   The question of why we should seek a criterion of the mental at all has been made pressing by some recent remarks of the linguist Noam Chomsky (2000, 75, 106), according to whom methodological naturalism mandates that we use terms like ‘mind’ and ‘mental’ on a par with terms like ‘chemical’, ‘optical’ or ‘electrical.’ Since we do not seek to determine the true criterion of the electrical or the mark of the chemical, by naturalistic parity of reasoning, Chomsky argues, we should no more seek a criterion of the mental. Whether we need criteria respectively for the chemical and for the optical, it is a genuine issue whether the English word ‘mental’ can justifiably apply to things as diverse as e.g., a pain and the belief that 5 is a prime number. As Richard Rorty (1979, 22) has put it, “the attempt to hitch pains and beliefs together seems ad hoc—they don’t seem to have anything in common except our refusal to call them ‘physical’”.  His conclusion is that the word ‘mental’ expresses no single property, let alone a ‘natural kind.’ On Rorty’s irrealist view, the word ‘mental’ is just part of an academic language-game with no scientific explanatory import.