By all odds the most influential mind-body theory in Western civilization has been mind-body dualism. Dualism was first developed as a philosophical theory by some of the Greek philosophers, notably Plato. It was adopted by most of the Christian thinkers of the first few centuries and subsequently came to share Christianity’s dominance of European civilization. In recent times it has been placed somewhat in the shadow, but it continues to be the working viewpoint of large numbers of people and as such demands serious consideration. The version of dualism discussed here is that of Descartes, by far the most influential dualist of modern times, but much of what is said applies to other forms of dualism as well.
Dualism begins by taking quite seriously the fact that human beings have both physical properties and mental properties—as opposed to theories like idealism and behaviorism which collapse the two types of properties into one. Furthermore, dualism gives a clear and straightforward explanation of the existence of the two types of properties: physical properties, it says, are properties of the body, while mental properties are properties of the mind.(Dualists sometimes use the word “soul” instead of “mind”; according to dualism the two words refer to the same thing.) The body is an ordinary physical thing, following the same laws which govern nature in general, but it has no mental properties, no awareness of any kind. Even a simple sensation, such as the pain felt when you scratch your finger, is not a property of the body but rather of the mind as influenced by the body.
The mind, on the other hand, has mental properties but no physical properties—it thinks, perceives and imagines but has no size, shape, mass, or even any spatial location. A person’s mind and her body, then, are about as different from each other as any two things could be—yet they are not disconnected; on the contrary, they are continually interacting with each other. For this reason, the full name of the theory is dualistic interactionism.
Whenever you have some sensory experience, such as stubbing your toe or seeing a traffic light, the sensory information, after being processed in the necessary ways by your brain, is “picked up” by your mind, and this is when you experience the stub or the red light. And whenever you decide to do anything, the decision, which occurs in your mind, is transmitted via the brain to the various muscles which carry out the decision. In fact, one can conceive of the mind-brain relationship as being like that between a computer operator and her computer. The brain is the “central computer” for the body, receiving information from the various sense organs and sending out instructions through the nervous system. The mind, as the operator, “reads out” information from the brain and decides on the course of action to be followed, which is then “typed into” the brain’s computer console and carried out by the appropriate parts of the body. Of course one must not think of the “operator” as physically present within the brain: The mind is completely nonphysical and is not literally located anywhere at all; but it does operate on the brain in such a way as to affect brain function and therefore bodily behavior.
Further points of interest concern the origin and destiny of the mind. Since the mind is seen as a completely nonphysical entity, the mind (or soul) cannot be generated through the biological process of reproduction. Instead, many dualists have held that each human person is endowed with a soul which is directly and individually created by God. And since the soul is nonphysical, there is no reason why its existence should be threatened by the death of its body. So dualism lends itself very readily to a belief in life after death. Christian theologians typically have held that the soul will be re-embodied in a changed, resurrection body; this view is consistent with mind-body dualism but is not required by it.
Many of the advantages of dualism are implicit in what has already been said. It recognizes the existence of both physical and mental properties of human beings, and it explains this fact in a straightforward way. It allows full scope for the scientific study of nature. (Descartes, the originator of dualism in the form here described, was deeply involved in the development of early modern physics.) At the same time, it recognizes the existence of an immaterial or “spiritual” part of man, so that certain aspects of human life (for example, morality and religion) cannot be fully comprehended by scientific study alone. It thus lends itself better than many other views to an affirmation of free will, although it does not require this. On the whole, dualism seems to harmonize quite well with a religious, specifically with a Christian, world view.
The most frequent objection to dualism proceeds by attacking the assertion of mind-body interaction which is central to the theory. Once we have conceived of mind and body as two entirely different types of reality, how is it possible for there to be the intimate and continuous cause-effect interaction required by dualism? How can the mind, totally lacking as it is said to be in any kind of physical reality, nevertheless bring about physical changes within the brain? And of course such changes, were they to occur, would come about in defiance of the laws of physics, which presumably govern physical processes in the brain as well as elsewhere.
