Full Idea
If there is a hidden structure, then generally it determines what it is to be a member of the natural kind, ...in all possible worlds. Put another way, it determines what we can and cannot counterfactually suppose about the natural kind.
Gist of Idea
The hidden structure of a natural kind determines membership in all possible worlds
Source
Hilary Putnam (The Meaning of 'Meaning' [1975], p.241)
Book Reference
Putnam,Hilary: 'Mind Language and Reality: Papers vol 2' [CUP 1975], p.241
A Reaction
This is the arrival of the bold new view of natural kinds (which is actually the original view - see Idea 8153). One must be careful of the necessity here. There is causal context, vagueness etc.
Related Idea
Idea 8153 By knowing one piece of clay or gold, you know all of clay or gold [Anon (Upan)]