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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]
     Full Idea: In each kind, whatever holds of something in itself and as such holds of it from necessity.
     From: Aristotle (Posterior Analytics [c.327 BCE], 75a30)
     A reaction: This seems to confirm the view that essential properties are necessary, but it does not, of course, follow that all necessary properties are essential properties (e.g. trivial necessities are not essential).
For essentialists two members of a natural kind must be identical [Ellis]
     Full Idea: Modern essentialists would insist that any two members of the same natural kind must be identical in all essential respects.
     From: Brian Ellis (The Philosophy of Nature: new essentialism [2002], Ch.1)
     A reaction: For this reason, animals no longer qualify as natural kinds, but electrons, gold atoms, and water molecules do. My sticking point is when anyone asserts that an electron necessarily has (say) its mass. Why no close counterpart of electrons?
The whole of our world is a natural kind, so all worlds like it necessarily have the same laws [Ellis]
     Full Idea: It is plausible to suppose that the world is an instance of a natural kind, ..and what is naturally necessary in our world is what must be true in any world of the same natural kind.
     From: Brian Ellis (The Philosophy of Nature: new essentialism [2002], Ch.6)
     A reaction: This is putting an awful lot of metaphysical weight on the concept of a 'natural kind', so it had better be a secure one. If we accept that natural laws necessarily follow from essences, why shouldn't the whole of our world have an essence, as water does?
Gold's atomic number might not be 79, but if it is, could non-79 stuff be gold? [Kripke]
     Full Idea: Gold could turn out not to have atomic number 79. …But given that gold does have the atomic number 79, could something be gold without having the atomic number 79?
     From: Saul A. Kripke (Naming and Necessity lectures [1970], Lecture 3)
     A reaction: The question seems to be 'is atomic number 79 essential to gold?', and on p.124 Kripke seems to say 'yes'. I agree. But how do we decide which features are essential to gold? Why do we think molten gold does count as gold?
'Cats are animals' has turned out to be a necessary truth [Kripke]
     Full Idea: 'Cats are animals' has turned out to be a necessary truth.
     From: Saul A. Kripke (Naming and Necessity lectures [1970], Lecture 3)
     A reaction: False! As Putnam has pointed out, we could yet discover that cats are subtly designed alien robots. This is a revealing error by Kripke, showing his desire to move from a useful logical clarification to an excessively amibitious metaphysics.
We can base the idea of a natural kind on the mechanisms that produce natural necessity [Harré/Madden]
     Full Idea: Natural necessity involves the concept of generative mechanisms and powerful particulars, and these in turn can be the basis of a useful notion of a natural kind.
     From: Harré,R./Madden,E.H. (Causal Powers [1975], 7.V)
     A reaction: Not sure about that. Say gold and silver are two kinds that lead to two outcomes. Each is a natural necessity. How do you distinguish them? Only by one being the gold-necessity and the other the silver-necessity. Circular?
Maybe the identity of kinds is necessary, but instances being of that kind is not [Mackie,P]
     Full Idea: One could be an essentialist about natural kinds (of tigers, or water) while holding that every actual instance or sample of a natural kind is only accidentally an instance or a sample of that kind.
     From: Penelope Mackie (How Things Might Have Been [2006], 10.2)
     A reaction: You wonder, then, in what the necessity of the kind consists, if it is not rooted in the instances, and presumably it could only result from a stipulative definition, and hence be conventional.