Full Idea
Everyone upon inquiry into his thoughts, will find that he has no other idea of any substance, but what he has barely of those sensible qualities, with a supposition of such a substratum as give support to those qualities, which he observes exist united.
Clarification
A 'substratum' is a supporting lower level
Gist of Idea
We think of substance as experienced qualities plus a presumed substratum of support
Source
John Locke (Essay Conc Human Understanding (2nd Ed) [1694], 2.23.06)
Book Reference
Locke,John: 'Essay Concerning Human Understanding', ed/tr. Nidditch,P.H. [OUP 1979], p.298
A Reaction
This is the orginal of the 'substratum' view of substances. The whole problem is captured here, because this is an empiricist trying not to extend his ontology beyond experience, but trying to explain unity, identity and continuity.