Full Idea
I now see a light, I hear a noise, I feel heat. Perhaps these things are false, since I am asleep. Yet I certainly do seem to see, hear, and feel warmth. This cannot be false.
Gist of Idea
My perceiving of things may be false, but my seeming to perceive them cannot be false
Source
René Descartes (Meditations [1641], §2.29)
Book Reference
Descartes,René: 'Discourse on Method/The Meditations', ed/tr. Sutcliffe,F.E. [Penguin 1968], p.107