TWO EPIPHENOMENALIST THREATS

If token physicalism is true, then tokens of an individual's propositional attitudes are brain-state tokens of the individual. If so, then the weak causal thesis of intentional realism is vindicated. Tokens of an individual's propositional attitudes are secured a causal role: if tokens of propositional attitudes are brain-state tokens and if brain-state tokens are causes, then so are tokens of propositional attitudes. However, token physicalism does not ipso facto vindicate the strong causal thesis according to which the semantic properties of propositional attitudes are causally efficacious.

Type physicalism might have vindicated the strong causal thesis, for type physicalists entertained the possibility that the semantic property of an individual's propositional attitude be identified with a physical property of the individual's brain (or central nervous system). On the view of most type physicalists, the identity in question was supposed to be a “synthetic” (or empirical) identity – on the model of the identity between water and H2O. The purported identity between a semantic property of an individual's propositional attitude and some physical property of his or her brain would have secured the strong causal thesis. Assuming the truth of the identity between the semantic property of an individual's propositional attitude and some physical property of his or her brain, then whatever causal efficacy the latter enjoys, the former must enjoy too. Type physicalism, however, is widely believed to have foundered on the phenomenon of multiple realizability. Putnam (1960) noticed that a given computer with some definite computational (or logical) property can be implemented (or realized) by a variety of physical devices with different physical properties.