Full Idea
I take it as a general rule that the things we conceive very clearly and very distinctly are all true, but that there is merely some difficulty in properly discerning which are those which we distinctly conceive.
Gist of Idea
Truth is clear and distinct conception - of which it is hard to be sure
Source
René Descartes (A Discourse on Method [1637], §4.33)
Book Reference
Descartes,René: 'Discourse on Method/The Meditations', ed/tr. Sutcliffe,F.E. [Penguin 1968], p.54