Full Idea
Certainty is not in the sense but in the understanding alone, when it has evident perceptions.
Gist of Idea
Understanding, not the senses, gives certainty
Source
René Descartes (Preface to 'Principles of Philosophy' [1647], p.177)
Book Reference
Descartes,René: 'Discourse on Method/The Meditations', ed/tr. Sutcliffe,F.E. [Penguin 1968], p.177