Meditation I
Descartes begins the first meditation by noting the the large number of false beliefs which he had adopted in his childhood. It is necessary to start over entirely, he realizes, if he wants to establish anything in the sciences which is firm and likely to last. In order to do this, he will suspend judgment about any of his beliefs which are in any way uncertain. To inspect each belief separately would take too long; he must find some way to undermine all of his beliefs at once.
The first way that Descartes tries to undermine his beliefs is by considering the fact that he remembers that his senses have deceived him before. If he has been misled by sensory information in the past (e.g. he judged that the stick in the water was bent, when in fact it was straight), then he may be deceived now, "and it is prudent never to trust completely those who have deceived us even once."
Descartes acknowledges, however, that this consideration doesn't undermine his beliefs very effectively, since it really seems as if he is sitting in his dressing gown by the fire, holding this piece of paper in his hands, and so on. How could his senses be misleading him about these things?
He goes on to suggest more powerful reasons to doubt that his beliefs are true. In general, his method is that of forming skeptical hypotheses. In the first meditation, he considers whether he is mad, dreaming, or deceived by an evil demon. If any of these scenarios were the case, many of his beliefs would be false. For instance, if he were mad, he might believe completely ridiculous things e.g. that his head is a pumpkin (he only considers the possibility that he is mad briefly and then seems to dismiss it). If he were dreaming, it would be false that he is sitting by the fire. He would on ...