 Recently, there has been some discussion of whether mental concepts are themselves natural kinds. The claim that concepts form a natural kind has been accepted by philosophers of psychology and cognitive psychologists alike (Margolis, 1994, 1995; Fodor, 1998, 2008). Recent literature has argued against that assumption (Griffiths 1997; Machery 2005, 2009; Piccinini and Scott 2006). Griffiths 1997 has argued that individual and familiar concepts such as emotion do not correspond to genuine natural kinds, and should therefore be eliminated from scientific vocabulary. Machery’s (2009, Ch. 3) heterogeneity hypothesis claims furthermore that the class of concepts do not delineate a homogeneous class; the kind ‘concept’. Rather, they divide into several distinct kinds that actually have little in common. Thus, it is a mistake to assume that there are many general properties of concepts, and that a theory of concepts should attempt to describe these. In general, then the notion of concept is not part of an adequate taxonomy of our mental representation.