Mind, identity theory of
Authors: Hill Christopher S. 
Published: February 2023

The doctrine that mental states are identical with physical states has long played a prominent role in theories of mind and consciousness. The most widely discussed version of the doctrine, known as ‘type physicalism’, is concerned with qualitative states of consciousness – that is, with states like pain, itching, the way oranges taste, the way onions smell, and the way scarlet objects look when seen in bright sunlight. Type physicalism asserts that all such states are identical with neural states of the brain. In other words, according to type physicalism, qualitative states like pain can be reduced to neural states without any residue: there is nothing to pain over and above a certain neural state.

Historically, the main reason for accepting type physicalism has been that qualitative states appear to be strongly correlated with neural states. For example, it appears that pain is strongly correlated with a certain pattern of activity that is spread across several parts of the brain, in the sense that pain occurs when and only when that pattern of activity occurs. If we assume that qualitative states are identical with neural states, we can explain why such correlations exist in a natural and appealing way. (Compare: if we assume that Superman is identical with Clark Kent, we can give a natural and appealing explanation of why appearances of Superman are strongly correlated with appearances of Clark.) Moreover, without the assumption of identity, it seems that we have to accept the correlations as brute facts. So type physicalism enjoys a prima facie appeal. Advocates of type physicalism have also given other arguments in support of their view.

But philosophical questions about the nature of the mind are never simple. In addition to being supported by prima facie plausible arguments, type physicalism is called into question by a number of arguments that dualists have devised to support their opposing position. To illustrate, dualists have pointed out that it is possible in principle to know everything about a certain brain process without knowing anything about the qualitative state that is correlated with it. (A neuroscientist could know everything about the process that is normally correlated with a certain type of colour experience, but be ignorant of the given type of experience because the scientist was colour blind.) It can seem that this would be impossible if qualitative states were reducible to brain states without remainder. Type physicalism is also challenged by a line of thought known as the multiple realization argument, which in effect denies that qualitative states are correlated with specific neural states. According to this argument, qualitative states like ours can be found in other creatures whose brains are very different than ours in point of structure or material composition. If that were so, the relation between qualitative states and specific brain states would be one–many rather than one–one. Type physicalism implies that the relation is one–one.

Thus there are arguments on both sides of the question of whether type physicalism should be accepted. The jury is still out on this important issue.