Full Idea
Of substance, we have no idea of what it is, but only a confused obscure one of what it does.
Gist of Idea
We don't know what substance is, and only vaguely know what it does
Source
John Locke (Essay Conc Human Understanding (2nd Ed) [1694], 2.13.19)
Book Reference
Locke,John: 'Essay Concerning Human Understanding', ed/tr. Nidditch,P.H. [OUP 1979], p.175
A Reaction
Locke seems to identityf 'substance' with 'real essence', about which he makes similar remarks. He was deeply pessimistic about our ability to unravel how the physical world works. Note that he isn't denying the existence of substance.