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.)