Full Idea
This I do say, that there are real constitutions in things from whence simple ideas flow, which we observe combin'd in them. But we distinguish particular substances into sorts or genera not by real essences or constitutions, but by observed simple ideas.
Gist of Idea
Things have real essences, but we categorise them according to the ideas we receive
Source
John Locke (Letters to William Molyneux [1692], 1693.01.20)
Book Reference
'Leibniz: Critical and Interpretive Essays', ed/tr. Hooker,Michael [Manchester 1982], p.179
A Reaction
This is the clearest statement I can find of Locke's position on essences. He is totally committed to their reality, but strongly aware of the empirical constraints which keep us from direct knowledge of them. He would be amazed by modern discoveries.