Full Idea
We come to the ideas of particular sorts of substances, by collecting combinations of simple ideas that exist together, and are therefore supposed to flow from the particular internal constitution, or unknown essence of that substance.
Gist of Idea
Particular substances are coexisting ideas that seem to flow from a hidden essence
Source
John Locke (Essay Conc Human Understanding (2nd Ed) [1694], 2.23.03)
Book Reference
Locke,John: 'Essay Concerning Human Understanding', ed/tr. Nidditch,P.H. [OUP 1979], p.296
A Reaction
This is Locke's concept of essence, as the source which gives rise to the other properties of a thing. Locke waxes sarcastic about this 'I know not what' in things, but he never actually denies it. He just thinks it is beyond our grasp.