Properties. Among the necessary truths about what is that follow from spatiomaterialism, the first set has to do with the nature of properties. Its main significance for issues in traditional philosophy is how it offers naturalists a solution to the problem of mind. By "consciousness," I mean the the fact that experience has an appearance to the subject, or that it is like something to be the subject. It cannot be explained without substances having phenomenal properties as well as physical properties, and ontological philosophy offers an explanation of phenomenal properties which entails that they have a necessary relationship to physical properties.

This implication of our ontology does not depend on recognizing the existence of space, but would follow from any form of materialism that took ontology to be explanatory and used the concept of substance introduced in Ontology: Substances. That makes it unique among the implications of ontological philosophy concerning the issues raised by traditional philosophical issues, for the rest depend on substantivalism about space.

In the case of phenomenal properties, the implications depend on our definition of the nature of substance, and the reason contemporary naturalists have overlooked this explanation is that materialism (or physicalism) is understood as realism about the theories of contemporary physics. Materialists posit the existence of whatever is required for the truth of the theories they believe, but they do not think much further about the nature of substances and properties. Thus, they take properties to be as ontologically basic as material substances, and that makes the relationship between physical and phenomenal properties seem puzzling.

Let us consider first what ontological philosophy implies about the nature of basic properties and their kinds before we take up the problem that follow from taking properties as just objects of knowledge.

Properties as aspects of substances. We have already seen how properties are related to the substances postulated by an explanatory ontology. They are aspects of substances, or part of what is assumed by postulating them which reason can pick out. We leave open questions about how rational beings like us are able to distinguish one aspect from another (until we discuss how reason comes to exist in a spatiomaterialist world like ours and see how reason depends on spatial imagination).

The basic properties of substances. We have already seen that substances, as substances, have two basic aspects, existence and essence. That is, they have the property of existence as well as an essential aspect to their nature. (See Ontology: Nature of substance.) But at this point, we must recognize two further aspects that may be involved in the essential aspect of the nature of substance as substance.

Existence. We have already seen how the existential aspect of substance as substance (or its property of existing) includes two properties, particularity and temporality. In other words, to say that a substance exists is to say that it has an existence that is distinct from other substances in the world (particularity) and that it endures through time temporality). (We take the temporal aspect to be endurance, because we have seen that endurance is the best ontological explanation of the nature of time, including both change and what makes the present different from past and future than perdurance. See Spatiomaterialism: Best explanation of time.)

Essence. Each substance must have an essential aspect in addition to its existential aspect, because in order to exist at all, it must exist in some determinate way. This was our reason for holding that substances have two basic aspects to their natures as substances, not only existence, but also an essence. It makes no sense to hold that something exists and to deny that it has any further aspect to its nature. But there may be two aspects to the essential aspect of the nature of substance as substance.

Intrinsic nature. This most basic aspect of its essential nature will be called its "intrinsic" essential property, for it is the kind of essential property that a substance has in virtue of existing as something distinct from all the other substances in the world. It is what the substance is in itself, or its way of existing on its own.

Extrinsic nature. But its intrinsic essential nature is not all there is to the essential nature of a substance, if the substance is part of the same world as other substances (and the existence of other substances is not entailed by its essential nature, as in the case of parts of space). Insofar as the world is made up of substances that exist independently of one another, and insofar as those substances are related to one another in some way other than simply being parts of the same world, each substance must also have extrinsic essential properties relative to those other substances. It may have different extrinsic essential properties relative to each kind of substance to which it is related, but its essential nature must have some such aspects.

Thus, the essential aspect of the nature of substance as substance includes two kinds of essential properties: an intrinsic essential property and extrinsic essential properties. In other words, each substance must exist some way in itself and it must also exist some way for other substances that exist independently of it as part of the same world.

The intrinsic and extrinsic aspects of the essential natures of substance can certainly be distinguished by reason. The world is made up of substances, and we can think about each distinct substance as it is in itself, whatever that may turn out to be, because in order to exist at all, it must exist in some determinate way. And if there are other substances whose existence does not depend on what it is in itself, we can also think about what it is for other substances, assuming that it is related to other substances in some determinate way in addition to merely being part of the same world with them (and that relation is not part of its intrinsic essential nature, as in the case of parts of space relative to one another).

What a substance is in itself cannot be reduced to what it is for other, independent substances, because if its extrinsic essential nature were all there is to its essential nature, there would be nothing to be related to other substances. Relations need relata, or something that already exists. The relata are substances, and since every substance has an essential aspect to its nature as well as an existential aspect, each relatum has an intrinsic essential aspect. Since substances already have intrinsic essential natures, their relationships to other, independent substances must be a further aspect of the essential aspects of their natures as substances. Thus, each substance must have properties of both kinds, though different kinds of substances making up the same world may have different kinds of intrinsic and extrinsic essential natures.

The basic properties of the two basic substances. Spatiomaterialism postulates the existence of two basic substances, matter and space, and it assumes that each bit of matter coincides with some part of space or other. But as we have seen, matter and space have opposite natures as parts of the world. Though in both cases, it makes sense to think of the substances as consisting of many particular substances, their parts are related to one another in opposite ways. Bits of matter can exist independently of one another, but no part of space can exist without all the other parts of space. That is, space has a unique kind of wholeness about it, which matter lacks. The parts of space are dependent on one another, whereas the parts of matter are independent of one another. Being opposite in this way is crucial to their roles in making up the natural world, for nearly every new necessary truth that is supported by ontological philosophy comes from how space contains all the bits of matter.

Matter and space are, however, different basic substances. The existence of one does not entail the existence of the other. We do not know what bits of matter would be like, if they did not coincide with space, or even if that is possible. But each has an existence that is distinct from the other. That is the basic assumption of spatiomaterialism. That is, there would be a difference between parts of space with which bits of matter coincide and parts with which no bits of matter coincide, even if that never actually happens, given what physics implies about the nature of matter. (As we will see, however, space can be empty.)

Both space and matter must, therefore, have all the basic properties that entities must have to be substances at all, including both kinds of existential properties and both kinds of essential properties. Space and matter have existential properties in the same way. But since each basic substance is made up of parts in opposite ways, each has intrinsic and extrinsic essential properties in different ways. To make this clear, let us generate a catalogue of all their basic properties, starting with matter.

Basic properties of matter. Matter is a basic kind of substance, and since it is related to every other substance (of both basic kinds) in a determinate way, it must have both an intrinsic and extrinsic aspect to the essential aspect to its nature as substance.

Intrinsic nature of matter. Matter must have an intrinsic nature, even if matter cannot actually exist without being contained by space, because it must exist in itself in a determinate way in order to have an existence that is distinct from space. (That intrinsic nature may, therefore, be what matter is in itself as it coincides with space, but it is nevertheless different from the aspect of matter by which it is related to space.) What is more, however, matter comes in particular substances that exist independently of one another, and thus, each material substance must have an intrinsic property independently of all the other bits of matter.

The intrinsic property of each bit of matter is simply whatever it is in itself, that is, as something that has an existence distinct from every other substances. This could be anything a substance might be in itself (though as we shall see, it is the aspect of the essential nature of matter that makes it possible to explain phenomenal properties.) Since there may be different forms of matter, with different essential natures, the intrinsic properties of matter may be various.

Extrinsic nature of matter. Each bit of matter must also have an extrinsic aspect to its essential nature, because it is related to other substances which exist independently of it as parts of a single world. But according to spatiomaterialism, the substances that exist independently of each bit of matter include both space and other bits of matter, and thus, each bit of matter can have two fundamentally different kinds of extrinsic essential properties: one by which it is related to space, and aspect, which presumably depends on the former, by which it is related to other bits of matter.

