Full Idea
Since I am clearly a thinking thing and not an extended thing, and on the other hand I have a distinct idea of a body, as merely an extended thing and not a thinking thing, it is certain that I am really distinct from my body, and can exist without it.
Gist of Idea
Mind is not extended, unlike the body
Source
René Descartes (Meditations [1641], §6.78)
Book Reference
Descartes,René: 'Discourse on Method/The Meditations', ed/tr. Sutcliffe,F.E. [Penguin 1968], p.156
A Reaction
How can he be 'certain' for this reason? This is a classic confusion of ontology and epistemology. Given that the mind is a special case, he should be asking WHY his thinking is clear to him, but his body isn't. Maybe it is because of his viewpoint.