In the first part of this article, I argue that Chrisopher Megone's (1998) natural-kind interpretation of Aristotle's argument that "the function of a human being is reason" does not resolve major puzzles about the argument, specifically the puzzles of why a human being has a function and why reason is that function. I attempt to resolve these puzzles by supplementing the natural-kind account with the doctrine that reason is the master regulatory natural function by which individuals enter into social life. In the second half, I critique Megone's value account of function and argue for a non-evaluative, causal-explanatory account of function and teleological explanation. Using the "black box essentialist" account of scientific concepts (Wakefield 1997), I provide a new analysis of the logical structure of the concept of a natural function that explains why values are intimately associated with judgments of function and dysfunction and yet why function and dysfunction are not intrinsically value concepts. I conclude by examining Megone's attempt to construct a quasi-Aristotelian approach to the concept of mental disorder. I argue that his value analysis of function does not offer a viable foundation for an adequate analysis of mental disorder that explains our intuitive judgments of disorder and non-disorder.