Extrinsic nature of matter relative to space. One kind of extrinsic essential property of matter is how it is related to space. Every bit of matter must be capable of coinciding with some part of space or other, since that is what spatiomaterialism assumes the basic relationship between matter and space to be. Given the essential nature of space, as we have seen, that gives each bit of matter certain spatial relations (in three dimensions) to every other part of space. And since every other bit of matter coincides with some part(s) of space or other, coinciding with space also gives each bit of matter certain spatial relations to every other bit of matter in space. They are all contained by space.

Each bit of matter coincides with a part or parts of space. No assumption has been made about how much space bits of matter can coincide with. There may be different forms of matter contained by space, and different forms of matter may coincide with larger or smaller areas of space. Bits of matter may even be spread out in space unevenly. It depends on further aspects of the extrinsic essential nature of matter relative to space which will be discussed later (in Change: Contingent laws of physics), when we take up the ontological explanation of physics and how space and matter endure through time. All we assume here is that each bit of matter has, at the moment of its existence, a unity about it, so that it exists as a whole distinct from all other bits of matter.

Furthermore, since both matter and space endure through time, there may also be a temporal aspect to the extrinsic essential nature of matter relative to space. For example, it is possible that part of the extrinsic essential nature of bits of matter relative to space is that they move across space in some determinate way.

Extrinsic nature of matter relative to matter. Simply being contained by space gives each bit of matter determinate spatial relations to every other bit of matter, but that is not a basic part of its extrinsic essential nature, because it is entailed by its extrinsic nature relative to space, being contained by space, and the nature of space. But since other bits of matter in space exist independently of it, there can be a basic extrinsic aspect to its essential nature that is relative to other bits of matter is space. For example, if one bit of matter coincides with a particular part(s) of space, it may not be possible for other bits of matter to be located there, or not possible for bits of matter of certain other kinds to be contained by that part of space. Furthermore, if motion is an aspect of the extrinsic essential nature of bits of matter relative to space, their spatial relations may change over time, and there may be regularities about how their motions affect one another (that is, they may exert forces by which they change one anothers motion). Indeed, if there are different forms of matter, there may be ways that bits of matter, because of their relative locations and motion, affect one another’s forms.

This is how physical properties are explained ontologically. The basic laws of physics describe regularities in the motion and interaction of basic particles, and the properties they must mention in order to predict or control what happens are called "physical properties." Hence, the truth of the basic laws of physics can be explained ontologically by the extrinsic essential natures of bits of matter relative to space and relative to other bits of matter, since their extrinsic properties include how the bits of matter move and interact with one another. Indeed, that is how spatiomaterialism will explain the basic laws of physics. In other words, physical properties will turn out to be extrinsic aspects of the essential nature of matter with respect to space, with respect to matter, or with respect to both.

It should be noticed, however, that this way of explaining physical laws makes a distinction between two different aspects of the extrinsic essential aspect of matter, implying that there is a difference between two kinds of physical properties. The physical properties having to do with spatial relations and motion are different from those having to do with interactions, because the extrinsic essential natures of matter relative to space is different from their extrinsic essential natures relative to other bits of matter. Indeed, this is, as shall see, the beginning of a deeper (that is, ontological) explanation of the truth of the basic laws of physics.

Basic properties of space. Space is also a substance enduring through time, and since, as a substance, it exists independently of matter, it must also have two aspects to its essential nature: an intrinsic and an extrinsic essential aspect to its nature as a substance. That distinction arises for space because of its relationship to matter, and unlike bits of matter, no such distinction between intrinsic and extrinsic properties can be made in the case of parts of space.

Space has an opposite nature from matter. It has a unique wholeness, because its parts cannot exist at all unless they are all related to one another geometrically in three dimensions. They are not independent substances. Since their relations to one another are part of the essential nature of each part of space, they do not need any further aspect of their essential natures by which to account for the relations to one another. Their essential natures include their relations to one another, and thus, there is no way to distinguish between an intrinsic and extrinsic aspect to their essential natures.

The reason for distinguishing an extrinsic from the intrinsic aspect of the essential nature of a substance was that when a substance exists together with other substances as parts of the same world, it needs some way of being related to them (beyond merely being parts of the same world). But since that was to assume that the substances exist independently of one another, we excluded substances whose essential natures entailed the existence of other substances, for they must already have relations to those other substances as part of their essential nature. That holds in the case of the parts of space.

To be sure, each part of space has an existence that is distinct from every other part of space. But they all have the same kind of essential nature, for they each have the same kind of relations to all the other parts of space. What makes the parts of space different from one another is the particular parts of space to which they have those relations. And since their relations to one another are part of their essential nature, they need only their essential natures to be related to all the other parts of space. That is why the existence of any part of space entails the existence of all the other parts of space.

It is possible to put this point paradoxically. Since the intrinsic nature of a substance is what it is in itself and its extrinsic nature is what it is for other substances, one might say that the intrinsic nature of each part of space relative to other parts of space entails its extrinsic nature, because what it for other parts of space is just what it is in itself as a part of space. But the paradox just emphasizes that no distinction can be made between the intrinsic and extrinsic natures of parts of space relative to one another.

In the case of space, therefore, the essential nature of each part of space as a part of space includes all its relations to other parts of space. That is the wholeness of space, and though it means that there is no distinction between the intrinsic and extrinsic aspects of the essential nature of each part of space relative to other parts of space, it also has implications for both the intrinsic and extrinsic essential nature of space relative to matter.

Intrinsic essential nature of space relative to matter. To exist independently of matter as its container, space must be something in itself. It must exist in a determinate way apart from space. That is the intrinsic essential nature of space relative to matter. But it is a nature that space can have only as a whole.

The essential aspect of the nature of space as a whole includes its being made up of parts with geometrical relations to one another in three dimensions, that is, being made up of all the locations in three dimensional space. This interdependence of the parts of space means that the essential nature of each part of space includes having geometrical relations to every other part of space. In both cases, the essential nature is the aspect the substances have in virtue of how they exist, and since the parts of space necessarily make up the whole of space, it is the same aspect of these substances that characterizes the essential nature of both part and whole. That aspect of the essential nature of space is the intrinsic nature of space.

There is, however, a part-whole relation involved in the essential nature of space. That is, the part is not identical to the whole, because it is only part of the whole. The whole is identical to all the parts. Thus, the existence of space as a whole entails the existence of each of its parts. But since all the parts must exist, if any one of them exists, the existence existence of any part of space also entails the existence of the whole. (Though there is a necessary relationship between them, it is, at this point, true because of what we mean by the terms used, that is, an analytic truth, not an ontologically necessary truth. It is an ontologically necessary truth about the world only if spatiomaterialism is the best possible ontological explanation of the world.)

Neither part nor whole is prior to the other. Space cannot be explained ontologically as a collection of parts of space, because no part of space can exist without the whole. Likewise the parts of space cannot be explained ontologically by the whole, because the whole of space is just all the parts of space.

What makes the parts of space different from one another is not their essential natures, but the particular parts of space to which each part has the geometrical relations entailed by its essential nature. This is to assume that all the parts of space have the same kind of essential nature, and that is the assumption we are making, since it is the simplest assumption we can make about the nature of space. But it does imply that space is infinite, both in its divisibility and its extent, and thus, the essential nature of space (or its intrinsic essential nature relative to matter) is an aspect of something that is infinite. (Of course, if it were to turn out that space is finite, as contemporary cosmology assumes, a much more complex assumption would have to be made about space, because if space has edges, the parts of space would have to have different essential natures. But space would presumably still have an essential nature that characterizes both part and whole equally, since they would still entail one another, and that would be its intrinsic nature relative to matter..)

The part-whole relation that holds for space is the unique wholeness of space, and since it is an assumption of spatiomaterialism, there is no genuine ontological explanation of it. But it is a remarkable essential nature, and since it is so basic to the spatiomaterialist explanation of the world (including its explanation of many further part-whole relations, as we shall see), a few comment might make it easier to grasp what is involved in taking space to be a substance.

