 A helpful way to think of the Ramsey sentence of a psychological theory is to regard it as defining a system's mental states “all at once” as states that interact with stimulations in various ways to produce behavior (See Lewis 1972; also see Field 1980 for a more technical elaboration of Lewis's method, and an account of some crucial differences between this kind of characterization and the one Lewis initially proposed.) This makes it clear that, in the classic formulations of functional theories, mental states are intended to be characterized in terms of their relations to stimulations, behavior, and all the other states that may be permissibly invoked by the theory in question, and thus certain functional theories may have more resources for individuating mental states than suggested by the crude definitions used as examples. The next three sections will discuss the potential of various sorts of functionalist theory for giving adequate characterizations of experiential and intentional states—and also for specifying the inputs and outputs of the system. 