherent spatial relations, it also fails to account for them in the way expected of an ontology. In short, its need to postulate a basic relationship with a temporally complex nature is itself a reason for rejecting an ontology.

Although materialism seems to be simpler than spatiomaterialism, therefore, there is, in either case, a way in which it is more complex than spatiomaterialism. Either its assumption about the essential nature of matter is more complex, or its assumption about the basic relationship by which material substances exist together as a world is more complex.

Thus, there is, at least, a standoff between spatial relationism and spatiomaterialism on grounds of simplicity. And that means that spatiomaterialism is favored by the empirical method, since spatiomaterialism has a greater scope (explaining the possibility of change by motion and by interaction).

Furthermore, the way in which spatiomaterialism is simpler than spatial relationism also a way in which it explains aspects of the world that spatial relationism can only assume. Spatiomaterialism can explain ontologically why spatial relations are always geometrically coherent (not just now, but in the future and past), whereas materialism must build that assumption either into the nature of the matter it postulates or into the nature of the basic relationship it assumes bits of matter have.

Though spatiomaterialism postulates two basic substances, rather than just one, its ontological causes are simpler than those of spatial relationism. But since both ontologies account for the same basic facts, that means that spatiomaterialism explains more with less. At the outset, we saw the greater scope of spatiomaterialism in its ontological explanation of why bits of matter have spatial relations and how change is possible (not to mention what will be show later, that they change spatial relations only by motion and do not interact at a distance). But in showing that spatiomaterialism is simpler than spatial relationism, despite initial impressions to the contrary, we have seen that its ontological causes explain another aspect of the world that materialism can only assume, namely, why bits of matter always have coherent spatial relations. In the end, therefore, it is how spatiomaterialism explain more less than makes the decision in favor of spatiomaterialism clear, at least, for naturalists who accept the empirical method.

Spatiomaterialism is better than spatiotemporalism. Substantivalism about spacetime entails, as we have seen, the perdurance theory about the temporal existential aspect of substances. But since we have already seen that the empirical method in ontology requires us to prefer the endurance theory to the perdurance theory of time, we ought to believe either spatial relationism or spatiomaterialism rather than spatiotemporalism. Both allow us to accept the endurance theory (thereby giving us an explanation of why the present is different from the past and future and allowing us to believe that change is real in the sense of properties coming into existence and going out of existence as time passes). But having established that the empirical method prefers spatiomaterialism to spatial relationism (that is, to materialism), we must conclude that spatiomaterialism is the best ontological explanation of the natural world (assuming, of course, that there is no fourth theory that is still better than spatiomaterialism).

Defenders of spatiotemporalism will, however, object to this conclusion. They believe they are forced to accept spatiotemporalism by contemporary physics. Einstein’s discovery of the special and general theories of relativity was a revolution that led to the overthrow of the Newtonian belief in absolute space and time in physics. It is clear that any ontology that holds that material substances endure through time entails that space and time are absolute. To hold that the substances constituting the world always exist at only one moment in their histories is to hold that they all exist at the same moment, for they are part of a single world and they must exist together to be parts of the same world. Thus, if there are substances with spatial relations to one another, the spatial relations they have at the present moment hold for every possible observer. This is even clearer, if space is also a substance, for in that case the spatial relations are all constituted by a substance that exists only at the present moment. Since that is to believe that space and time are absolute, to choose to believe spatiomaterialism, or for that matter spatial relationism, would be a counterrevolution in physics. Thus, it is not likely to attract many followers among physicists and philosophers of science.

What led physics to reject Newtonian absolute space and time in favor of the spacetime of Einstein’s relativity theories was the empirical method of science. Physicists were merely inferring to the best explanation of what they could observe about the world. The special and general theories both predicted quantitatively precise measurements that were not expected by classical Newtonian physics, and they have been confirmed repeatedly. Nor does anyone dispute the mathematical simplicity of Einstein’s theories. The special theory was a paragon of simplicity by comparison with the cobbling together of ad hoc constraints by which Lorentz had proposed to explain the same phenomena. The general theory of relativity was based on the special theory, and though its mathematics was novel in physics, there is no question about its elegance. These two theories were clearly the best explanation that physics offered of the space and time found in the natural world, and that caused a revolution in physics, because neither theory had any use for absolute space or absolute time. All that was required for them to be true was spacetime, that is, the ontological equality of all points in space and time.^iv

The empirical method in science is, however, different from the empirical method in ontology, and thus, what is the best scientific explanation may not be the best ontological explanation. Science tries to explain what happens, and thus, it infers to the best efficient-cause explanation of what can be observed. Its criteria of truth are prediction and control. But there is, as we have seen, a difference between efficient-cause and ontological-cause explanations. Ontology tries to explain everything in the world, not only what happens there, but also what exists there — including the properties and relations of the objects found in the world, and how it is possible for anything to happen in the first place. Such things are explained ontologically by showing how they are constituted by basic substances and relations among them. Thus, empirical ontology tries to explain the world most completely using the fewest and simplest substances with the fewest and simplest relations. Though the goal of explaining the most with the least is the same, the kinds of explanation involved are different.

