Whether engaged in high-level scientific activity or in the ordinary business of living, we spend a great deal of our time sorting the objects we come across into kinds. Philosophers are concerned with the kinds of kinds into which we sort these objects, and with the principles that distinguish one kind of kinds from another. One kind of kinds that has loomed large in recent philosophical discussions is that of so-called natural kinds. And one conception of natural kinds has dominated discussion in the contemporary philosophies of science, language, and mind, and this conception will concern us here. But first, some background.

Historical discussions of natural kinds (Ayers 1981) usually start with Aristotle and his conception of the individuals that are members of a kind of substance in virtue of the fact that they share a certain property (an essence) with all and only the other members...