The parts of space are puzzling. Mathematicians call them points because the simplest parts of space have no spatial dimensions. But since they make up space as a whole, there are infinitely many of them in any finite distance. That is called the "continuousness" of space, or its infinite divisibility. But since it has been assumed as part of the essential nature of space, there is no ontological explanation of it in spatiomaterialism. It is just another aspect of the wholeness of space.

As explained above, the wholeness of space implies that parts of space do not have extrinsic essential natures relative to one another. This is because what forces us to recognize that any substance has an extrinsic nature is that it can exist independently of other substances and is nevertheless related to them in some more determinate way than simply being parts of the same world with them. An extrinsic essential property characterizes what the substance is for the other substance, or what it contributes to how they are related. But since parts of space cannot exist independently of one another, they lack extrinsic essential natures as parts relative to other parts of space. Their relations to one another are part of their essential natures. The existence of one part of space entails the existence of all the others.

To say that the parts of space lack extrinsic essential natures relative to other parts of space makes it seem that they do have intrinsic essential natures relative to other parts of space. After all, since each part of space does have an existence that is distinct from every other part of space, it must have something in itself. But since what it is in itself includes it geometrical relations to every other part of space, its intrinsic nature seems to be just its essential nature as a part of space. Thus, it is less misleading to say that no distinction can be made between extrinsic and intrinsic natures of parts of space relative to other parts of space. That is just the unique part-whole relation about space.

It is the unique wholeness of space that makes it odd to think of space as a substance. Space does not seem to be a substance because it is everywhere. That makes it seem like nothing to us, because we are, as rational beings, parts of the world (that is, located in space), and we use the structure of space as a way of thinking about the world. We think of material objects as what is substantial about the world, and we take for granted that such substances have have spatial relations to one another, because that is also a most basic aspect of our way of thinking about the world. (That is, spatial imagination is built into every perception). But the appearance that space is nothing is just the essential nature of space (both part and whole). That is just its intrinsic nature relative to matter. And it is because the parts of space exist in such a way that they make up a three dimensional whole that the bits of matter that coincide with parts of space are related to one another. Thus, to see as nothing is, in effect, to grasp its intrinsic nature relative to matter. That is how it appears from "inside space," so to speak.

On the other hand, to think of space as a substance is, in effect, to see space from the outside, rather than from the inside. It gives us the same angle on space that space itself gives us on material objects, because it provides a context in which we can see how space is related to other things, most relevantly, how it is related to bits of matter.

It may help, therefore, to step back a bit and think about what we are doing in taking space to be a substance. We are recognizing that space is an ontological cause of the things that are found in the natural world that is different from matter, that is, as a separate principle, along with matter, in explaining everything. Space is something self-subsistent that helps constitute the world. It may not be possible to have a deeper understanding of the intrinsic essential nature of space relative to matter than what we know by its role, along with matter, in explaining the world ontologically. That is the step that is required, as I have suggested, to see the world from the outside. But "from the outside" is itself a spatial metaphor. You cannot see space from the outside, for taken literally, the outside of anything is always inside space itself. Thus, as I have suggested, it may be better to think of substantivalism about space as what we must assume in order to have a God’s Eye View of the world. After all, space is something that God would have had to create, along with matter, in order to create the natural world. But neither can that description be taken literally, since, as naturalists we deny that there is any being that transcends the world. Thus, the best we can do is, perhaps, just to recognize that the existence of space as a substance enduring though time is just an independent, basic assumption of the most complete ontological explanation that we can give of the world. Everything else in the world is located within the three dimensions of space. That is the bottom of our understanding of the nature of the world, according to ontological philosophy.

Extrinsic essential nature of space. Just as bits of matter have an extrinsic essential nature that allows them to coincide with space space, so space must have an extrinsic essential nature that allows it to coincide with bits of matter. But since space is a whole with parts that differ from one another as different locations in its three dimensional structure, it is not clear whether this extrinsic essential property characterizes the essential aspect of space as a whole or its parts.

Particular bits of matter clearly coincide with particular parts of space. But if any bit of matter coincides with more than one part of space, coinciding with bits of matter is also clearly something that parts of space must do jointly. Furthermore, it is only because many different bits of matter are all contained by the same whole space that coinciding with space gives them spatial relations to one another. Thus, what coincides with them seems to be space as a whole as well as its parts. That is, bits of matter are contained by space

On the other hand, coinciding with bits of matter is something space does to each bit of matter separately, not how space relates to matter as a whole, because matter is not a whole, but just all the bits that exist. To be sure, space coincides with all the bits of matter in the world. But that is just the spatiomaterialist assumption about how these two basic substances exist together as a world, not something that characterizes the essential natures of space as a whole and matter as a whole.

What makes the nature of space problematic is its unique wholeness, or how space is made up of parts and yet is still one. For our purposes, therefore, it is enough to recognize that the capacity to contain bits of matter is the extrinsic essential nature of space, both whole and part, though each bit of matter coincides with some part (or contiguous parts) of space or other(s). And if different varieties of material substances are contained by space in different ways, it must have all the extrinsic essential properties required to do so.

Furthermore, space must also have extrinsic essential properties corresponding to all the extrinsic essential properties of bits of matter relative to space. That is, it must give bits of matter motion through space, if that is how they coincide with space, and it must enable them to interact in all the ways that are involved in the extrinsic essential natures of various kinds of bits of matter relative to other bits of matter. These are also extrinsic essential properties that space both has as a whole and in each part.

Nor is that necessarily all there is to the extrinsic essential nature of space (though relativistic physics holds, in effect, that it is). Since space is a substance, which exists independently of matter, it is possible for space to interact with bits of matter in other ways. Indeed, that is what we shall need to assume in order to explain ontologically how Einstein’s special and general theories of relativity are true. The basic assumption of our ontological explanation of relativity will be is that light always has a determinate velocity relative to space itself, and in explaining special relativity, we will hold that space imposes certain (Lorentz) distortions on material objects moving through space with high velocity. In the case of general relativity, we will assume, further, that the accumulation of large quantities of matter in space alters the velocity at which light moves in nearby regions of space.

This is to hold that the parts of space can contain bits of matter in different ways in the regions around centers of gravity But that is not to say that are any changes in the relations among the parts of space itself. It is only to say that there is a change in how bits of matter coincide with space in those regions. In short, the assumption we shall make in explaining Einsteinian relativity is that space has an absolute, uniform Euclidean three dimensional structure, and that that structure is not changed even though the extrinsic essential nature of space includes interactions with matter that change the state of certain parts of space and, thereby, change how bits of matter coincide with space in those regions. (See Change: Special theory of relativity and Change: General theory of relativity.)

In a more speculative way, I will suggest that space also plays a role in explaining the truth of quantum mechanics, the basic particles recognized by physics, and certain issues in cosmology. Those roles would characterize further the extrinsic essential nature of space, both part and whole. (See Change: Quantum mechanics and Change: Cosmology.)

Properties as Objects of Knowledge. Ontological philosophy explains properties as aspects of the substances it postulates. But when philosophers begin their argument from the point of view of the cognitive subject by reflecting on how they know, they see properties as objects of knowledge, and that gives rise to philosophical problems, including problems about the nature of properties. To take properties as objects that are known in some way is, in effect, to see them as more basic than substances, because the objects that have them seem to be nothing but something that has properties of certain kinds that are present to the subject and to which he can refer. This is the source of the problem of mind. It can be seen that there is a difference between two basic kinds of essential properties (which ontological philosophy explains as the difference between intrinsic and extrinsic essential properties), but epistemological philosophy has no way to explain how they are related to one another because it takes properties to be basic. In its contemporary form, as we shall see, it infects materialism. But let us begin by seeing how the problem of mind arises.

The Problem of mind. The problem of mind arises when naturalists discover that there is a basic difference between properties which was not obvious at first. In our naive or natural attitude toward the world, we take the natural world to be simply what we perceive, as if the objects in space, including our own bodies, were simply what they appear to be. This is a form of realism, because it is to assume that those objects in space would exist even if we were not perceiving them. But it is naive, because it assumes that the objects being perceived actually have the properties that they appear to have in perception, including not only their locations, shapes, and dispositional properties (such as how they move and interact), but also their colors, odors, sounds and tactile properties, such as hot and cold, wet and dry.

