Full Idea
I can think of countless geometrical figures, concerning which there can be no suspicion of their ever having entered me through the senses.
Gist of Idea
I can think of innumerable shapes I have never experienced
Source
René Descartes (Meditations [1641], §5.64)
Book Reference
Descartes,René: 'Discourse on Method/The Meditations', ed/tr. Sutcliffe,F.E. [Penguin 1968], p.143