Descartes begins Part I of the Principles by calling all of our beliefs into doubt. This exercise is meant to free us from our reliance on the senses, so that we can begin to contemplate purely intellectual truths.
The doubting is initiated in two stages. In the first stage, all the beliefs we have ever received from sensory perceptions are called into doubt. In the second stage, even our intellectual beliefs are called into doubt.
Descartes presents two reasons for doubting that our sensory perceptions tell us the truth. First of all, our senses have been known to deceive us. Examples of the sort of systematic deception he has in mind here include phenomena such as the bent appearance of a straight stick when viewed in water and the optical illusion of smallness created by distance. The second doubt that Descartes brings to bear on sensory perceptions is more dramatic. Descartes claims that even in optimal viewing conditions (i.e. close by, no intervening water, etc.) we cannot trust our senses. The reason is that when we sleep we often have sensations indistinguishable from those that we have when we are awake. We admit that those dreaming sensations do not correspond to reality, so why are we any more certain of our waking sensations? How do we know that any particular sensation is not just a dream, a sensation stemming from causes unbeknownst to us? This second argument is popularly referred to as the "Dreamer Argument."
Descartes next casts doubt onto our mathematical demonstrations and other self- evident truths. In order to do this, he first points out that people are sometimes known to make mistakes when it comes to these subjects. In addition, he claims, for all we know, God (or some lesser being) is manipulating our thoughts, causing things to seem certain when really they are not. This argument is commonly referred to as the "Evil Demon Argument".
After attempting to undermine all of our beliefs, Descartes identifies one belief that resists all such attempts: the belief that I myself exist. This stage in Descartes' argument is called the cogito, derived from the Latin translation of "I think." It in only in the Principles that Descartes states the argument in its famous form: "I think, therefore I am." This oft- quoted and rarely understood argument is meant to be understood as follows: the very act of thought proves existence, because one cannot possibly think without existing.
The cogito is arguably the most famous argument in philosophy, but what is it really supposed to prove? What is Descartes' purpose in beginning his magnum opus with such a trivial piece of knowledge? In order to see the answer to this question, it is important to view the cogito in its context.