Since we know on empirical grounds, that spatiomaterialism is the best ontological explanation of the world, empirically minded ontological naturalists must prefer it to spatiotemporalism, if it is possible. Thus, the only relevant question is whether it is possible that spatiomaterialism is true, given that Einstein’s special and general theories of relativity are the best efficient-cause explanations of all the relevant phenomena.

The answer is Yes. It is possible to explain all the observational predictions of what will happen that is entailed by either the special or the general theory of relativity on the assumption that space is a substance enduring through time and, thus, absolute. To be sure, spatiomaterialism must make certain additional assumptions about the nature of space and matter and how they interact, which are, in effect, new laws of nature. But it is possible. (And the fact that spatiomaterialism is able to explain the truth of Einstein's two theories is further reason for preferring it over spatial relationism, because spatial relationism cannot explain them. It can only assume them in the same way it does the geometrical coherence of spatial relations.)

The spatiomaterialist interpretation of Einstein’s special and general theories of relativity is given a detailed defense below (in Change: Special theory of relativity and Change: General theory of relativity), as one of the implications of spatiomaterialism for physics. But we can see the possibility of such an interpretation in the abstract, and since this may seem unlikely to some, let me sketch briefly just how the truth of Einsteinian relativity will be explained ontologically by spatiomaterialism.

Given the endurance theory of time, what substantivalism about space implies is that only one moment in the history of each location in absolute space exists. That is the present moment, and it is the same for all of them, since the parts of space are all parts of the same world. Thus, all that a spatiomaterialist interpretation requires is that each and every part of space (along with the bits of matter coinciding with parts of it) be in a state at the present moment that is consistent with Einstein’s two theories. What that means is that, among all the possible inertial frames, which relativity takes to be equivalent, one, and only one, is true. This is not to say that it is possible to determine by some measurement which one it is. That is clearly precluded by Einstein’s theories; if it weren't, they could not be called relativity theories. But it is equally clear that there can be an inertial frame at absolute rest, even though it is not possible to detect which one it is.

What makes such an easy accommodation possible is that empirical science and empirical ontology have different criteria of truth. Since the empirical method of science seeks the best efficient-cause explanation of what happens in the world, its criterion of truth depends on predicting and controlling what happens, and thus, given that inertial frames are all equivalent in that regard, it can take the truth to be what is the same for all of them. In ontology, however, the empirical method seeks the best ontological-cause explanation of what exists in the world. Its criterion of truth is the simplest substances and relations that will explain everything in and about the world, and thus, it must explain how all the different inertial frames could be part of the same world. That is something that science can take for granted, because one observer can always predict what coordinates will be assigned by other observers. And since the reasons for believing that there is an absolute frame of reference are ontological, the lack of any difference in the predictions made from different inertial frames is not a reason to doubt that it exists.

The detailed spatiomaterialist explanation of the two relativity theories shows, from the point of view of the inertial frame at rest in absolute space (whichever one that is), how it is possible that all the other inertial frames are observationally indistinguishable from it. Here is the gist of the explanations given in CHANGE.

The special theory of relativity implies that the various possible inertial frames (that is, the various possible unaccelerated material objects that might be used as the basis for measuring distances in space and intervals of time) are all equivalent, making it impossible ever to determine by measurement which one is at rest in absolute space. But the undetectability of absolute rest does not mean that there is no such thing. Indeed, as Lorentz began to show early in the twentieth century, it is possible to explain the observational equivalence of inertial frames which makes absolute rest undetectable on the assumption that all the material objects are located in an absolute space in which light has a constant velocity. Lorentz showed that it would not be possible to detect absolute motion by measurements of the velocity of light, if material objects with a high velocity relative to absolute space suffered several distortions (including the shrinking of their lengths in the direction of motion, the slowing down of their clocks, and increase in their mass at a certain rate). It is also possible to show that, if observers on all inertial frames accept Einstein's definition of simultaneity at a distance (and synchronize their clocks on the assumption that the velocity of light is the same both ways, back and forth, in every direction -- that is, as if they were at rest in absolute space), those same "Lorentz distortions" will make all inertial equivalent even when it comes to their measurements of one another. (Observers on both of any pair of inertial frames will see the other's clocks slowed down, the other's measuring rods as shrunken, etc.)

