mind–body problem The relation between mind and body lies at the heart of much philosophy.

Action, which (it seems) originates in the mind and results in bodily movements, raises many questions, most notably that of free will. It may be that every physical event is the inevitable result of past circumstances together with the laws of nature (determinism). If so, how could it ever be up to us, who have no control over the past or the laws of nature, what we do? As it happens, the behaviour of subatomic particles appears to obey only statistical laws, analogous to those governing games of chance; and so most physicists nowadays reject determinism. But it is no more evident how we could affect the paths of elementary particles via our intentions than it is how we could affect the distant past or laws of nature. A perennial philosophical research project is to try to explain what it is to have a choice about what one does that is compatible with determinism and other physical theories.

Another traditional problem about mind and body is what it takes for us to persist from one time to another: what makes some past or future being you, rather than someone else? (What would it take, say, for you to continue to exist after your biological death?) One answer is that you are that past or future being that has your mind — roughly speaking, the one whose life you can remember, or who can remember your life. A rival view is that our identity through time consists in some physical relation not involving the mind: you are that past or future being that has your body, or that is the same biological organism as you are. If your mind were destroyed in a way that left the rest of you relatively intact — if you were to lapse into a persistent vegetative state, for example — then according to the bodily approach the resulting being would be you, whereas on the psychological approach you would cease to exist.

But the paradigmatic ‘mind–body problem’ has to do with the nature of the mind itself. In particular, are mental phenomena in any sense physical? Do human beings think and feel solely by virtue of the workings of their brains and other organs? If so, how do thought and consciousness arise out of that biochemical machinery?

Mental phenomena are thought to have features that resist explanation in physical terms. For example, we know about our own sensations in a way that no one else possibly could, whereas physical phenomena are always publicly accessible, at least in principle. Some mental states seem to have an essentially subjective character that no physical state could have. Thought is intrinsically purposeful in a way that physical phenomena, which are fundamentally mechanical, could not be. And so on.

One response to these convictions is to deny that any physical body could be the subject of thought: mentality is an entirely non-physical phenomenon. You and I, since we do think, are therefore immaterial things (or at least have an immaterial part). On this view, associated with the French philosopher René Descartes (1596–1650), a perfect physical duplicate of you, with a brain identical to yours, might fail to be conscious, or even to have any psychological features at all, for such features reside in an immaterial thing that even the most sophisticated physical object might lack.

Cartesians have found it difficult to explain how the mind relates to the physical world. One problem is how something with no physical properties could affect, or be affected by, physical objects, as appears to occur in action and perception. Another is that having psychological features seems to admit of degrees, whereas something either definitely has or definitely lacks an immaterial part. Thus, there seems to be no principled way of saying when, in the development of a human being from an embryo, the organism comes to be associated with a Cartesian soul, or what animals besides human beings have one. And Cartesianism makes it surprising that damage to the brain should cause unconsciousness, rather than merely preventing one from moving or perceiving anything. Quite aside from these difficulties, Cartesians are as much in the dark about how non-physical things manage to think and feel as their opponents are about how physical things can do so.

Some anti-Cartesians deny that the mind exists at all. Logical behaviourists claim that when we use the language of psychology we are merely talking about how people are disposed to act, and not about any internal states. Eliminative materialists argue that the ‘folk theory’ of belief, sensation, consciousness, and other familiar mentalistic concepts is fundamentally mistaken and will one day be replaced by a science of behaviour that bears no more resemblance to traditional psychology than modern chemistry bears to alchemy.

Most of those working on the mind–body problem today believe that the mind is in some sense physical; but there is much disagreement about what this comes to. One view (the so-called mind–brain identity theory) is that psychological states are simply special physical states: for example, pain was once thought to be the firing of C-fibres. A more popular strategy is to identify mental features with functional properties: what makes something a pain is its characteristic causes (injury) and effects (groaning, worry, actions one believes will alleviate it) — much as what makes something a carburettor is the role it plays in the functioning of an internal combustion engine. Thus, pain might be a different state, as it were, in different creatures.

Others argue that, although mental phenomena are produced by the brain (and not by anything immaterial), they cannot be explained in physical terms, or at least not in the terms of current physical science. One intriguing suggestion is that the human mind is simply incapable of understanding itself, though there is nothing intrinsically mysterious about it.

Eric T. Olson