Full Idea
While I decided to think that everything was false, it followed necessarily that I who thought thus must be something; the truth 'I think therefore I am' was so certain that the most extravagant scepticism could never shake it.
Gist of Idea
In thinking everything else false, my own existence remains totally certain
Source
René Descartes (A Discourse on Method [1637], §4.32)
Book Reference
Descartes,René: 'Discourse on Method/The Meditations', ed/tr. Sutcliffe,F.E. [Penguin 1968], p.53