The latter properties are distinctive, for they are qualitative properties, or properties that are simply a quality of some kind that is immediately present to the perceiver. When I perceive that a leaf is green, for example, the surface of the leaf appears green, and the greenness is an object of my immediate awareness. What I mean by "green" is that kind of quality that seems to inhere in the surface of the leaf, and I cannot define "green" any more precisely than that, because what I mean is something that is intrinsic to the object I am aware of. The quality is what makes it the kind of object it is. Such qualitative properties are now often called "qualia," and they are involved in everything we perceive, including not only the colors that objects have to vision, but also the odors they have to smell, the sounds they have to hearing, and certain tactile properties they have to touch. Such qualities, or qualia, also characterize one’s own body, but one’ own body has additional qualities that are perceived in a different way, such as pains, tickles, itches, and the like, for they are not perceivable by others.

The problem of mind arises when it is recognized that the qualia that are immediately present to us in perception are not located in the objects we perceive in the space in and around our bodies, but are somehow part of us as subjects, most closely connected to our brains. That is, the mind become a problem with the acceptance of critical realism.

Naturalists are forced to recognize that qualia are subjective in this sense when they discover that perception is a physical process in which the objects stimulate sensory organs and that somehow gives rise to the qualia we have. In each sensory modality, what causes the experience is a chain of causes and effects that starts in the object being perceived, proceeds through the body, making events occur in the brain, and the qualia come at the end of that causal chain. Thus, qualia must somehow be part of one’s brain. And if we follow this argument to its conclusion, naturalists also come to recognize that the space in which sensory qualia seem to be located is itself also merely phenomenal and, thus, distinct from the space in which the physical objects actually exist.

This discovery about perception is called "critical realism" (or "representative realism"). It is realism, because it holds that the objects being perceived really do exist in physical space as the causes of the appearances we have in perceiving, including our bodies. But it is critical, because it does not take the qualia that make up those appearances to be properties in the objects that give rise to them, but rather as parts of the subject, where their function is apparently to represent those properties in the material objects in real space to the subject. Likewise, it is critical because it recognizes that the spatial relations that appear to hold among the qualia in perception are different from the spatial relations that hold among the material objects in real space.

Thus, critical realism about perception makes it clear that objects with physical properties in real space exist somehow "beyond" the (complex) phenomenal properties we have. Since material objects in real space have physical properties, it is to discover that we must distinguish the qualia and their configurations in phenomenal space from physical properties. They are what we call "phenomenal properties."

Critical realism gives rise to the so-called problem of mind, for it seems that the subject to whom the configurations of qualia appear is a radically different kind of entity from the material objects in real space. Material objects have physical properties, including not only the physical dispositions that make them causes of the qualia that appear in perception, but also relations in real space. But the subject is radically different, because he is something to which phenomenal properties appear.

When we reflect on the perceptual appearances we have as perceiving subjects, furthermore, we recognize that they play distinctive roles in our processes of knowing and doing. There are other appearances similar to perceptual appearances, albeit fainter and less detailed, which play other roles. Traditionally, the former are called "ideas of perception," and the latter are called "ideas of memory and imagination." But they, and perhaps other appearances that our mental processes have to us in thinking and feeling emotions, are all phenomenal properties.

To acknowledge this fundamental difference from material objects, the subject calls himself "mind" and contrasts it with his body, which is just an object in space (albeit a special one, since it is the one through which he acts). The mind-body is problem is how the mind and body can be parts of the same world, that is, what are their natures and how are they related to one another.

Theories of mind. This problem about the nature of mind is arguably the source of all the problems encountered in modern philosophy, and it arises in contemporary philosophy as the problem about the relationship between physical and phenomenal properties. The question is how to explain the natures of the two radically different kinds of properties that are known from the point of view of the critical realist as parts of the same world. There is not much of a problem for ontological philosophy, and so let us consider why before we derive the various positions on the nature of mind defended by traditional, epistemological philosophy.

Ontological theories of mind. A solution to the problem of mind would pay back one of the mortgages we took out on spatiomaterialism, for it would explain how beings like us are conscious. And it can be found in the differences among the basic properties that are entailed by spatiomaterialism, or indeed, that are entailed by any materialism that accepts our notion of substance and takes ontology to be explanatory. Physical properties are different from phenomenal properties as the extrinsic essential natures of bits of matter are different from the intrinsic essential natures of matter.

Material objects in space with their physical properties present no problem, for they are precisely what a naturalistic ontology is intended to explain, and though we will put off the detailed ontological explanation of physical properties, we have already seen how they will be explained as aspects of the extrinsic essential natures of bits of matter in space.

As naturalists, we assume that the subjects who perceive the world are themselves material objects in the world. And we have good reason to believe that they are rather special material objects, for they are animals with complex brains. Spatiomaterialism will throw much light on how the brain is responsible for the behavior and cognitive processes that we ordinarily believe take place in experiencing subjects like ourselves. (See Change: Evolutionary stage 6 and following.) But they are basically explanations of how the brain is a machine that enables subject to have the beliefs, desires, and behavior that we do, and for now, let us take it for granted that there is such an explanation.

Assuming, therefore, that the brain can account for the behavior and cognitive capacities of subjects like us in the natural world, all that is needed to solve the problem about mind is an explanation of the existence of phenomenal properties that shows how it is possible for material objects to have them. The obvious explanation of the nature of phenomenal properties, given the kinds of basic properties that substances have, is that they are the intrinsic essential aspect of the nature of some bits of matter that help make up the brain. That would mean that phenomenal properties are related of physical properties as the intrinsic essential nature is related to the extrinsic essential nature of some bits of matter that help make up the cognitive subject. Since bits of matter must have both kinds of essential properties, this ontological explanation would imply that there is an ontologically necessary relationship between physical and phenomenal properties. That explanation of how the connection is necessary is what solves the problem about mind that plagues contemporary philosophy, as we shall see below: Properties: Ontological theory of the necessary connection.)

This is enough to show that consciousness is possible, if spatiomaterialism is true, though it depends, of course, on showing that there is a form of matter that helps to constitute the conscious subject whose intrinsic essential nature can plausibly account for all the phenomenal properties. Since they include not only sensory qualia, but the complex configurations of them in phenomenal space, there is more to the explanation of consciousness than this ontological explanation of the basic properties of substances. To explain those complex phenomenal properties is to explain what I will call the "unity of consciousness." We cannot do that, however, until we have considered the forms of matter entailed by spatiomaterialism (as we shall in Change: Contingent laws of physics), and explained how the brain works (in we shall in Change: Evolutionary stage 6 and following). For the spatiomaterialist explanation of the unity of mind, see Change: Unity of consciousness.)

In order to suggest how such an explanation is plausible, however, let me just say here without further defense that the relevant form of matter will turn out to be the photons that are generated by the active mammalian brain. That is, the firing of neurons involves the rapid acceleration of charged objects (ions), and since in mammals, many such neurons fire in a synchronized way (throughout the projection from the thalamus to the neocortex), the whole brain is like a complex antenna generating photons with a very complex structures in space and time. The intrinsic essential aspect of the nature of those bits of matter can explain phenomenal properties, including not only the simple qualia but also how they appear to be configured in phenomenal space, not to mention the differences between perception and memory and imagination.

Epistemological theories of mind. Epistemological philosophy does not attempt to explain things ontologically, except as an afterthought to an argument that attempts to justify knowledge of some kind, that is, as realism about the objects of which it tries to show that we have knowledge. Instead, it uses reflection on how we know to introduce a theory about the nature of reason, and and starting with some kind of knowledge that is taken as unproblematic by that theory, it tries to justify knowledge of something else. Success is realism, but realism leads to metaphysical dualism, that is, an ontology that postulates kinds of substances that are so utterly different from one another that it is not possible to explain how they are related to one another at all. And the ontological problems of realism lead, as we have noted, to anti-realism, the denial that we have the kind of knowledge defended (which may entail it own distinctive metaphysics).

