Full Idea
What other qualities necessarily co-exist with a substance we cannot know, unless we can discover their natural dependence; which in their primary qualities we can go but a very little way in, and in secondary qualities we know no connexion at all.
Gist of Idea
We can only slightly know necessary co-existence of qualities, if they are primary
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
His concept of essence is precisely that which gives rise to the collection of a thing's properties, so his doubts here are consistent. I take the modern position to be an optimist reading of Locke, that actually we can identify the substances.