In other words, if we think of the effect of space on the material objects it contains as the "ether" in which Newtonian physicists thought that light had a fixed velocity (or what I will call an "inherent motion" in space), and if we assume that the motion of material objects through the ether has certain distorting effects on them and their physical processes, then all of the observational consequences of Einstein’s special theory of relativity follow. We will have explained all the phenomena without referring to spacetime. Thus, it is not necessary to give up the assumption that space is a substance enduring through time to explain what is described by Einstein's special theory of relativity.

The general theory of relativity is a theory about gravitation formulated in terms of spacetime. It holds, in effect, that matter accumulation in spacetime imposes a curvature on spacetime, and that in curved spacetime, the path of inertial motion is not straight, but curved, or in other words, accelerated. But all the predictions that follow from assuming that spacetime is curved can also be made on the assumption that the velocity of light relative to space varies from place to place in space. That is, the spatiomaterialist interpretation of Einstein’s special theory of relativity is, in effect, an ontological interpretation of what is meant by "spacetime,"and that is what makes it possible to explain the observational adequacy of Einstein’s general theory of relativity on the assumption that space is a substance enduring through time.

As suggested above, talk of "spacetime" can be replaced by talk of an ether, in which the velocity of light is equal both ways in every direction, though that is really just a way of describing how space interacts with the matter it contains. In our ontological explanation of the special theory, we assume that the ether is at rest in absolute space and we explained all the other inertial frames as observers on the one that is at absolute rest. In order to explain the general theory, we assume that the ether itself can have a velocity in space, one that varies across space according to the accumulations of matter nearby. That means that the absolute velocity of light varies from location to location in absolute space (that is, at different locations in the inertial frame at absolute rest, from which we are giving this explanation). But it also means that material objects, which interact with one another by way of electromagnetic interactions through the ether, are accelerated with the ether, and such a moving, accelerated ether is what "curved spacetime" comes down to ontologically, for as we shall see, it explains all the observational predictions of the general theory of relativity. It could all be just the effect that space has on the light and matter it contains, if the right states for space to have such effects are imposed on space by the accumulation of matter in space. Precisely the same observational predictions follow from this theory as from Einstein’s general theory of relativity, and thus, it is possible that space is a substance enduring through time, that is, absolute.

These sketches of the spatiomaterialist explanation of the truth of Einstein's two relativity theories may be too brief for most people to follow easily. But they are included here because even a suggestion of the nature of these arguments may clarify what is meant by saying that it is possible that spatiomaterialism is true, notwithstanding the Einsteinian revolution in physics. But at this point, it is still just a promise, and thus, we accept the obligation to show in detail how it is possible as we take up showing what holds necessarily, if spatiomaterialism is true. It is like taking out a mortgage in order to construct the ontological foundation for this philosophical argument. If it should turn out, as we build the edifice of ontological philosophy, that relativistic phenomena cannot be explained on the assumption that space is a substance existing in time, spatiomaterialism will have been falsified and we will not be entitled to use it as a foundation to support any conclusions about the world. We will have to concede that we do not have a new way of doing philosophy after all.

As it stands, however, spatiomaterialism is a better ontological explanation of the natural world than either spatial relationism or spatiotemporalism, because the latter two theories have opposite failings. Spatial relationism (that is, materialism) can explain why the present is different from the past and future (and, thus, can hold that change is real), but it cannot explain spatial relations. Spatiotemporalism can explain spatial relations, but it cannot explain why the present is different from the past or the future, that is, except as another kind of relation like that of space (and, thus, cannot hold that change is real). Spatiomaterialism, however, can explain both spatial relations and why the present is different from the past and the future (and, thus, can hold that change is real).

