Mind: A Brief Introduction (John R. Searle) / quote 57 of 138
Type-type versus token-token identity
The type-type identity theory says “Every type of mental state is identical with some type of physical state.” By their own lights this is a bit sloppy, because the identity in question is between actual concrete tokens and not abstract universal types. What they meant is: for every mental-state type there is some brain-state type such that every token of the mental type is a token of the brain type. The token identity theorists simply said: for every token of a certain type of mental state, there is some token of some type of physical state or other with which that mental state token is identical. They, in short, did not require, for example, that all token pains had to exemplify exactly the same type of brain state. They might be tokens of different types of brain states even though they were all tokens of the same mental type, pain. For that reason they were called “token-token” identity theorists as opposed to “type-type” identity theorists. Token-token identity seems much more plausible than type-type identity theory.