The present paper constitutes a development of the position that illness, whether bodily or mental, should be analyzed as an incapacitating failure of bodily or mental capacities, respectively, to realize their functions. The paper undertakes this development by responding to two critics. It addresses first Szasz's continued claims that (1) physical illness is the paradigm concept of illness and (2) a philosophical analysis of mental illness does not shed any light on the social and legal role of the idea. Then, in reply to Wakefield, the aim is to defend the account as an interpretation of Aristotle and to argue that this Aristotelian view of mental illness is preferable to one that rests on a supposed value free account of human function. More generally the discussion points to the fact that both Wakefield and Szasz rely on a number of metaphysical assumptions about the supposedly empirical nature of medical diagnosis, about the relation between facts and values, and about mind and body (among others), which are open to challenge. In particular the paper indicates an Aristotelian approach to the fusion, in the natural world, of so-called facts and values, and the relevance of this fusion to the analysis of the concept of illness. This suggests the debate over distinct conceptions of that concept must both illumine and be illuminated by these deeper metaphysical questions.