Why a scientific realist cannot be a functionalist

According to functionalism, mental state types consist solely in relations to inputs, outputs, and other mental states. I argue that two central claims of a prominent and plausible type of scientific realism conflict with the functionalist position. These claims are that natural kinds in a mature science are not reducible to natural kinds in any other, and that all dispositional features of natural kinds can be explained at the type-level. These claims, when applied to psychology, have the consequence that at least some mental state types consist not merely in relations to inputs, outputs, and other mental states, but also in nonrelational properties that play a role in explaining functional relations. Consequently, a scientific realist of the sort I describe must reject functionalism.