What are natural kinds?
I’d say they don’t exist at all! When people talk about natural kinds, they refer to the things that are logically primitive of reality; those things which necessarily instantiate their object, and are the building blocks of the subject matter of science.
So, what candidates do we have of natural kinds? Gold, and other chemical elements, for one. Species? Species are subject to change, so maybe not. What about things in physics? Electrons, for instance, or heat (if defined by say, behaving in a way that such and such…)
Natural kinds are the talk of the town for philosophers of biology, and metaphysicians who have aspirations to talk of science.
I’m sure an interesting case can be made about mathematical kinds; but I don’t want to address that. What world are we invoking by positing the existence of natural kinds qua properties? Presumably, I hope, the real world. However, I have a very small suspicion that it’s not the real world. Why? Because we might want to say something about laws, and that is where we get into modality…
It seems to go against my Humean sentiments to posit these things called properties.