causal relevance, a three-place relation between event types, and circumstances, and argue for a logical independence condition on properties standing in the causal relevance relation relative to circumstances. In section 3, I apply these results to show that functionally defined states are not causally relevant to the output or state transitions in terms of which they are defined. In section 4, I extend this result to what that output in turn causes and to intervening mechanisms. In section 5, I examine (...) the implications of this result for functional theories of mental states. In section 6, I distinguish between functional descriptions of properties and functional definitions of properties, and argue the former present no obstacle to mental states being causally relevant to behavior, but that this is so because they do not treat mental states as functional states. In section 7, I examine the nature of explanations that appeal to functional states or properties. section 8 identifies some difficulties that arise in thinking about specifically conscious mental states as functional states. section 9 is a brief conclusion. (shrink)