Though both modern and contemporary philosophy start by reflecting on how we know, the problem of mind took different forms for each period, because they had different explanations of how we know, that is, different theories about the nature of reason. Modern philosophers had a theory about the nature of reason that was based on reflecting on how individual minds know, and so its realism led to mind-body dualism. Contemporary philosophers had a theory about the nature of reason that was based on reflecting on knowledge as an intersubjective process, and so its realism led to property dualism (and puzzles about the relationship between physical and phenomenal properties). Let us consider each in turn.

I will give a brief account of the problem of mind in modern philosophy in order to provide a context in which to understand the approach of contemporary philosophy.

Modern philosophy. Though the ancient atomists were critical realists, naive realism otherwise dominated ancient and medieval philosophy. It was the rise of modern science that led to the rediscovery of critical realism. Modern science presupposed an ontology that ascribed only physical properties to objects in nature, and it implied that perception depends on a chain of causes and effects starting in the object and ending somewhere in the brain. Though modern scientists and philosophers alike recognized that sensory qualia are parts of the subject, it was Descartes who first saw how to use it to pursue a new form of epistemological philosophy. Descartes so-called method of doubt was to deny everything that it was possible to doubt. As a critical realist, that led him to doubt the existence of his own body and the natural world in which it exists.

Realism about the external world: mind-body dualism. Descartes could not doubt that he was having ideas, and thus, he argued that he had indubitable knowledge of his own existence. Descartes affirmed the certainty of this knowledge by asserting, "I think, therefore I am." From this foundation, Descartes introduced a theory about the nature of reason that implied that any ideas that are equally clear and distinct are true, and thus, he set out to show that we could know both the existence and nature of the external world. Given his goal, the success of modern realism was realism about the world of material objects in space. Descartes' plan was to justify modern science philosophically, that is, from a foundation that is prior to what science learns about what happens in the natural world from observation. But apart from other difficulties in his argument, his project foundered on the problem of mind-body dualism.

The rational method he used was discussed in Method, and the dualistic ontology to which it led was discussed in Ontology. Descartes' dualism of mind and body was the problem of mind in modern philosophy. Critical realism made it clear that physical properties are fundamentally different from phenomenal properties, and that made it seem that the objects with those properties were substances with opposite kinds of essential natures, namely, mind and body. As Descartes saw it, body is always divisible into smaller parts, whereas mind has a unity that does not permit division, because all the qualia that seem to be located in space have an appearance for the subject at the same time. And whereas mind can think in this sense, body cannot, for it has only the properties that physics ascribes to it (which Descartes thought came down to extension, that is, geometrical properties). The difference in their essential natures left no plausible explanation of how they interact, and attempts to solve it (such as Spinoza’s claim that substance can have two opposite essential natures and Leibniz’s claim that nothing exists but minds, or "monads," as he called them) were embarrassing failures.

Anti-realism about the external world: idealism. The inability to explain how mind and body are related as parts of a single world doomed attempts to justify knowledge of the natural world, and the British empiricists (Locke, Berkeley and Hume) followed the skeptical argument to its conclusion, doubting in the end that the natural world is anything but perceptual ideas (or impressions of sensation, as Hume put it). Locke did not recognize that the principle of empiricism (that all our knowledge about the natural world comes from experience) leads to skepticism about the existence of the natural world, but Berkeley embraced this skeptical conclusion ontologically and defended idealism explicitly. However, Hume and the subsequent tradition of empiricism merely dismissed all attempts to explain the natural world ontologically as meaningless metaphysics (though idealism is all that empiricism has to offer to those who look for a theory about what exists).

The second phase of modern philosophy struggled with the problem of mind in a different way. Kant held that science has knowledge only of the phenomenal world, and thus, he was not a realist. But he was still a dualist, because he believed that, in addition to mind, there are things in themselves in addition to the phenomenal world. Hegel sought to overcome Kant's dualism and defend the claim of reason to know the real nature of what exists, but the only way he could do was by defending absolute idealism (that is, by holding that everything, including the natural world, can be reduced, dialectically, of course, to an idea at the bottom).

Spinoza stands out among modern philosophers, because his way of denying mind-body dualism was to deny that body is a different substance from mind. He took mind and body to be related as two attributes of the same substance. (That is close to the implication of ontological philosophy, except that Spinoza believed that the world is a single substance. He could not explain the relationship between the attributes of thought and extension as the relationship between the intrinsic and extrinsic essential aspects of substances, because there are no relationships among substances in his view.)

Contemporary philosophy. Naturalism is the attitude of contemporary philosophy. In the twentieth century, continuing advancement by science in explaining the natural world, discovering laws of nature and various mechanisms embodying them, made the abstruse and inconclusive arguments of philosophy of modern philosophy seem fundamentally misguided. Philosophers abandoned the Cartesian method and its metaphysical problems in favor of an explanation of how we know that derives from reflecting on knowledge as an intersubjective process, and that brought with it a commitment to naturalism. And contemporary philosophers accepted natural science, with some reservations, as the most adequate way of knowing we have. Thus, the problem that mind poses for contemporary philosophers can be seen as a question about how a science of consciousness is possible. Contemporary philosophers assume, as naturalists, that what modern philosophers called "mind" must somehow be part of the natural world, and though they could dismiss mind-body dualism, it was harder to deny the difference between physical and phenomenal properties. Those who affirm the existence of phenomenal properties as well as physical properties are called "property dualists." For naturalists, therefore, the question became how phenomenal properties can be included as something characterizing the natural world being explained by science, even though science refers only to physical properties.

For present purposes, let us take "physical properties" to include functional properties, such as "being a clock" or "conveying signals." They may not be reducible to physical properties, but since no one denies that they "supervene," at least, on physical properties, all the causal connections in particular cases come down to basic physical properties. And the issue is how phenomenal properties are related to physical or functional properties.

Contemporary philosophers have taken great care to show that phenomenal properties are different from physical properties, for example, in famous arguments by Thomas Nagel, Frank Jackson, and Saul Kripke. By asking what it is like to be a bat, Nagel (1979, 1986) was pointing to a subjective aspect of experience that cannot be known by the "view from nowhere", that is, by natural science. Jackson (1982) made it clear that phenomenal qualities, or qualia, are themselves objects of knowledge by pointing out that Mary, a neurophysiologist who studied the physical mechanism of color perception in a laboratory devoid of red objects, would come to know something more about the perception of red when she left the room and actually saw something red, namely, how red appears to the subject. And Kripke (1980) showed that properties rigidly designated by how they appear to subjects cannot be identical to physical properties because the connection is not metaphysically necessary, as it would have to be, if they were identical.

For those who are inclined to take natural science as revealing the basic nature of the world, the problem of mind is how there can be a science of consciousness. It is most obviously problematic when science is understood as using a method that bases its conclusions on observation in one way or another. This reliance on observation is a basic tenet of its empirical method as traditionally understood, for example, by empiricists, logical positivists and most practicing scientists. (Though there are well known problems in the philosophy of science about the theory-ladenness of observation statements, it is agreed on all sides that observation depends on perception, that is, on the use of our sensory organs to discover the states of objects in space.)

The reason that this tenet of the empirical method makes consciousness a problem for science is that phenomenal properties are apparently knowable only by reflection. We have seen how the difference between physical and phenomenal properties was discovered -- that is, by reflecting on the causal explanation of perception from the point of view of the perceiving subject. But it also seems that our only "evidence" that psychological states involve phenomenal properties comes from each of us reflecting on our own psychological states. The nature of simple qualia, for example, what red qualia are like, is not revealed to observation. They are private to each individual subject.