Matter. Naturalists believe that the world is just what is in space and time, and having seen that we should, if possible, believe that substances are in time in the sense of enduring through time, and that substances are in space in the sense of either being parts of space itself or coinciding with parts of space, the final issue to be settled is about the nature of the substances that coincide with space and endure through time. The simplest theory is obviously materialism, the belief that matter is the only kind of basic substance that coincides with space. But some phenomena seem to require immaterial substances as well. Our ontological causes would be more complex, if we had to postulate both material and immaterial substances as coinciding with space. But if the scope of our ontological theory is increased by postulating immaterial substances, it can be argued that there is a tradeoff between simplicity and scope that keeps the empirical method from requiring naturalists to accept materialism. In this case, therefore, we must decide whether there are any phenomena that require us to postulate immaterial substances as well as material substances. Let us set the stage by considering more carefully what materialism holds.

Materialism. Materialism holds that none but material substances coincide with parts of space. Matter comes in particular bits, and by "matter," we shall mean only substances whose behavior in space makes the laws of physics true. Thus, we assume that bits of matter move and interact in the regular ways required by the basic laws of contemporary physics and that there are enough different kinds of bits of matter to account for all the kinds of entities mentioned by those laws, from electrons and nucleons (or triplets of quarks) to force fields and photons. We will see what essential nature material substances that coincide with space must have for this to be true. (See Contingent Laws under Local Regularities under Change.) But given that it is true, materialism may also be called "physicalism," because the properties mentioned by the basic laws of physics are called "physical properties."

More abstractly, bits of matter are "basic" substances in the sense that they are the most elementary substances of their kind. Since each has an existence that is distinct from all the rest, they are "particular" substances. They are "concrete" in the sense that no bit of matter can be in two different locations at the same time. And they are "independent" of one another in the sense that the existence of one bit of matter does not, in general, depend on the existence of the others. That is, bits of matter can also move independently of one another and interact locally (though, as we shall see in Change: Forms of matter, there are some varieties of matter that cannot exist except in conjunction with matter of a different variety).

Since spatiomaterialism holds that bits of matter are in space in the sense of being contained by space as a substance, we shall take the basic laws of physics to be descriptions of regularities about their motion and interaction that result from their being contained by space, that is, as ontological effects of both space and matter. That is different from what spatial relationism assumes about the nature of matter, because spatial relationism can simply define the essential aspect of material substances by the basic laws of physics, implying that there is nothing more to be known about their natures and that bits of matter have an essential nature that is irreducibly temporally complex. But since we take space to be a substance, we are assuming that at least some of the regularities described by basic laws of physics can be explained ontologically, that is, by how the essential nature of space works together with the essential nature of matter, because of how matter and space coincide, to constitute those regularities. That is why we took spatiomaterialism to have a greater scope than spatial relationism: it could explain why bits of matter have spatial relations and how change is possible, rather than just assuming it. That is also how spatiomaterialism can promise to explain the truth of Einstein's relativity theories, as just mentioned. And it is how we will explain the other laws of physics in Contingent Laws under Change. Indeed, the possibility of such explanations is what we assumed by taking ontology to be a kind of explanation, rather than merely realism about science. But that means that spatiomaterialism must take matter to be a kind of substance that, working together with space as the substance with which it coincides, makes the basic laws of contemporary physics true.

In addition to explaining why efficient-cause explanations are true, moreover, ontological-cause explanations can also explain why rational-cause explanations are true, making all the kinds of explanations mentioned in Method parts of a single explanation of the world in the end and reducing the social sciences by way of natural science to spatiomaterialism.

Although spatiomaterialism implies that there is more to be known about the essential nature of matter, what is relevant for present purposes is that it agrees with materialism (or physicalism) about physics being causally complete. What happens in the world is just what comes about, given the initial and boundary conditions that prevail, as the result of bits of matter moving and interacting according to the basic laws of physics. That is how all efficient causes bring about their effects, according to materialism, and spatiomaterialism expects to be able to explain why those causal connections hold. Naturalists who follow the empirical method must prefer that kind of ontology, if it is possible, because it is the simplest explanation of what happens in nature. The only question is whether it is possible.

Immaterialism. It is not possible, according to critics of materialism, because there are aspects of the natural world that require us to postulate immaterial substances in space. Though all naturalists deny the existence of anything outside space and time, all the kinds of phenomena mentioned in Naturalism: Problems as posing a problem for naturalism also pose a problem for materialism. That is, consciousness, goodness and holiness, the phenomena that lead, respectively, to the belief in Cartesian minds, Platonic Forms, and a transcendent God, can also be used to argue for the existence of substances whose natures are not described by the basic laws of physics. However, to postulate mental substances, teleological substances, or spiritual substances would be to give up materialism in favor of a more complex ontology, one with immaterial substances that coincide with space and endure through time, along with material substances.

