Full Idea
Neither our imagination nor our senses could ever assure us of anything, if our understanding did not intervene.
Gist of Idea
Understanding, rather than imagination or senses, gives knowledge
Source
René Descartes (A Discourse on Method [1637], §4.37)
Book Reference
Descartes,René: 'Discourse on Method/The Meditations', ed/tr. Sutcliffe,F.E. [Penguin 1968], p.58