Full Idea
When someone says 'I am thinking, therefore I am, or I exist', he does not deduce existence from thought by means of a syllogism, but recognises it as something self-evident by a simple intuition of the mind.
Gist of Idea
The Cogito is not a syllogism but a self-evident intuition
Source
René Descartes (Reply to Second Objections [1641], 140)
Book Reference
Descartes,René: 'Meditations on First Philosophy etc.', ed/tr. Cottingham,John [CUP 1986], p.68