Full Idea
The idea of whiteness or bitterness, as it is in the mind, exactly answering that power which is in any body to produce it, has all the real conformity it can, or ought to have, with things without us. This conformity is sufficient for real knowledge.
Gist of Idea
The constant link between whiteness and things that produce it is the basis of our knowledge
Source
John Locke (Essay Conc Human Understanding (2nd Ed) [1694], 4.04.05)
Book Reference
Locke,John: 'Essay Concerning Human Understanding', ed/tr. Nidditch,P.H. [OUP 1979], p.564
A Reaction
I take this to say that consistent covariation with certain things in the world is the best criterion we can find for our knowledge of secondary, and hence primary, qualities. Why they two covary is beyond our ken. Sounds right.