Notice that, although space is not a material substance, it is not an immaterial substance in the sense relevant here. Space is not a material substance in the sense that it has an opposite essential nature to matter. (Whereas bits of matter are independent of one another, parts of space cannot exist without one another.) But here we are concerned with the causal completeness of physics, and by "immaterial substances," we mean only substances that coincide with space. What makes them immaterial is that they do not move and interact as described by the basic laws of physics.

Though space is not a material substance, it is not an immaterial substance in the relevant sense, because substantivalism about space does not itself deny the causal completeness of physics. On the contrary, it affords an ontological explanation of why the basic laws of physics are true and, thus, an explanation of the connection between cause and effect in efficient-cause explanations.

Ironically, however, as it will turn out, all that needs to be added to materialism in order to explain the problematic phenomena that lead to belief in immaterial substances is substantivalism about space. As we shall see, that is because it shows the ontological necessity of global regularities, as well as the local regularities described by the basic laws of physics. It order to see what spatiomaterialism must do, let us consider more carefully each of the reasons for believing in immaterial substances.

Mental substances. The first challenge to materialism comes from the existence of conscious beings like us. As explained in Naturalism: Consciousness, the basic phenomenon that leads to belief in the existence of mind is "consciousness," which will be understood here as the fact that it is like something to perceive the world and experiences of other kinds. The appearances involved in perception are something distinct from what exists in the natural world independently of us, and when we reflect on how we know about them, it seems that the appearances themselves are responsible for our being aware of them and for the judgments we make about them. That is what led Descartes to believe that minds are immaterial substances not located in space. Though we must, as naturalists, deny the existence of Cartesian minds, we must give an ontological explanation of the natural world that explains the phenomenon of consciousness.

To be conscious is to have qualia or phenomenal properties. Since they are properties of a radically different kind from the physical properties by which the essential nature of matter is defined, materialism seems to be incapable of explaining consciousness. There are several alternatives.

Eliminative materialism. What materialists can do is explain away the phenomenon. That is the position called "eliminative materialism." It assumes that everything that conscious subjects do in the world can be explained by the brain and other forms of efficient causation. That means that there is no way to show that someone else is conscious by how they behave or anything else that happens in the world. Thus, consciousness eludes the method of empirical science, since the only acceptable evidence for scientific explanations is what is known by perception. Eliminative materialism would "solve" the problem of consciousness by simply denying the existence of phenomenal properties. It holds that belief in them is the result of a confusion (see Dennett) or the lack of an adequate scientific explanation of the brain (see Churchland.) This position is not easily refuted, since the evidence for consciousness is strictly private, in the sense that it depends on first-person reflection.

The willingness to reduce conscious subjects to what materialism can explain is, however, the sort of attitude that has given reductionistic materialism such a bad name. Most naturalists (like Chalmers) doubt that eliminative materialists are taking consciousness seriously, for naturalists are themselves parts of the natural world and they can know that they are conscious by reflection, even if natural science cannot.

Emergentism. At the other extreme is emergentism. It is possible for naturalists to give up materialism and hold that what explains this phenomenon are mental substances that coincide with space along with material substances. Emergentism is different from the belief in Cartesian minds, because it takes the mental substances to be in space, and for spatiomaterialists, to be contained by space as a substance is to coincide with some part(s) of it. But emergentism agrees with the Cartesian view about mental substances making a difference to what happens in the world. It holds that mental substances are partly responsible, at least, for behavior that is ordinarily attributed to conscious mind, such as rational behavior. Such a view, however, denies materialism, for it denies the causal completeness of physics. It implies that there are substances in space and time that do not obey the laws of physics, thereby denying that physics can, in principle, explain everything that happens in nature.^v

It may seem that emergentism is not a form of immaterialism, because what emergentists mean by "conscious mind" cannot be a substance by our definition. We are assuming that substances never come into existence nor go out of existence over time, but emergentists typically hold that conscious mind comes into existence at some point because of the complexity of physical causes, for example, at some stage in the evolution of the brain. However, these views are not incompatible, because the way in which conscious mind emerges can be explained by assuming that matter itself has a (temporally complex) nature that allows its nature to change from being the kind described by the laws of physics to being a kind that gives consciousness a causal role in the world. That is to hold that there are immaterial substances in space, for it implies that there are substances that do not obey the basic laws of physics. That may mean that there are no material substances, only immaterial substances that appear at times to be material. In any case, it is a naturalistic theory. But since bits of matter would have to follow more complex laws than those of physics, the existence of emergent minds would require a more complex ontology, and thus, naturalists have good reason to prefer a less disruptive explanation of consciousness, if it is possible.

