Descartes - "I think, therefore I am"

In the Second Meditation, Descartes famously states, "cogito ergo sum.” I think, therefore I am. This is the first foothold for him in his search for certainty. He posits that a thing that thinks must exist, and he points out that, whatever else he may be, he is a thinking thing. Therefore, he must exist.

Now, Descartes has already called into question the trustworthiness of the senses in an argument from illusion, and also that of the imagination. So how can he perceive anything? To answer this question, he considers a piece of wax. He realizes that he is using his senses to detect it, but also acknowledges that, without thinking, his senses give him nothing but disjuncted images. How is it that he can identify a piece of wax in solid and liquid states, and in any shape? Is it his imagination? Descartes says no, because there are an unimaginable number of configurations. The body has failed to give a clear idea of what the wax is. What, then, can illuminate what the wax is?

Descartes says it is the light of the mind. He concedes that the wax may not actually exist externally, that it might be a figment of his imagination. But the act of scrutinizing the wax with his intellect goes beyond sense perception and imagination. It gives him the truest knowledge of the wax. It is only through the intellect that he can understand what the wax is. And in this act of understanding by way of the intellect, Descartes can be sure that he himself exists.