 Within this broad characterization of functionalism, however, a number of distinctions can be made. One of particular importance is the distinction between theories in which the functional characterizations of mental states purport to provide analyses of the meanings of our mental state terms (or otherwise restrict themselves to a priori information), and theories that permit functional characterizations of mental states to appeal to information deriving from scientific experimentation (or speculation). (See Shoemaker 1984c, and Rey 1997, for further discussion and more fine-grained distinctions.) There are other important differences among functionalist theories as well. These (sometimes orthogonal) differences, and the motivations for them, can best be appreciated by examining the origins of functionalism and tracing its evolution in response both to explicit criticisms of the thesis and changing views about the nature of psychological explanation.