Epiphenomenalism. Epiphenomenalism is a compromise between eliminative materialism and emergentism. It holds that all the causal roles of conscious mind are really the work of the brain and, thus, can ultimately be explained by matter alone. Thus, it cleaves to materialism and believes in the causal completeness of physics. But it also holds that processes involving physical properties of those kinds "give rise" to phenomenal properties. That is how it explains the phenomenon of consciousness. Since those phenomenal properties have no effects, in turn, on what happens in the world, it is called "epi-phenomenalism." That is, phenomenal properties are effects of physical properties without ever themselves being causes of anything. Such a view avoids postulating any immaterial substances, since the substances in space would always obey the laws of physical. But it would have to assume that material substances can have properties that are not mentioned by the basic laws of physics. Thus, it accepts what is called "property dualism," while cleaving to materialism (or physicalism). Matter must have phenomenal properties as well as physical properties.

Epiphenomenalism is, however, an unhappy compromise, because phenomenal properties are fundamentally different from the properties by which materialists define the essential natures of material substances. They are not entailed by anything that physics can discover about the world. Thus, it is possible to conceive of a physical world in which organisms with brains exactly like our own did not have phenomenal properties. That is, there may be zombies. Or to use Kripke’s famous metaphor, epiphenomenalism makes it seem as though, God, after creating the physical world, had to go back and tack phenomenal properties onto material substances in order to make beings like us conscious. Thus, even though epiphenomenalism allows naturalists to avoid immaterialism, there is still reason to believe that materialism is not the deepest truth about the nature of existence in the natural world, because consciousness is still something found in the world that does not seem to be constituted by material substances.

In order to be the best ontological explanation of the natural world, therefore, spatiomaterialism must explain consciousness. That is, it must explain the relationship between physical and phenomenal properties in a way that shows phenomenological properties to be ontologically necessary.

And it can. Indeed, that will be the first necessary truth derived from this ontological foundation. (See Properties.)

However, since this is only a promise at this point, we are taking out a second mortgage on the house of ontological philosophy in order to construct its foundation (that is, in addition to explaining why Einsteinian relativity is true), and only if we pay off both mortgages will we have a clear title to a new way of doing philosophy. But as it now stands, if we do pay them back, the empirical method will require us to accept spatiomaterialism as true, and we will not be able to deny the necessary truths that follow from it. This argument will be a new way of doing philosophy.

Teleological substances. Another problem with naturalism is the existence of a real difference between good and bad, that is, a difference in the objects or events themselves that make it true that some ought to exist and others ought not. That is the phenomenon that led Plato to believe in the existence of Forms in a realm of Being, and the same phenomenon that theists believed they could explain by the existence of a God who created the natural world. Though as naturalists, we must deny both of those supernaturalistic explanations, we do need an explanation of the phenomenon itself. If it cannot be explained by materialism, goodness will count as evidence for the existence of immaterial substances.

Hedonism. The time-honored way for materialists to explain the phenomenon of goodness is by offering a causal explanation of what is good, such as psychological hedonism, that is, the view that beings like us cannot help but seek pleasure. But that is to hold, in effect, that pleasure is what is good without explaining why the good is good in the sense that it ought to exist. It would only explain why hedonistic beings like us inevitably pursue it.

Furthermore, hedonism does not explain moral goodness, for it does not explain why we ought to do what morality requires when it does not maximize our expected pleasure, that is, when it is not in our self-interest.

Nor is the goodness of morality explained by theories, like Hume's, that take human nature to include a moral sentiment, which inclines one to do what is moral when it conflicts with self interest. Such a psychological disposition may explain why human beings are moral, but not why they ought to be.

Non-cognitivism. The other traditional naturalistic attempt to explain the phenomenon of goodness is to hold that it is an illusion. The appearance that there is an objective difference between good and bad could comes from projecting our feelings about things onto the world, so that they appear to be properties of the objects themselves. This view has had many defenders in the Twentieth Century (such as Ayer).

These ways of answering the challenge of goodness are, once again, what has given materialist reductionism a bad name. They do not convince everyone, and those who continue to believe in a real difference between good and bad, in which the good really ought to exist regardless what we may happen to (or be determined to) believe about it, will accuse materialists of leaving something out of their supposedly complete explanation of the world. Thus, although materialism is the simplest ontological explanation of the natural world, the empirical method cannot force us to accept it as true as long it cannot explain goodness as something that beings like us find in the natural world.