There are ways of observing the brain in operation, and new ways are being developed. But no one has found a way of using such observations to demonstrate that brain states involve phenomenal properties. Indeed, neurophysiologists don’t expect their methods ever to show either the existence of phenomenal properties or how qualia appear.

To be sure, there is evidence for the existence of phenomenal properties in what people say about their psychological states. But that evidence depends on scientists interpreting the other’s talk of qualia and phenomenal properties as references to objects of the same sorts they each know privately by reflection on their own phenomenal properties. The verbal behavior itself does not seem to depend on anything but physical causes.

The difference between reflection and perception makes it doubtful, therefore, that science will ever be able to know about phenomenal properties.

Anti-realism about phenomenal properties: eliminative materialism. One quick way of dealing with this problem is simply to deny there are any phenomenal properties. This is, in effect, anti-realism about phenomenal properties from the point of view of science, though it is usually called "eliminative materialism," by the kind of ontology it defends.

In one version, eliminative materialism holds that the need for talk of phenomenal properties will eventually be eliminated, at least from science, as science explains all the phenomena relevant to psychology in its own terms. That would show that our traditional talk about phenomenal properties (and psychological states, such as perceptions, beliefs, desires, and the like) is just a mis-description of what really exists, which is fully described by physical properties. (See Churchland 1995.)

It is also possible to argue that we are fooling ourselves to think that traditional talk about phenomenal properties is meaningful in the first place. (See Dennett 1991 and Rorty 1979.)

But eliminative materialism does not show how a science of consciousness is possible. Rather, it holds that a science of consciousness is not necessary because there is nothing to be explained. The problem of mind arises only for those who are realists about phenomenal properties and believe that they exist in addition to physical properties.

Realism about phenomenal properties: property dualism. During most of the century, empiricism in psychology took the form of behaviorism, the attempt to explain human beings in terms of laws describing their observable behavior. Consciousness was thereby banished from science. But that is puzzling to contemporary naturalists, for they expect natural science to explain everything in the natural world, and they know, as reflective beings, that they themselves are conscious. They are realists about phenomenal properties, and that makes them property dualists, because they recognize the existence of phenomenal as well as physical properties. And the problem of mind can be seen at the attempt to show how science can study consciousness, that is, how it can justify theories that refer to the phenomenal properties of psychological states. There are several possibilities.

Emergentism. The most obvious way for science to include consciousness would be to take mind to be an immaterial substance that is located in space along with bits of matter. Or if we call everything located in space "matter," it is to hold that some bits of matter have phenomenal properties that play a causal role in the natural world. If phenomenal properties of bits of matter did somehow make a difference to what happens in nature, they would be not only effects of physical causes, but they would themselves be efficient causes, and their existence could be detected empirically. Science could know about them in the same way it knows about other unobservable entities, such as electrons and force fields. Bits of matter with phenomenal properties would have to be mentioned by the best explanations of what can be observed through perception alone.

It is conceivable, at least, that phenomenal properties would have to be introduced by some branch of science, such as neurophysiology. The mechanisms found in the brain might provide no way of explaining, for example, why human beings say that they have phenomenal properties or why they call certain sensations green and others red. If all possible physical explanations were ruled out, the best explanation might be to hold that reports about phenomenal properties depend causally on how psychological states appear to the subject having them, which would mean that phenomenal properties are efficient causes. Phenomenal properties would then be unobservable entities of neurophysiology.

Any such neurophysiological discovery would, however, have serious implications for physics. The grounds for believing that there are phenomenal properties playing a causal role would be that no physical mechanism can explain certain verbal behaviors, and that would imply that there are efficient causes at work in brains that are not physical properties. This would be shocking, for physics is thought to be causally complete, in the sense that physical properties are sufficient, in principle, to explain every kind of event that happens to what is located in space.

It might be argued that the reason physics has not noticed the causal role played by phenomenal properties is that they are emergent and make a difference only in highly complex physical objects, such as brains, which evolve (or in complex functional systems generally). But in order for phenomenal properties to be effective in brains, neurophysiology would have to predict something different from what physics would predict for the same situations on the basis of physical properties. Thus, physics would have to come to believe that some material objects have properties in addition to the physical properties that it has already recognized and that these new properties affect how physical entities move or interact in certain situations. In other words, this kind of emergentism would be causal. Such a discovery would contradict physics as we know it. At a minimum, it would show that physics is not causally complete.

A science of consciousness could, therefore, be established by a scientific discovery of the kind that even the most hidebound defender of the traditional view of the empirical method could not deny. That would be a scientific solution to what has heretofore seemed to be a philosophical problem about mind. There is, however, no evidence at present suggesting that phenomenal properties should be introduced as unobservable (that is, not directly perceivable) theoretical entities of neurophysiology. It seems quite unlikely to contemporary naturalists, considering how radically physics would have to be mistaken. And if phenomenal properties are, as ontological philosophy suggests, the intrinsic essential properties of certain kinds of matter involved in the function of the brain, they have no causal role. All the causal roles are played by extrinsic essential properties, that is, the physical properties already recognized by science.

Epiphenomenalism. Another way founding a science of consciousness would be to accept reflection as a form of observation in science. Though reflection has long been the province of philosophy, this avenue is open to naturalists who think of philosophy as "continuous with" science.

This trend in recent philosophy of science explicitly abandons epistemology in the traditional sense of providing an a priori foundation for the justification of science and its method (Kitcher 1992; Rosenberg 1994). Instead of "first philosophy," it proposes to use the results of science itself to justify and improve the methods of science, which has given it the name "naturalized epistemology" (after Quine 1969). For example, scientific discoveries about the mechanisms of human cognition could be used to improve evidence gathering methods in science as much as discoveries about the accuracy of any measuring instrument. But those same human beings have a capacity for reflection as well as perception, and thus their reflection on phenomenal properties could be considered a way of gathering evidence about the natural world which is just as legitimate as their perception of physical properties. To naturalists of this kind, therefore, it may seem there is no obstacle to a science of consciousness. Indeed, these days, cognitive scientists often use reports about reflection on phenomenal properties as evidence, a practice recently defended by Goldman (1997).

To recognize reflection, including what can be known only by reflection, as part of the data base of natural science is, however, a trivial solution to the problem of mind. It overcomes the epistemological obstacle to a science of consciousness by, in effect, redefining "science" to include a form of knowledge that has traditionally been taken as the foundation of by philosophy.

It will not be acceptable to naturalists who cleave to a more traditional notion of empirical science as based on observation by perception. They will dig in their heals from fear of opening the door to other forms of private knowledge in science, such as the intuitions by which rationalists justified their metaphysical systems. And attempts to draw a new line of demarcation between science and philosophy that will include reflection on phenomenal properties but exclude the supposed certainty of clear and distinct ideas would seem like mere gerrymandering.

Even if there were no epistemological objections to reflection, however, this avenue to a science of consciousness would lead to ontological problems for science. It would complicate the scientific view of the natural world in a way that is quite problematic, for it would be to acknowledge the existence of properties that simply do not fit together intelligibly with the properties already recognized by science. The latter come down to properties mentioned by physics. Specifically, physical (and functional) properties seem to be responsible for all the behavior and internal processes found in complex organisms like us. Thus, to acknowledge the existence of phenomenal properties on the grounds that they can be "observed" in nature through reflection on what experience is like would be to recognize that some natural objects, human beings, at least, have properties of a fundamentally different kind from those already recognized by natural science. And if physicists are correct in believing it to be possible, in principle, to explain everything that happens in nature by the efficient causes picked out by physical properties, two facts about these properties follow. One is that phenomenal properties are somehow effects of the physical (or functional) properties of such organisms. The other is that having phenomenal properties cannot itself have any effect, in turn, on physical or functional properties. In other words, phenomenal properties would be epiphenomenal relative to physical (and functional) properties.

