See [Descartes' Epistemology](http://plato.stanford.edu/entries/descartes-epistemology/#4) : 4. *Cogito Ergo Sum*, for some key points, including :

>In short, the success of the cogito does not presuppose Descartes' mind-body dualism.

>[...] much of the debate over whether the cogito involves inference, or is instead a simple intuition (roughly, self-evident), is preempted by two observations. [...] As Descartes writes:

>>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 recognizes it as something self-evident by a simple intuition of the mind. (*Replies 2*, AT 7:140).