Descartes uses arguments that are situated in the context of doubt (often known as hyperbolic or methodic doubt). Moreover, these arguments have one specific aim (at least in the first meditation, and the beginning of the second), and that is to reach certainty. Once he uncovers one indubitable item ("I am, I exist"), he spends the rest of the second meditation seeking to extend the certainty that he exists as a thinking thing - to see what are the modes of thinking, and whether or not there is anything else as certain as thinking.
Descartes has a standard for certainty: what ...
Discusses Rene Descartes and certainty.