Epiphenomenalism is, at best, an inelegant ontology. It takes phenomenal properties to be "nomological danglers," in Feigl’s (1958) famous terms. Epiphenomenalists can insist, of course, that there is a causal necessity about the connection between physical (and/or functional) properties and phenomenal properties. But it would be just an assumption, for they have no explanation of why physical (or functional) properties give rise to phenomenal properties. Nor any explanation of why phenomenal properties should be impotent.

Thus, if the goal of science is to discover all the most basic laws of nature, epiphenomenalism would mean that those most fundamental laws include not only the basic laws of physics, which describe efficient-cause connections, but also psychophysical laws, which describe a regular connection between physical (and/or functional) properties and phenomenal properties. (For example, see Chalmers1996, pp. 87, 170-1, 274-5.)

Or, to use Kripke’s (1980, p. 153-5) famous metaphor, God, in creating such a world, would have to go back, after creating all the physical objects and putting them together as a natural world, and tack on the phenomenal properties. The extra effort required belies their odd status. No one finds epiphenomenalism satisfactory. (It repels even Chalmers 1996, p. 160.)

Necessary connection between physical and phenomenal properties. Ontological philosophy provides, as we have seen, a way of avoiding the problem of epiphenomenalism. Though it accepts property dualism, it reveals a necessary connection between physical and phenomenal properties, and that would found a science of consciousness, because it would show that phenomenal properties are already part of what exists according to science. Contemporary philosophers recognize that demonstrating a necessary connection between physical and phenomenal properties would solve the problem with epiphenomenalism (and thus, the most basic aspect of the problem of mind), but they have not been able to take this avenue all the way to a science of consciousness, because cannot see how it is possible to show that phenomenal properties are necessarily connected to something science already mentions in its physical (and/or functional) descriptions. The obstacle they encounter comes from the epistemological approach to philosophy, which contemporary naturalists have inherited, for in this case, ontology as mere realism makes it seem that properties are more basic than substances. Let us see how they fail to find any way to demonstrate a necessary connection between physical and phenomenal properties before we compare epistemological to ontological philosophy.

Necessity in Epistemological Philosophy. Contemporary analytic philosophy offers various ways in which a necessary connection might be established. Let us consider them.

A priori necessity. The original form of necessary truth in contemporary Anglo-American analytic philosophy was analytic truth, or propositions that are true by virtue of the meanings of the terms involved. That would bean a priori connection between physical and phenomenal properties, but it is not a possible foundation for a science of consciousness, for the inability to see an intelligible connection between them is the very problem of consciousness. What we mean by "phenomenal properties" is so different from what we mean by "physical properties" (or by "functional properties," for that matter) that it seems almost absurd even to compare them. That makes it easy to conceive of possible worlds that are physically like our own, but which lack phenomenal properties altogether. That is, there could be a world of zombies, or beings that are physically and functionally indistinguishable form us except for not being conscious. It is also possible to conceive of worlds with phenomenal properties but no physical properties, for that is the view that was defended in modern philosophy as idealism. Hence, no necessary connection can be established a priori.

Causal necessity. Any necessary connection between physical and phenomenal properties must, therefore, be a posteriori. It must be something we can somehow discover about the world from experience. But it cannot be a mere causal necessity of the sort that laws of nature are supposed to have. That sort of necessity would reduce either to causal emergentism or to epiphenomenalism, depending on which causal connections phenomenal properties were supposed to have (that is, being effects of physical properties that are also causes of them, or else being effects that are not causes). It is their inadequacy that forces naturalists to look for a metaphysically necessary connection.

Theoretical identification. The more popular model for discovering necessary connections is theoretical identification in science, such as the discovery that water is identical to masses of H₂O molecules. Thus, just as the solidity of ice was discovered to be identical to the stability of the crystal structure formed by weak hydrogen bonds among adjacent H₂O molecules when their kinetic energy fall below a certain point, so phenomenal properties might turn out to be identical to physical properties of some other kind.

However, physical and phenomenal properties cannot be related in this way, because theoretical identification is a necessary connection. As Kripke(1980) showed, in order for two (rigidly designated) properties to be identical, it must be impossible to conceive one without the other. For example, if the solidity of ice is identical to a certain kind of crystalline structure of H₂O in the actual world, then the identity must hold in any possible world where either exists. It is not, however, impossible to conceive of worlds in which beings physically and functionally like us lack phenomenal properties altogether. No scientific theory can identify the two kinds of properties, and so a world of zombies is still possible.

Supervenience. If the reduction involved in theoretical identification does not provide an avenue to a science of consciousness, science does not offer many other models for showing a necessary connection between physical and phenomenal properties. One possibility is supervenience, which is a weaker relation than the complete reduction involved in the theoretical identification of apparently different physical properties. What has forced philosophers to recognize supervenience is the existence of functional properties. Though a functional property may be identical to certain physical properties in particular cases or classes of cases, there are many other ways that the same functional property can be realized by physical properties and, thus, no general identity between properties at the two levels. For example, there are many kinds of physical mechanisms that can function as clocks. And physical properties that do are said to "realize" a clock. But supervenience cannot be how phenomenal properties are related to physical properties, for that would require phenomenal properties to be identical to physical properties in particular cases, and that is what does not seem to be the case. Thus, a zombie world still seems possible.

A process of elimination leads to the conclusions that, if there is a metaphysically necessary connection between physical and phenomenal properties that can be discovered by experience, it must a new kind of relationship, not previously recognized by science. That is what ontological philosophy offers, and though it is beyond the reach of epistemological philosophy, David Charlmers comes close.

Chalmers (1996, p. 135) considers the possibility that "there are properties essential to the physical constitution of the world that are not accessible to physical investigation." The existence of such intrinsic properties is plausible to him, because all the properties mentioned by physics are basically relational, characterizing entities by their causal connections and other relations to one another. Even physical properties that seem to be inherent in the objects that have them, such as mass, energy, spin, and charge, are measured by the causal relations they have to one another. Thus, whatever has physical properties could also have an intrinsic nature.

However, Chalmers has no way to understand how they might be related to physical properties, because he does not think of substances as anything more than the properties they have. That makes properties ontologically basic, and so he tries to describe the relationship by saying that intrinsic properties might " ‘realize’ the extrinsic physical properties, and that the laws connecting them might realize physical laws" (155). And describing the significance of discovering some such relationship, he says that, if intrinsic properties were "constitutive of physical properties" (136), then even though a zombie world may seem to be physically identical to ours, it would actually be different physically, for it would lack some "inaccessible essential properties, which are also the properties that guarantee consciousness" (135). This is the view of phenomenal properties to which Chalmers himself inclines (153-156), though it has also been suggested by others.ⁱ

As Chalmers recognizes, however, to suggest that intrinsic properties are a special kind of phenomenal (or proto-phenomenal) property underling all physical (and functional) properties is not to show that there is a necessary connection between intrinsic and physical properties. It is merely to point to a possibility. Chalmers (135) rightly calls it "speculative metaphysics." Though it may be coherent, it is no more than speculation, because without the concept of substance to explain the nature of properties, it is just a vague possibility. And since nothing makes it inconceivable that a world physically like our own would lack intrinsic properties, this view reduces to property dualism — a point that Chalmers makes by invoking Kripke’s metaphor: "After ensuring that a world identical to ours from the standpoint of out physical theories, God has to expend further effort to make that world identical to ours across the board" (136). Zombies are still possible.ⁱⁱ

Metaphysical Necessity in Ontological Philosophy. What keeps epistemological philosophy from discovering a necessary relationship between physical and phenomenal properties that would found an empirical science of consciousness is the implicit assumption that properties are basic. What enables ontological philosophy to show that phenomenal properties are an essential part of the natural world investigated by science is reducing properties to substances. Physical properties, as we have seen, characterize the extrinsic essential natures of all forms of material substances, and if phenomenal properties characterize the intrinsic essential nature of some form of matter that helps constitute the conscious subject, phenomenal and physical properties would be related as intrinsic and extrinsic aspects of the essential natures of the substances constituting the world. That relationship does seem to be metaphysically necessary in the sense relevant in this debate, though in our terms it is an ontologically necessary truth, since the necessity of its truth comes from its being an implication of the ontology we have found to be true on empirical grounds.

