 Since kinds are revealed by science, a science can revise which kinds it holds exist: phlogisticated air was regarded as a kind until after Lavoisier’s chemical revolution. A science can even question a whole category of kinds, or sort and classify kinds differently as the science changes and new knowlege is gained. Before being superseded in this regard by the chemical elements, biological species were taken to be the best exemplars of kindhood. Yet now it is somewhat controversial to state that species are natural kinds. As the world changes, its kinds may change too. Rapidly mutating micro-organisms demand new classification systems. Kinds in the social sciences, such as economics or sociology, are even more problematic, since the changing norms and practices of individuals and societies may also be held to be constitutive factors in kind membership, and these norms and practices may themselves respond to our classification of people into kinds. These examples are troublesome because there is some tension between the existence of kinds and the mutability of the particulars which are supposed to fall under those kinds. In the case of atoms or galaxies, the particulars under study are typically long-lived and only slowly changing; viruses and economic structures, on the other hand, are more dynamic.