Full Idea
It could be that if I were to cease all thinking I would then utterly cease to exist. …I am therefore precisely nothing but a thinking thing; that is, a mind, or intellect, or understanding, or reason.
Gist of Idea
If I don't think, there is no reason to think that I exist
Source
René Descartes (Meditations [1641], §2.27)
Book Reference
Descartes,René: 'Discourse on Method/The Meditations', ed/tr. Sutcliffe,F.E. [Penguin 1968], p.105