There is much less in this objection than is generally thought. To begin with, of course the dualist will affirm that the physical processes within the brain are not completely predictable by physical law; the laws of physics describe the behavior of particles and the like when no nonphysical influence (such as that of the mind) is acting upon them. It may be true that there is some difficulty in imagining just how this influence operates, but what of that? There is no reason to think that reality is limited by what is easy for us to imagine. If it could be proved, from premises which are evidently true, that mind-body interaction is impossible, then the dualist would be in trouble. But no such proof has been given.
But there are other difficulties with dualism that are not so easily brushed aside. For one thing, dualism, in spite of its affirmation of mind-body interaction, is hard pressed to explain the extent of the mind’s dependence on the body as we actually find it. From the standpoint of dualism, it is readily understandable that physical damage to the brain or nervous system should interrupt the flow of sensory information from the body, as well as the mind’s ability to initiate bodily actions. But why should consciousness itself be interrupted by a blow on the head or by the action of drugs? And how does dualism account for the profound changes of personality and character which may result either from physical damage to the brain or from chemical imbalances within it? It seems likely that any adequate account of these phenomena will have to recognize that the mind is dependent upon the brain in a way that is more fundamental than dualism is willing to allow.
Another group of objections to dualism arises from the intriguing yet baffling problem of the souls of animals. Do animals have souls or don’t they? Descartes, impressed with the difficulties which arise if we attribute souls to animals, decided that they do not. This means that animals are purely physical automata, with physiological reactions but no actual feelings, sensations or experiences of any kind. When your dog jumps up to greet you as you come home, or yelps when you step on his tail, it may seem to you that the dog really is feeling joy or pain, but nothing of the sort is true. What you observe is entirely the result of automatic physiological reactions within the dog’s body. But clearly this is absurd! In order to avoid the absurdity, the dualist must affirm that animals do indeed have souls—not, to be sure, souls just like those of human beings, but souls all the same.
This, however, opens the way to further problems. Where do all these souls come from? It may seem not unreasonable that God should individually create a soul for each human being, but do we want to say this also about rabbits, toads and termites? And what happens to the souls of animals when the organisms perish? Are they also, like human souls, naturally immortal? If not, why not? And what of those organisms, like starfish, which can be cut into parts with each part subsequently developing into a complete organism? Before such a division there is one starfish and therefore one soul. Afterward, there are two starfish and, presumably, two souls. Where did the second soul come from? For the dualist, no good answer seems available.
These are not, I think, merely frivolous objections. Rather they point to a serious difficulty with dualism. The dualistic view draws, and is intended to draw, a very strong contrast between man as a spiritual being and “mere” physical nature. But the gulf thus fixed between matter and spirit means that the entire living creation other than man is left unprovided for—and it may be that this can only be rectified by abandoning or fundamentally modifying dualism.
Man is a wholly material being: This is the central thesis of materialism. Materialism, like dualism, comes in several varieties; currently the most popular variety is the mind-body identity theory. This theory does not deny that humans have both mental and physical attributes but says that both are attributes of the same thing—namely, the living human organism. A human being is his body, and the body is the person.
Another way to look at materialism is this: In discussing dualism, we said that the brain can be likened to the central control computer of the body, and the mind to its operator. But does every computer need an operator? We are familiar in fiction, if not yet in everyday life, with computers that “set up on their own” and operate independently of human control. Think, for example, of Hal, the psychotic computer in 2001: A Space Odyssey. For the materialist, the human brain is a self-operating computer. The thoughts and other mental properties of humans are simply properties of complex, highly organized physical systems—namely, human brains. Whether manmade computers will ever be able really to think—as opposed to simulating thought processes—may be an open question: Does consciousness exist only in biological systems, such as humans and other animals, or would it also arise in a properly constructed assemblage of microchips and integrated circuits? But whether or not ordinary computers can think, the “meat computer” which each of us carries around in his or her head can and does.
