Full Idea
A universal is not an object of any sense, and therefore cannot be imagined; but it may be distinctly conceived.
Gist of Idea
Universals are not objects of sense and cannot be imagined - but can be conceived
Source
Thomas Reid (Essays on Intellectual Powers 5: Abstraction [1785], 6)
Book Reference
Reid,Thomas: 'Inquiry and Essays', ed/tr. Beanblossom /K.Lehrer [Hackett 1983], p.245
A Reaction
If you try to imagine whiteness, what size is it, and what substance embodies it? Neither are needed to think of whiteness, so Reid is right. A nice observation.