The problem of natural kinds forms the busy crossroads where a number of larger problems meet: the problem of universals, the problem of induction and projectibility, the problem of natural laws and de re modalities, the problem of meaning and reference, the problem of intertheoretic reduction, the question of the aim of science, and the problem of scientific realism in general. Nor do these exhaust the list. Not surprisingly then, different writers confront a different ‘problem of natural kinds,’ depending on which background issue is for them the principal issue at stake. The issues of essentialism, meaning, and reference, for example, have tended to dominate recent discussions of natural kinds (Kripke , Putnam , , Mellor , Churchland , Shapere ). But evidently these are only part of the puzzle.