Full Idea
Reason now persuades me that I should withhold my assent no less carefully from opinions that are not completely certain and indubitable than I would from those that are patently false.
Gist of Idea
Reason says don't assent to uncertain principles, just as much as totally false ones
Source
René Descartes (Meditations [1641], §1.18)
Book Reference
Descartes,René: 'Discourse on Method/The Meditations', ed/tr. Sutcliffe,F.E. [Penguin 1968], p.95