  A fourth sort of argument suggests that a definition of art stating individually necessary and jointly sufficient conditions for a thing to be an artwork, is likely to be discoverable only if cognitive science makes it plausible to think that humans categorize things in terms of necessary and sufficient conditions. But, the argument continues, cognitive science actually supports the view that the structure of concepts mirrors the way humans categorize things – which is with respect to their similarity to prototypes (or exemplars), and not in terms of necessary and sufficient conditions. So the quest for a definition of art that states individually necessary and jointly sufficient conditions is misguided and not likely to succeed (Dean 2003). Against this it has been urged that psychological theories of concepts like the prototype theory and its relatives can provide at best an account of how people in fact classify things, but not an account of correct classifications of extra-psychological phenomena, and that, even if relevant, prototype theory and other psychological theories of concepts are at present too controversial to draw substantive philosophical morals from (Rey 1983; Adajian 2005).