Troubles with functionalism

One characterization of functionalism that is probably vague enough to be acceptable to most functionalists is: each type of mental state is a state consisting of a disposition to act in certain ways and to have certain mental states, given certain sensory inputs and certain mental states. So put, functionalism can be seen as a new incarnation of behaviorism. Behaviorism identifies mental states with dispositions to act in certain ways in certain input situations. But as critics have pointed out (Chisholm, 1957; Geach, 1957; Putnam, 1963), desire for goal G cannot be identified with, say, the disposition to do A in input circumstances in which A leads to G, since, after all, the agent might not know that A leads to G and thus might not be disposed to do A. Functionalism replaces behaviorism's "sensory inputs ' with "sensory inputs and mental states '; and functionalism replaces behaviorism's "dispositions to act" with "dispositions to act and have certain mental states." Functionalists want to individuate mental states causally, and since mental states have mental causes and effects as well as sensory causes and behavioral effects, Functionalists individuate mental states partly in terms of causal relations to other mental states. One consequence of this difference between functionalism and behaviorism is that there are possible organisms that according to behaviorism, have mental states but, according to functionalism, do not have mental states.