The foundation of Descartes argument is based on the first person narrative style of his Meditations on First Philosophy. It is much more than a style; it is his main way of reasoning. It begins with finding an unshakeable point. To find this certainty one must throw out all that one knows and doubt everything, then build from there. He finds this certainty in his conscious awareness of his own thought. Only he can be certain of his own existence because only he can know of his conscious intelligence. It's feasible to question other's existence because certainty begins only with knowledge of oneself. .
As soon as Descartes creates the basis of his argument he can make more inferences as he moves on to different meditations. He now has an immovable point "I am, I exist, this is certain. But for how long? For as long as I am thinking." So what constitutes thinking? According to Descartes, thought includes doubting, understanding, affirming, denying, willing, refusing, and also imagining and sensing. This list is a substantial amount of thought which he analyzes much further. The thought that is most related to his argument is sensory perceptions. .
His famous concept of the evil demon is just one of his thought experiments. Here he claims that all his senses and thoughts are controlled by an evil demon within his mind. This is his first method of doubt. If we can doubt all of our senses, than how can we have any certainty? He answers this question with the "piece of wax" argument. Since a piece of wax can have an infinite amount of forms (shape, smell, taste, color, sound) he confirms with our intellect we can make a judgement of "purely mental scrutiny." This intellect includes out sensory perceptions. So by doubting, just as Plato did, Descartes destroyed the old way of thinking and created a new one.
