interested in diVerences amongst mental states which reXect diVerences in
their causal powers. And where the causal powers of two token mental
states are identical, we shall therefore want to regard them as being of the
very same type. (We understand ‘causal powers’ here to include the poten-
tialcausesas well as the potentialeVectsof the state in question. We also
assume that psychology is not – unlike geology, for example – an historical
science; that is, it does not individuate the kinds with which it deals in
terms of their actual causal history. See chapter 7 for further discussion.)
Now the notion of a causal power is counterfactual-involving. To talk of
the causal powers of state S is to talk, not just of what S actually causes, but
also of what Swouldcause (or be caused by) in various hypothetical and
counterfactual circumstances. Seen in this light, it is obvious that the
causal powers of the states of the two twins are the same. For if Peterewere
to be on Twin Earth, then he would behave exactly as Petertwdoes (even
under an intentional description); and if Petertwwere on Earth, he would
behave exactly as Peteredoes. Similarly, if Mary were to be sitting where
Joan is, then she would be behaving as Joan does, and vice versa. In fact, it
is the causal powers of mental states which supervene on (non-relationally
described) brain states. Then if we insist that mental state types should
supervene on causal powers, it will follow that content is narrow. For
otherwise there would be (relational) diVerences between mental states
(widely individuated) which would not reXect diVerences in their causal
powers.
This looks as if it might become a powerful argument in support of
narrow content. But why should one accept that mental states are only
distinct where their causal powers are? This will follow if we think that
mental states are, basically, the theoretical posits of an explanatory proto-
science (that is, if we accept some or other version of the ‘theory-theory’ of
mental states, as we argued in chapter 4 we should). For science, in general,
types entities and states by their causal powers, taking no interest in
diVerences between states not reXected in diVerences in their causal
powers. (At least, this is true of sciences which are a-historical.)
No doubt folk-psychology may bemore thana proto-science, and may
also take an interest in (merely relational) diVerences amongst mental
states not reXected in their causal potential. (Indeed, we will argue as
much in section 5 below.) But to the extent that folk psychology isat least
attempting to do the work of a scientiWc theory – typing states by their
causal powers, and explaining events as caused by the states so distin-
guished – to that extent we have reason to categorise thoughts narrowly,
in terms of a non-relationally individuated notion of content. Moreover,
if we are to extract from folk psychology a notion of contentWtto
subserve a content-based scientiWc psychology, then it would appear,
Explanation and causation 149