Full Idea
Our ranking, and distinguishing natural substances into species consists in the nominal essences the mind makes, and not in the real essences to be found in things themselves.
Gist of Idea
We distinguish species by their nominal essence, not by their real essence
Source
John Locke (Essay Conc Human Understanding (2nd Ed) [1694], 3.06.11)
Book Reference
Locke,John: 'Essay Concerning Human Understanding', ed/tr. Nidditch,P.H. [OUP 1979], p.444
A Reaction
Note that, as far as I can see, Locke never denies the existence of real essences, or even that we might occasionally know them. He is here merely describing, fairly accurately, I think, his empiricist view of how these categories have come about.
Related Idea
Idea 12531 Nominal Essence is the abstract idea to which a name is attached [Locke]