 Given this history, it is helpful to think of functionalist theories as belonging to one of three major strains — “machine functionalism”, “psychofunctionalism” and “analytic functionalism” — and to see them as emerging, respectively, from early AI theories, empirical behaviorism, and logical behaviorism. It's important to recognize, however, that there is at least some overlap in the bloodlines of these different strains of functionalism, and also that there are functionalist theories, both earlier and more recent, that fall somewhere in between. For example, Wilfrid Sellars's (1956) account of mental states as “theoretical entities” is widely regarded as an important early version of functionalism, but it takes the proper characterization of thoughts and experiences to depend partially on their role in providing a scientific explanation of behavior, and partly on what he calls the “logic”, or the a priori interrelations, of the relevant concepts. Still, it is instructive to give separate treatment to the three major strains of the doctrine, as long as these caveats are kept in mind.