Mind, identity theory of

1. Origin of the identity theory

The identity theory of mind holds that each and every mental state is identical with some state in the brain. My desire for coffee, my feeling happy, and my believing that the dog is about to bite are all states of my brain. The view is not that mental states and brain states are correlated but that they are literally one and the same. Despite its name, the identity theory of mind is strictly speaking not a view about the mind as such, but about mental phenomena. However, most protagonists of the identity theory, and most contemporary philosophers of mind if it comes to that, hold some version of the view that the mind is a construction out of its states in somewhat the way that an army is a construction out of its soldiers.

The identity theory of mind arose out of dissatisfaction with dualism, and with behaviourism as an attempt to avoid dualism. According to dualism, mental states are quite distinct from any material states, including brain states (see Dualism). The most famous challenge to dualism is to give a satisfactory account of the causal interactions between mental states and material states, and most especially to give a satisfactory account of causation from mental states to bodily occurrences. We believe that sometimes my desire for ice cream causes my arm to move in such a way that an ice cream is in my mouth, that my pangs of hunger cause me to tighten my belt, and so on. But how do states allegedly ‘outside’ the material world cause material goings on like arm movements? How do they do this in such a way as to avoid violating the various conservation laws in physics? And how do they do this in such a way as not to conflict with what the physical sciences, and especially neuroscience, tell us about how bodily movements are caused?

The last question is particularly pressing. The success of the physical sciences in explaining phenomena in their own, physical, terms has been striking. We now know that lightning is not caused by Thor’s actions, that epilepsy is not caused by demonic possession, and that plants do not grow because they contain a vital essence but because their cells divide (see Vitalism). It is hard to believe that bodily behaviour is unique in resisting in-principle explanation in purely physical terms. The dualist can respond to this challenge by denying the common-sense view that mental states sometimes cause bodily behaviour, a position known as epiphenomenalism. This position holds that although physical states on occasion cause mental states, mental states themselves never cause anything, being mere epiphenomena of the brain states that, along with the appropriate material surroundings, are the true causes of the behaviour we associate with mental states. Apart from flying in the face of common-sense, this position makes it hard to see why the mind evolved (see Epiphenomenalism).

Behaviourism treats mental states in terms of behaviour and dispositions to behaviour (see Dispositions). Its inspiration comes from facts such as these: that those creatures we credit with mental states are precisely those manifesting sophisticated behaviour and possessing sophisticated behavioural capacities; that psychology became a serious science when psychologists started to investigate the mind via the investigation of behavioural capacities; and that there are conceptual links between mental states and behaviour – it is, for instance, part of the concept of an intention that having an intention goes along with behaving in a way that tends to fulfil it, and it is part of the concept of intelligence that the intelligent are better problem solvers than the unintelligent (see Behaviourism, analytic).

Behaviourists delight in pointing out that it is hard to see how dualists could explain the last two points. Why should investigation of an immaterial realm be especially aided by looking at behaviour? How could the way things are in some immaterial realm be conceptually linked to brute behavioural facts? However, for our purposes here the crucial point is that behaviourists, like dualists, have trouble over the causation of behaviour.

The common-sense position is that mental states are causally responsible for behaviour – the itch causes the scratching – and responsible for behavioural dispositions and capacities – people’s intelligence is responsible for their capacity to solve hard problems. But then mental states are not the same as behaviour and behavioural dispositions and capacities, being rather their underlying causes. And so they are, says the identity theorist. Mental states are those brain states that all the scientific evidence points to as being causally responsible for the behaviour, and behavioural dispositions and capacities, distinctive of those creatures we credit with a mental life.