In order to explain the phenomenon of goodness, it may be argued that naturalists must postulate teleological substances of some kind, such as Aristotle did by holding that there are final causes as well as efficient causes at work in nature. To suppose that forces of any kind are responsible for the goals pursued by biological organisms generally or by human beings would be to hold that there are substances that somehow guide change in nature to bring about certain states or goals. They could not be material substances, because substances whose essential natures are described by the basic laws of physics do not have such forward-looking effects (unless, of course, they have very special initial and boundary conditions as parts of mechanisms, which would need to be explained). In order to account for final causation, for example, Aristotle postulated essential forms as a component of each particular substance in space. Indeed, the actualization of the essential form that exists potentially in substances of its natural kind was supposed to be the end for the sake of which "natural change" takes place.

Nor was it just their role in final causation that made them immaterial substances. Though essential forms are located in space and time as a component, along with matter, of the particular substances that have them, the same essential form must be able to exist simultaneously in different particular substances with different locations in space at the same time. Thus, they are universals, not concrete material substances.

It may be possible to materialize teleological causation (as " vitalists" like Hans Driesch did) by postulating "entelechies" (instead of essential forms and final causes) and holding that each entelechy can exist at only one location in space at a time. But still, any substances exerting teleological forces would be unlike the substances that materialists accept, because in order to guide motion and interaction toward certain goals, they would have to work in more complex ways than provided by the basic laws of physics. And even if they did, making what is good objective, it would still be necessary to show how that explains why the goals pursued are good.

The defense of teleological substances has been rare ever since the discovery earlier in this century that Darwin was on the right track in explaining natural teleology as a result of evolution. Darwin showed how the natural selection of random variations in reproducing organisms could explain why change seems to occur for the sake of ends in them. The existence of traits serving specific functions was a result of the differential survival and reproduction of organisms having the traits, while other organisms, lacking the traits, died out. In other words, it is merely an adaptation to the environment. And when the role of genes in the inheritance of traits became clear, it was even harder to believe that immaterial substances were responsible for the goal-directed traits of biological organisms -- and harder still when DNA molecules were found to be playing the role of genes. Since nothing but efficient causes are involved in the mechanism of inheritance and their evolution by natural selection, it was no longer plausible to believe in the existence of teleological substances.

This evolutionary explanation of the goal-directedness of biological traits is not, however, an explanation of the phenomenon of goodness. The consensus among contemporary Darwinists is that Darwin’s theory has nothing to do with progressive evolution. As we mentioned earlier, they believe that the cause of natural selection is externally caused changes in the environment, which makes the course of evolution seem accidental. What is more, since organisms must make do with whatever random variations turn up when the environment changes, it also suggests that evolved traits are not generally the best way to serve the functions required, but merely what enabled them to survive difficult periods. (For a fuller discussion of contemporary Darwinism, see Change: Accidentalism.) Thus, to those who believe that there is a real difference between good and bad, one that explains why the good ought to exist, the contemporary Darwinist explanation of the ends pursued by organisms seems more like an attempt to debunk their belief in goodness than an explanation of its nature.

Goodness remains, therefore, a source of doubt about materialism. Though materialism may be part of the simplest explanation of the natural world, there will be naturalists who do not accept it, as long as it cannot explain why things are good in the sense that they ought to exist. They have reason to believe that teleological substances of some kind are required to explain this phenomena. The tradeoff between simplicity and scope prevents the empirical method from deciding.

In order to hold that the empirical method requires naturalists to believe that materialism is true, therefore, and that there are no immaterial substances in space, it will be necessary to explain the phenomenon of goodness to the satisfaction of those who believe in an objective difference between good and bad. That is, it will be necessary to give an explanation of the goals pursued by beings like us (and by other organisms) that explains why those goals ought to be pursued.

In order to establish this foundation for ontological philosophy, therefore, we must take out a third mortgage on the necessary truths supported by it. Not only must spatiomaterialism explain the truth of Einstein's two relativity theories and the nature of consciousness, but it must also explain the nature of goodness. And if it turns out that we cannot pay off these mortgages, it will not be clear that spatiomaterialism is the best ontological explanation of the natural world. We will not be entitled to claim that any truths founded on its are necessary relative to what is ordinarily believed.

