We talk as if there are natural kinds and in particular we quantify over them. We can count the number of elements discovered by Sir Humphrey Davy, or the number of kinds of particle in the standard model. Consequently, it looks at first sight at least, that natural kinds are entities of a sort. In the light of this we may ask certain questions: is the apparent existence of natural kinds real or an illusion? And if real, what sort of (...) entity are natural kinds? Are they sui generis? Or can they be identified with or reduced to some other kind of entity? In this essay I shall look at possible reasons for asserting that either kinds are no sort of entity, or, if they are entities, their existence is equivalent to some fact not involving kinds. Richard Boyd seems to take the view that the apparent existence of natural kinds is an illusion. (shrink)

In this paper I examine two ways of defining the rigidity of general terms. First I discuss the view that rigid general terms express essential properties. I argue that the view is ultimately unsatisfactory, although not on the basis of the standard objections raised against it. I then discuss the characterisation in terms of sameness of designation in every possible world. I defend that view from two objections but I argue that the approach, although basically right, should be interpreted cautiously.

I criticize the treatment of natural kinds as some sort of object, advocated in a recent paper by Alexander Bird. The arguments he gives for regimenting an illustrative statement featuring chemical kinds in his preferred manner are not conclusive, and his criticisms of an alternative strategy involving universally quantified sentences fail. This is important because of the widespread but poorly supported assumption that expressions of natural kinds should be treated as singular referring terms.