Full Idea
We have the ideas but of three sorts of substance; 1. God. 2. Finite intelligence. 3. Bodies.
Gist of Idea
We can conceive of three sorts of substance: God, finite intelligence, and bodies
Source
John Locke (Essay Conc Human Understanding (2nd Ed) [1694], 2.27.02)
Book Reference
Locke,John: 'Essay Concerning Human Understanding', ed/tr. Nidditch,P.H. [OUP 1979], p.329
A Reaction
Given Locke's scepticism about our ability to know of substances, this seems a bold claim, and can only really be a report of contemporary culture and language.