Psychophysical identity theorists often say that the identifications they anticipate between mental and neural states are essentially like various uncontroversial theoretical identifications: the identification of water with H2O, of light with electromagnetic radiation, and so on. Such theoretical identifications are usually described as pieces of voluntary theorizing, as follows. Theoretical advances make it possible to simplify total science by positing bridge laws identifying some of the entities discussed in one theory with entities discussed in another theory. In the name of parsimony, we posit those bridge laws forthwith. Identifications are made, not found.

In ‘An Argument for the Identity Theory’, I claimed that this was a bad picture of psychophysical identification, since a suitable physiological theory could imply psychophysical identities – not merely make it reasonable to posit them for the sake of parsimony. The implication was as follows:

1. Mental state M = the occupant of causal role R (by definition of M).

2. Neural state N = the occupant of causal role R (by the physiological theory).

3. ∴ Mental state M = neural state N (by transitivity of =).

If the meanings of the names of mental states were really such as to provide the first premise, and if the advance of physiology were such as to provide the second premise, then the conclusion would follow. Physiology and the meanings of words would leave us no choice but to make the psychophysical identification.

In this sequel, I shall uphold the view that psychophysical identifications thus described would be like theoretical identifications, though they would not fit the usual account thereof.