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26. Natural Theory / B. Natural Kinds / 6. Necessity of Kinds

[natural kinds in some way have to be as they are]

7 ideas
Whatever holds of a kind intrinsically holds of it necessarily [Aristotle]
For essentialists two members of a natural kind must be identical [Ellis]
The whole of our world is a natural kind, so all worlds like it necessarily have the same laws [Ellis]
'Cats are animals' has turned out to be a necessary truth [Kripke]
Gold's atomic number might not be 79, but if it is, could non-79 stuff be gold? [Kripke]
We can base the idea of a natural kind on the mechanisms that produce natural necessity [Harré/Madden]
Maybe the identity of kinds is necessary, but instances being of that kind is not [Mackie,P]