One merit of materialism is its simplicity. Instead of explaining human life in terms of an immaterial mind whose nature is obscure and whose very existence is controversial, it limits itself to the familiar material organism whose existence is indisputable and whose characteristics are readily amenable to scientific study. In fact, the desire to have a thoroughly scientific understanding of human nature is one of the strongest motivations for adopting materialism. The dualist’s immaterial mind is seen, rightly, as a barrier that would prevent us from fully integrating human life and activity into a unified scientific perspective in which the laws of physical science are the fundamental operating principles of the universe. The phenomenal success of science to date makes such a unified perspective a reasonable hope, and materialism is the mind-body theory which best accords with this hope. It should also be pointed out that materialism is completely free from the objections noted against dualism: All the objections result from the gulf between mind and matter, and materialism never allows that gulf to open in the first place.
But on further consideration the advantages of materialism may seem less clear. Materialism is simpler in that it has one basic type of substance instead of two, but there remains the duality of physical and mental properties. Materialists have tried in various ways to eliminate distinctively mental properties, but none of these attempts seems to be very successful. And it is not clear that the elimination of mental substance represents a real gain in simplicity if we must then ascribe to the physical substance properties quite unlike those it is known to have in all other contexts. Explaining how consciousness, feeling and other mental attributes arise from combinations of physical particles may not be a great deal easier than explaining the origin and nature of the immaterial mind.
Materialism’s claim to produce a complete scientific understanding of human nature is also open to question. It has to be emphasized that such an understanding does not now exist; scientific progress has indeed been remarkable and must not be ignored, but it is by no means clear that it is leading in the direction of a single “unified science” in which all human thought and behavior is explained in physicalistic terms. And the tenability of this objective is called into question by some of the arguments presented in the chapter “Freedom and Necessity”: It was argued there that physical determinism (with or without a random element due to quantum indeterminacy) entails not only the denial of moral responsibility but also the denial of human rationality—a consequence which is clearly unacceptable to the materialist, who relies heavily on scientific knowledge. Unless this argument can be met, the materialist will be forced to admit that there is after all something about human beings which cannot be captured in explanations couched in terms of the physical sciences.
It would seem that the disagreement between the dualist and the materialist, like that between the determinist and the libertarian, is connected with deep-rooted motivations which do not easily yield to philosophical argument. Persons who are deeply committed to a scientific world view may acknowledge the difficulties of materialism but will tend to cling to it anyway as they work for solutions of the difficulties. Similarly, persons committed to a religious and humanistic world view will continue to affirm at least some of the tenets of dualism in spite of their recognition of the difficulties of that viewpoint. One’s views on the mind-body problem will tend to be strongly influenced by one’s general perspective on the way things are. But the converse is true as well: surely one of the acid tests for a world view is whether it is able to provide a consistent, coherent and acceptable account of the nature of humanity. Both materialism and dualism seem to leave something to be desired in this regard.
There is no logical limit to the number of sections in this chapter. Unlike the free-will controversy, the mind-body problem does not divide up into a limited number of clear-cut, mutually exclusive alternatives. But of the many additional views which could be considered, only one more will be pursued here. Is it possible, we may ask, to develop a mind-body theory which will combine some of the advantages of both dualism and materialism while avoiding many of the disadvantages of each?
The difficulties of dualism arise from the gulf which is created between mind and matter when we assert that mind is a separate element added to the physical organism “from outside.” Materialism, on the other hand, reduces man entirely to a physical organism functioning according to natural laws, with the result that crucial aspects of human existence—morality, rationality, aesthetic experience, religion—inevitably are either slighted or denied altogether. What seems to be needed, then, is a view in which the human mind or soul is grounded in the human biological organism without being reduced to that organism.
Suppose we say, first of all, that the human mind or soul is produced by the human brain and is not a separate element added to the brain from outside. This of course agrees with materialism, but the difference from materialism is apparent when we add that while the mind is produced by the brain and dependent upon it, nevertheless the mind is distinct from the brain and its activities are not completely explainable in terms of brain function. This statement, in contrast with the first one given, indicates the element of truth in dualism which is denied by materialism.