Full Idea
Doubtless I exist if I persuade myself of something. But there is some powerful and cunning deceiver who is deliberately deceiving me. Then too there is no doubt that I exist, if he is deceiving me.
Gist of Idea
I must even exist if I am being deceived by something
Source
René Descartes (Meditations [1641], §2.25)
Book Reference
Descartes,René: 'Discourse on Method/The Meditations', ed/tr. Sutcliffe,F.E. [Penguin 1968], p.103