However, he has still not doubted everything, as there are some things that still apply in dreams, such as mathematics and geometry. Even in dreams, two plus three will still equal five, and a square will always have four sides. The third wave of doubt allows Descartes to doubt whether this is the case. He contemplates whether there is an all powerful, evil demon deceiving him and if there is the slightest possibility of this, Descartes must disregard all previous beliefs. There is a chance that the demon is misleading him to believe that a quadrilateral has four sides, when in fact it has seven. Descartes has now reached the point where he believes everything can be doubted.

At the beginning of Meditation Two, Descartes is unsure of how to escape the doubt he finds himself drowning in as a result of the previous day’s doubts. He starts by considering whether he can deceive himself. It appears that he can doubt his senses, whether he is dreaming and whether 2+3=5, but he cannot doubt the fact that he is thinking. Although his thoughts may be deceived, he is persuaded that he exists, merely by having thoughts at all. He has thus presented the Cogito as an argument for his existence.

Descartes claims that even though the demon may be deceiving him, he must exist in order to be deceived. If he is doubting, he is thinking, and so long as he thinks, he is something, and that something exists. Every time Descartes expresses or conceives the words ‘ego sum, ego existo’, it is necessarily true.

Thus far, Descartes has proved that he exists but has not discussed at length what it is t