Full Idea
Supposing that the real essences were discoverable, ..yet we could not reasonably think that the ranking of things under general names was regulated by those internal real constitutions, or any thing else but their obvious appearance.
Gist of Idea
If we discovered real essences, we would still categorise things by the external appearance
Source
John Locke (Essay Conc Human Understanding (2nd Ed) [1694], 3.06.25)
Book Reference
Locke,John: 'Essay Concerning Human Understanding', ed/tr. Nidditch,P.H. [OUP 1979], p.452
A Reaction
This makes Putnam's water and twater the same! I take it there is no rule here. The authority of science has gradually substitute H2O as the criterion for water, so real essence rules, but it doesn't have to.