A quick catch-up

At the beginning of Meditation Two Descartes found something that was certain and could not be doubted. He said

I must finally conclude that this proposition, I am, I exist, is necessarily true whenever it is put forward by me or conceived in my mind.

From there he goes on to clarify what is meant by 'I'. Since he is 'not admitting anything except what is necessarily true' it is possible that he doesn't have a body. However, it is not possible that he doesn't have a mind that thinks and, be careful, for Descartes this doesn't mean he has a brain. He says

I am, then, in the strict sense only a thing that thinks; that is, I am a mind, or intelligence, or intellect, or reason -- words whose meaning I have been ignorant of until now. But for all that I am a thing which is real and which truly exists. But what kind of a thing? As I have just said -- a thinking thing.

The problem though is how does one move from knowledge of yourself as a thinking thing to knowledge of other things. This is why Descartes needs to introduce the notion of clear and distinct perception.