Metaphysically necessary truths are understood as holding in every possible physical world, and the connection proposed by ontological philosophy is necessary in that sense, for it would hold in any possible physical world in which the basic laws of physics are descriptions of how elementary material substances move and interact. Their basically relational nature indicates that physical properties characterize the extrinsic essential natures of those substances. But since substances cannot have such properties unless they have some way of existing apart from the relations, they must also have an intrinsic essential nature. Thus, Zombies would be impossible. Any being with all the same physical (and functional) properties would necessarily also have intrinsic properties.

To use Kripke’s (1980, 153-4) vivid image, God would not have to go back and tack on intrinsic properties after he had created the physical world, for if God had created the world by combining many material substances in the first place, those substances would already have intrinsic natures of some kind or other. In fact, it would not be possible for God to create a physical world out of multiple substance without intrinsic properties, even if he wanted to.

This is not to say, however, that there is no possible physical world without intrinsic properties. It is possible for a world to have all the same physical (and functional) properties as our own and yet to lack intrinsic properties.

That would be the case, for example, in a physical world that is not constituted by substances at all, as the empiricists’ so-called "bundle theory" of substances would have it. (That is, however, just the form of idealism that one finds when one looks in empiricism for a theory of what exists.)

Even if the physical world must be constituted by substances of some kind in order to exist independently, it could lack intrinsic properties, for it could be constituted by substances that are mere substrata for physical properties (assuming that it is coherent to suppose there can be substances without any inherent properties at all).

Nor would intrinsic properties be needed if the world were constituted by a single substance in which particular properties have spatiotemporal locations.

The necessity of the relationship between intrinsic and extrinsic properties depends, in other words, on an ontological assumption that is not itself necessary, namely, that the world is constituted by many particular substances existing together in some way. There was no such condition on the kind of metaphysical necessity that Kripke discussed, for he was considering only the possibility of properties being identical. That is, if phenomenal and physical properties were identical, there would be no possible physical world without phenomenal properties. But the way in which ontological philosophy demonstrates a metaphysically necessary connection does not come from discovering the identity of two apparently different properties. It comes from discovering that material substances must have two aspects to the essential aspect of the nature as substances. That is, it depends on a theory about the nature of the substances constituting the world that can be justified empirically. (As we have seen, the foundation of ontological philosophy is established by accepting naturalism, taking ontology to be explanatory, and using the empirical method to decide which possible ontological explanation is true.)

Kripke's model for identifying properties with one another comes from discoveries in science that physical properties picked out on the macro-level (such as the solidity of ice) are identical to physical properties picked out on the micro-level (such as the hydrogen bonds among H₂O molecules under certain temperature and pressure conditions).

Ontological philosophy, by contrast, discovers how properties characterizing one aspect of the essential nature of substances (their extrinsic essential nature) are related to another aspect of the essential nature that such substances must have (their intrinsic essential nature). The nature of material substances is what connects them.

Moreover, this ontological explanation of properties is what realists about physics would have to accept, if they took up the ontological issue about their nature at all, for the assumption that there are substances whose aspects are properties is certainly more plausible than any of the alternative theories about substances: the bundle theory, the substratum theory, or the assumption that the whole world is a single substance. And if physical properties are simply the extrinsic essential aspects of the various material substances making up the actual world, naturalists will come to recognize that every possible physical world is made of multiple substances and, hence, that material substances have intrinsic properties in every possible physical world.

This ontological explanation of the necessary connection between physical and phenomenal properties is not a priori, but a posteriori, because it is discovered. As Kripke agues, that means that it must be possible for it to appear that there are possible world in which it does not hold. Kripke showed how such an appearance of contingency is caused in the case of theoretical identification. But it is also possible on this ontological explanation of phenomenal properties to explain how it is possible for it to appear that there are possible worlds in which this ontologically connection does not hold. The illusion of contingency about their relationship comes from failing to recognize that the physical world is constituted by multiple substances and seeing how properties are reducible to them. That is why Chalmers dismisses the belief in intrinsic properties as mere speculation.ⁱⁱⁱ

Relative to a necessary connection established by the identity of properties, the connection established by this ontological argument for its necessity is limited. From the ontological necessity of the connection between intrinsic and extrinsic essential natures of substances it does not follow that there is a ontologically necessary connection between phenomenal and physical properties, not even if phenomenal properties are a kind of intrinsic essential nature of certain substances in our world. Since intrinsic and extrinsic properties characterize different aspects of the essential aspect of substances, it is conceivable that in another possible physical world made of multiple substances, substances would have the same physical properties as ours, and yet have different kinds of intrinsic properties. That is, different worlds could be constituted by different kinds of material substances.

Thus, beings that are physically similar to us in another world constituted by multiple substances might have phenomenal properties with, for example, an inverted spectrum of color qualia. Or they might have more radically different kinds of intrinsic natures. All that is ontologically necessary is that beings like us physically in any possible world made of substances have intrinsic natures of some kind. Though a zombie world is not ontologically possible, an inverted spectrum world is.

Despite this limit to what is necessarily true, however, it is still possible to found a science of consciousness on this ontological explanation of properties, for it implies that, in any possible physical world made of the same kinds of substances as those constituting our world, there are no beings physically and functionally like us that do not also have phenomenal properties like ours. That is enough to found a science of consciousness, because our science is about the actual world. It would be gratuitous to hold that physically indistinguishable material substances in the actual world are different kinds of substances in this sense, especially since they are convertible into one another. Thus, the kinds of phenomenal properties on which one reflects will be the same as those on which other subjects reflect, if the relevant physical properties in the brain are the same.

This ontological explanation of phenomenal properties also explains how they are objects of knowledge. It phenomenal properties are the intrinsic essential nature of some form of matter making up conscious subjects, we can explain why there is something more for Mary to learn about perception when she leaves the black and white neurophysiology laboratory in which she has spent her life and finally sees something red. When she sees something red, the process she has been studying all her life is for the first time embodied in her. Some bit of matter that helps constitute Mary herself has an intrinsic essential nature of a kind whose extrinsic essential nature has been one of the objects of her study. Thus, Mary learns what it is like to be a certain bit of the matter involved in that process.

The property that Mary discovers is, however, an epiphenomenal property on this theory. If phenomenal properties are kinds of intrinsic properties, they are never the efficient cause of anything that happens in the world. The efficient causes are all properties characterizing the extrinsic essential natures of substances, and since they determine what happens, they determine the kinds of bits of matter that exist and, thereby, all the intrinsic properties in the world. But phenomenal properties are not mere "nomological danglers," because intrinsic properties earn their claim to reality for natural science by being necessary aspects of the same substances whose extrinsic essential natures are physical properties.^iv

Finally, this ontological reduction of properties also solves Nagel’s problem about the relationship between the "view from nowhere" and the subjective aspect of experience, or "what it is like." By the "view from nowhere," Nagel means the scientific view of the natural world, and if this ontological interpretation of physics is correct, that is the view of the world as being made up of material substances related spatially as parts of the same world. The problem, as Nagel sees it, is that the scientific view leaves out the subjective aspect of experience.

That problem is solved, however, if the world is made up of substances in the sense assumed here, for the subjective aspect of experience turns out to be the intrinsic aspect of the essential nature of certain elementary material substances making up the subject as an organism in nature. What is left out of the "view from nowhere" is not the existence of phenomenal properties, but only their nature. To know their nature, it is necessary to be the substances making up the subject, because what it is like for the subject is the kind of intrinsic essential nature of the relevant bit of matter.

It is still necessary, however, to explain another aspect of the nature of consciousness, namely, its unity, or why so many different kinds of qualia all appear to the same subject and that same time in perception. That is explained in Change: Unity of Consciousness. But that depends on the implication of spatiomaterialism for science, and before taking up science, we must explain why mathematics is true.