Full Idea
Since I do not observe that any other thing belongs necessarily to my nature or essence except that I am a thinking thing, I rightly conclude that my essence consists in this alone, that I am a thinking thing, or substance whose essence is thinking.
Gist of Idea
Since I only observe myself to be thinking, I conclude that that is my essence
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
This actually appears to be my favourite confusion - of episemology with ontology. Compare 'whenever I see him he is smiling, so he must be happy'. Personally I am happy to say that my essence is thinking, as long as it needn't be conscious.