Full Idea
He who decides to doubt everything cannot nevertheless doubt that he exists while he doubts.
Gist of Idea
Total doubt can't include your existence while doubting
Source
René Descartes (Principles of Philosophy [1646], Pref)
Book Reference
Descartes,René: 'Philosophical Essays and Correspondence', ed/tr. Ariew,Roger [Hackett 2000], p.226