Full Idea
The schools seem to intimate the confession of all mankind, that they have no idea of the real essences and substances, since they have not names for such ideas.
Gist of Idea
The schools recognised that they don't really know essences, because they couldn't coin names for them
Source
John Locke (Essay Conc Human Understanding (2nd Ed) [1694], 3.08.2)
Book Reference
Locke,John: 'Essay Concerning Human Understanding', ed/tr. Nidditch,P.H. [OUP 1979], p.475
A Reaction
He observes that schools timidly coined a few abstract terms for essences, but that they never caught on. This is an interesting criticism of essentialism from ordinary language. If a term names something real, it ought to 'catch on'.