In his theory of categories, Aristotle introduces a distinction between two types of universals, i.e., kinds and attributes. While attributes determine how their subjects are, kinds determine what something is: kinds represent unified ways of being which account for the existence and identity of particular objects. Since its introduction into the philosophical discussion, the concept of a kind has attracted criticism. The most important objection argues that no separate category of kinds is needed because all kinds can be reduced to (...) conjunctions of ordinary attributes. The present paper explores one possible response to this reductionist challenge on behalf of the Aristotelian—one which takes issue with the view that conjunctive properties license a reduction of kinds to attributes. The aim of this exploration is not only to defend an Aristotelian doctrine of kinds—i.e., to convey a better understanding of why we actually need an irreducible category of kinds—but also to define the Aristotelian position more precisely; i.e., to shed more light on the concept of kinds and the very the idea of kindhood. (shrink)