 In a series of influential papers, David Lewis (1970, 1972) defended a particular approach to the semantics of theoretical terms. He applied that same approach to everyday psychological vocabulary (e.g., “belief” and “desire”), and thereby obtained a functionalist theory of mental states. Accordingly, when thinking about the semantics of everyday psychological vocabulary, Lewis treats the conjunction of commonsense platitudes about mental states as a term-introducing theory. It is possible to identify folk psychology with that conjunction. Alternatively, we could think of folk psychology as a systematization of a set of platitudes. Either way, Lewis’s approach lends credence to the idea that FP is structured in the same way as other kinds of theory.