Full Idea
There used to be a 'problem of Abstract Ideas' because it was assumed that an idea ought, somehow, to be a mental image; if some of our ideas appeared not to be images, this was a paradox and some solution must be found.
Gist of Idea
If ideas have to be images, then abstract ideas become a paradoxical problem
Source
H.H. Price (Thinking and Experience [1953], Ch.VIII)
Book Reference
Price,H.H.: 'Thinking and Experience' [Hutchinson 1953], p.234
A Reaction
Berkeley in particular seems to be struck by the fact that we are incapable of thinking of a general triangle, simply because there is no image related to it. Most conversations go too fast for images to form even of very visual things.