It will, however, turn out that spatiomaterialism can pay off this mortgage. There is a better explanation of the difference between good and bad than contemporary Darwinists offer, and ironically, what makes it possible is the recognition that space is a substance. The key, once again, is how substantivalism about space entails the ontological necessity of global regularities, for evolution is the "Reproductive Global Regularity."

Spiritual substances. The final reason for doubting that materialism (or we are assuming, spatiomaterialism) is the best ontological explanation of the natural world is what we called the phenomenon of "holiness," which leads people to believe in the existence of a transcendent God. Though, as naturalists, we must deny the existence of a transcendent God, the phenomenon that gives rise to belief in God calls for explanation, and if we cannot explain why people believe that is something worthy of worship without postulating spiritual or other immaterial substances in space, the empirical method will not force naturalists to accept spatiomaterialism. There will again be a tradeoff between simplicity and greater scope that makes it unclear whether spatiomaterialism or some from of immaterialism is the better ontological of the natural world.

In this case, once again, a common materialist response to the challenge is to hold that what needs explaining is not the phenomenon of holiness, but rather the belief in God itself. Thus, people are said to have a psychological need to believe in God, either as a result of conditioning (behaviorism), psycho-sexual development (Freudianism), an instinct selected for other functions (sociobiology), or some other irrational cause. This is materialist reductionism in the pejorative sense. It does not take seriously the source of the belief in the sacred, at least, not in the eyes of those who believe there is something worthy of worship.

This sort of explanation is not required by naturalism, that is, the denial of supernaturalism, for religious people can be naturalists. Though naturalists cannot believe in the existence of a transcendent God of any kind, they can insist that there is something immaterial in the natural world that is worthy of worship. It is not obvious, after all, that what is holy must exist outside space and time. It could be a spiritual substance in space, if not the world itself.

The existence of spiritual substances is not, however, compatible with materialism. A spiritual substance must have effects that are different from what happens as bits of matter move and interact according to the basic laws of physics, for otherwise there would be no reason to believe that a spiritual substance exists, much less that it is worthy of worship. Thus, it must not be a material substance in our sense.

Nor is it sufficient to declare that the world itself is worthy of worship. There must be something about the world that makes it holy, and naturalists have never explained what it is.

Spinoza's pantheism was rejected by traditional theists for this reason. His metaphysics explained why goals are pursued by beings in the world, but it denied that pursuing them was a result of free will and it failed to explain why those goals are good.

It may not seem necessary, in the case of holiness, to take out a fourth mortgage to establish spatiomaterialism as the foundation for a new way of doing philosophy, because if spatiomaterialism can explain everything but how there is something worthy of worship in the natural world, it could be argued that what we have discovered is that there is nothing sacred in space.

However, that would not work, if there were naturalists who continued to believe in the sacred, because they would insist that it can be explained by some kind of immaterialism. And if they were not just being willful or arbitrary, but argued with us, giving reasons for believing in spiritual substances of some kind, we could not claim that the empirical method forces naturalists to believe that spatiomaterialism is true. There would be a tradeoff between the simplicity of materialism and the scope of immaterialism, and we could not, in good conscience, defend any of the necessary truths of ontological philosophy.

Thus, we will take out a fourth mortgage on the foundation needed to do philosophy in this new way. It may seem wildly optimistic at this point, or even foolish, to promise an explanation of holiness. But as we shall see, spatiomaterialism does show that there is something in or about the natural world that is worthy of worship. This fourth mortgage will be paid back in the sense that either the religiously inclined will agree that it explains what they are getting at, or else we will have sufficient grounds for holding that they are not being fully rational about all the relevant issues in rejecting it. The dispute may continue at that point, but it will be about their rationality, not about whether spatiomaterialism is the foundation for a new way of doing philosophy.

This completes the construction of the foundation of ontological philosophy, though we carry quite a burden with us as we take up the project of using spatiomaterialism as a foundation for necessary truths. In order to hold that spatiomaterialism is the best ontological explanation of the natural world, we must explain why Einsteinian relativity is true, why beings like us are conscious, how there is a real difference between good and bad, and how there is something in the natural world that is worthy of worship. If we can pay off those mortgages, however, the edifice that we shall construct on that foundation will stand. What spatiomaterialism implies about the world will hold necessarily relative to science and our ordinary ways of reasoning about what to believe, including empirical science, ethics, and the whole gamut of ordinary cognitive endeavors. And the use of an empirical naturalistic ontology as a foundation for necessary truths will have proved itself to be a new way of doing philosophy.