Friday, August 11, 2006

One Thing of Two Kinds

It's commonly assumed in natural kind discussions that nothing can belong to two natural kinds unless one of the natural kinds is a species of the other. It's difficult to see how this would work, though; as Brian Ellis notes in Scientific Essentialism (p. 56n2), in the case of chemical kinds it seems to be quite clearly false. Copper sulfate, for instance, is both a cupric compound and a sulfate; there are excellent reasons for considering both cupric compounds and sulfates to be genuine natural kinds if anything is a natural kind; and there seems to be no reason for thinking that one is a natural kind and the other is not. But neither is a species of the other; they simply happen to overlap at copper sulfate. So a thing can instantiate two different non-nested natural kinds (as indicated by different types of diagnostic features.).

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