In addition to metaphysical issues dealing with the immortality of the soul, God's existence, freedom of the will, and more is the question, “How do mind and body interact?” The answer one gives depends upon on what theory of reality one accepts. There are at least two prevailing theories.

What exists? The question is clear, but several compelling answers have been offered. If an individual believes that only matter exists, that person is a materialist and accepts a theory known as materialism. On the other hand, if a person believes that only minds and mental states and ideas exist, then she is an idealist and she embraces the theory known as idealism. Finally, if a person accepts that matter and mental entities exist, then he is a dualist and accepts the doctrine of dualism.

Dualistic Interractionism

The name dualism comes from the theory that there are two kinds of reality — material and mental. Take consciousness as an example of one kind of reality. Right now you may be thinking of the page before you, but that consciousness is not solid or material. In addition, consciousness is not extended in three-dimensional space. In fact, it does not occupy space at all. In contrast to physical bodies, mental entities have no width, breadth, depth, shape, height, color, velocity, or temperature. Can you imagine someone calling a belief blue or describing an emotion as being two inches by three inches in dimension? No, nor can you imagine a thought as being porous or a notion as being hard.

Further, your consciousness is only observable by yourself. It cannot be measured, and it isn't clear how consciousness — possessing all nonphysical properties — could cause anything. Only a person can think his own thoughts, feel his own emotions, and suffer his own pain.

As distinct as mental and physical entities are, your ordinary thinking tells you that they interact in some fashion. If a person touches a hot stove, the burned skin communicates some message to the central nervous system. This pain message would seem to have no size or shape or be public in any way. So the stove is very public, while your sustained pain is private.

Materialism

Thomas Hobbes, one of the early empiricists, said that nothing but matter in motion exists. According to one brand of materialism, even the mind — be it spirit, soul, or consciousness — is matter in motion. Further, materialism is a monistic theory, since it claims that only one kind of substance exists. Thus, matter and the universe do not in any way possess the nonphysical characteristics of mind, such as purpose, intention, awareness, intelligence, wishing, and so on.

On this materialistic view, then, there is no interaction to explain. Dualism must be false since it believes in two kinds of reality. If you trip and turn your ankle, the pain state that you feel is purely physical. It is no more nonphysical or private than the hurt ankle itself. One kind of materialism is logical behaviorism.

The English philosopher Gilbert Ryle (1900–76) held logical behaviorism to be true. In fact, Ryle famously criticized Rene Descartes's dualism as the theory that there is a “ghost in the machine.” Recall that Descartes was a dualist who believed that mind and body were two fundamentally different kinds of substances. He said that minds are private, immeasurable, and so forth, while bodies are extended, public, and so on. Despite their differences, they interact. But Ryle voiced an objection to Descartes's view.

Ryle said the opinion that the mind is a substance rests on a category mistake. Mind is surely a noun, Ryle conceded. But it refers to no known object. So Descartes confuses the discourse about bodies with discourse about minds. Mental states like belief, desire, and intention are not internal causes of behavior, as Descartes said. Rather, Ryle thought they were just dispositions to behave in certain ways. So when you talk about mental states like thoughts, beliefs, and desires, you are really referring to overt bodily behavior. When you say “John is thinking deeply,” you are not implying that this activity of thinking is taking place in some private place that only he knows of. You are asserting that you can see it in his behavior. Hence the doctrine is given the name logical behaviorism.