Full Idea
There is no discoverable connection between any secondary quality, and those primary qualities that it depends on.
Gist of Idea
We can't know how primary and secondary qualities connect together
Source
John Locke (Essay Conc Human Understanding (2nd Ed) [1694], 4.03.12)
Book Reference
Locke,John: 'Essay Concerning Human Understanding', ed/tr. Nidditch,P.H. [OUP 1979], p.545
A Reaction
I take this to be an accurate account of the problem, and it pinpoints what may be the single most recalcitrant mystery facing human understanding - why do red things look RED?