Some people believe that water is necessarily H2O. To say it is necessarily H2O is to impose a modality constraint upon it; but this says very little and many theses can be realised from this.
1. We may say that such facts are true are constitutive of any world at all
2. We may say that given the regularity of laws of causation; we may ascribe necessity to physical phenomena (in terms of chemical reactions…so, mutatis mutandis, water necessarily boils at 100 degrees)
1. Does a necessity claims such as this impose a conceptual indispensability to chemical natural kinds
2. In virtue of what would such a necessity obtain? (worldly status? essential properties? wider metaphysics…dispositions, causal powers etc.) I think the Kantian view that I am going to advocate would claim that a constraint upon the necessity of kinds is tacit upon accepting the taxonomy itself; if we deny the construal of “hydrogen-two-oxygen”; we may deny the claim; to accept the construal however, means that we accept that it is an object of possible experience
It is this latter concern that a Kantian philosophy of science must confer necessity to natural kinds; its a bit a weak conception, because it requires empirical confirmation that the taxonomy is correct.
Sinistre* (and Michael)