Functionalism is perhaps the most prominent theory of mind today. The central thesis of functionalism is that the standard mental relations (or properties or states) are uniquely determined by their causal roles in functioning organisms. That is, the principles of psychology specify the characteristic way that (behavioral or physiological) input, the standard mental relations such as belief and desire, and (behavioral or physiological) output are causally arranged; and the central idea of functionalism is that, e.g., belief's characteristic causal role can be fulfilled by exactly one relation-namely, belief itself. Clearly, then, the most direct way to refute functionalism would be to show that there are relations that demonstrably differ from the standard mental relations and that, nevertheless, could fulfill the same causal role as those mental relations.