  Predicate dualism is the theory that psychological or mentalistic predicates are (a) essential for a full description of the world and (b) are not reducible to physicalistic predicates. For a mental predicate to be reducible, there would be bridging laws connecting types of psychological states to types of physical ones in such a way that the use of the mental predicate carried no information that could not be expressed without it. An example of what we believe to be a true type reduction outside psychology is the case of water, where water is always H2O: something is water if and only if it is H2O. If one were to replace the word ‘water’ by ‘H2O’, it is plausible to say that one could convey all the same information. But the terms in many of the special sciences (that is, any science except physics itself) are not reducible in this way. Not every hurricane or every infectious disease, let alone every devaluation of the currency or every coup d’etat has the same constitutive structure. These states are defined more by what they do than by their composition or structure. Their names are classified as functional terms rather than natural kind terms. It goes with this that such kinds of state are multiply realizable; that is, they may be constituted by different kinds of physical structures under different circumstances. Because of this, unlike in the case of water and H2O, one could not replace these terms by some more basic physical description and still convey the same information. There is no particular description, using the language of physics or chemistry, that would do the work of the word ‘hurricane’, in the way that ‘H2O’ would do the work of ‘water’. It is widely agreed that many, if not all, psychological states are similarly irreducible, and so psychological predicates are not reducible to physical descriptions and one has predicate dualism. (The classic source for irreducibility in the special sciences in general is Fodor (1974), and for irreducibility in the philosophy of mind, Davidson (1971).)