The Identity Thesis and Neuropsychology

Abstract

The psycho-neural identity thesis (IT) is roughly the assertion that psychological states are identical with certain physical states of the nervous system. There are, of course, many more precise formulations of IT. In particular, there are two versions of IT depending on whether we take 'states' to refer to types or tokens. Token materialism is a weaker claim than type materialism, assuming that a token of type x may be identical with a token of type y even though x and y are different types. Thus, type materialism entails token materialism, but not conversely. In this paper, IT shall be construed as asserting that psychological states are type-identical with certain neural states. One well-known argument for IT is based on an appeal to the theoretical nature of the claim. Psycho-neural identities are thought of as reduction-functions or bridge-laws necessary for the micro-reduction of psychological theories to neurological ones. Thus, the justification for IT is of the same kind as the justification for such scientific identities as temperature is molecular mean kinetic energy. While there is much to be said about this way of understanding IT, we may simply notice that it presupposes that psycho-neural identities are type-to-type identities.' In a recent paper ([1]), N. J. Block and J. A. Fodor argue that type materialism is very likely false. While they advance three distinct arguments in behalf of this claim, we shall consider only one of these in detail. This argument is based on an appeal to the Lashleyan doctrine of neurological equipotentiality, which holds that any of a wide variety of psychological states can be served by any of a wide variety of neural structures. Thus, a given psychological state may in fact be associated