A Posteriori Necessities & Natural Kinds

A new article by Nigel Leary ('Natural Kinds: Thick Essentialism or Promiscuous Realism?') suggests to me that Kripkean kind identities may or may not be true a posterori necessary statements. I don't think Leary's argument is incorrect per se, but I do think that perhaps he makes an incorrect equivocation.

The point that Leary makes is that Kripkean identity statements like "Water is H2O" are necessary statements only if we think that 'the essence of a chemical element is its chemical structure'. In short, these identity statements are only necessary insofar as we grant some form of essentialism. But then, the idea that such statements are necessary collapses, since we must assume a form of necessity (namely, essentialism) in order to have necessary statements. "Water is H2O" is a necessary statement iff an object's chemical structure is essential to it.

(Or perhaps the necessity fails because 'Twin Earth' thought experiments aren't wholly out of line, in the sense that they might really show something that is possible in the case of 'Water' being composed of 'XYZ'. But this is more questionable, so I won't get into it here.)

I suppose the pertinent question is: is an object's chemical structure necessary to it? I'm not so sure that it is. And to this extent, I agree with Leary that the Kripkean strategy for deriving a posteriori necessary truths might be, to a large extent, a circular, and so ultimately unsucessful strategy.

But I will say that I think that broadly construed Kripkean a posteriori necessary identity statements are mostly true, at least the ones that concern natural kind identities. This is because, as far as I'm concenred, natural kinds are correctly defined dispositionally. 'What it is to be' a natural kind is just to have a certain dispositional nature. If this is correct - and I do think it is - it is easy to see why I can allow that statements like "Water is H2O" might not be necessary, though they certainly concern natural kinds.

However, Leary is equivocating a bit. To say that a natural kind has a chemical structure essentially could mean that it has a categorical property as a matter of necessity. But it could also mean that it has a certain dispositional nature of necessity - namely, the dispositional nature proper to the pertient mixture of hydrogen and oxygen. If having this particular mixture brings along (or better yet, just is) a certain dispositional nature, I should think that the statement "Water is H2O" expresses a necessary truth.