Taxonomy, ontology, and natural kinds

When we ask what natural kinds are, there are two different things we might have in mind. The first, which I'll call the taxonomy question, is what distinguishes a category which is a natural kind from an arbitrary class. The second, which I'll call the ontology question, is what manner of stuff there is that realizes the category. Many philosophers have systematically conflated the two questions. The confusion is exhibited both by essentialists and by philosophers who pose their accounts in terms of similarity. It also leads to misreading philosophers who do make the distinction. Distinguishing the questions allows for a more subtle understanding of both natural kinds and their underlying metaphysics.