 The exclusion of this possibility grounds a decisive difference from the way natural kinds are standardly construed in the wake of Locke and Kripke. Recall that, in this Kripkean construal, lumps of matter are instances of chemical kinds because of their satisfaction of intrinsic necessary and sufficient conditions, viz. their atoms possessing a certain number of protons. The same conditions also individuate the chemical kinds themselves. Chemical kinds are thus spatiotemporally unrestricted sets. This means that there are no metaphysical barriers to the chance generation of members of the kind, independently of whether the kind is instantiated at any contiguous time or place. Nitrogen could come to exist by metaphysical happenstance, should an element with the atomic number 14 somehow come into being, even in a world in which up to that point no nitrogen has existed (Hull 1978: 349; 1984: 22).