Full Idea
The complex ideas that our names of the species of substances properly stand for are collections of qualities, as have been observed to co-exist in an unknown substratum which we call 'substance'.
Gist of Idea
Complex ideas are collections of qualities we attach to an unknown substratum
Source
John Locke (Essay Conc Human Understanding (2nd Ed) [1694], 4.06.07)
Book Reference
Locke,John: 'Essay Concerning Human Understanding', ed/tr. Nidditch,P.H. [OUP 1979], p.582
A Reaction
Locke refers to a substratum, but this is not actually a 'bare' substratum, as he believes in real essences (see other quotations), but believes we have absolutely no chance of knowing them.