In the first Meditation

In the first Meditation, Descartes employs ‘the method of doubt.’ He wants to find a foundation of knowledge that is so secure, it can stand up against the doubts of the strongest skepticism. Descartes presents what is, in effect, a dialogue between a person employing common sense and a skeptic. The person relying on common sense believes that there are various reliable sources of knowledge, while the skeptic claims that there is no secure foundation for knowledge. The skeptic in this debate acts in the same way as an ancient (historical) skeptic. That is, the skeptical procedure to isolate internal contradictions in the positions of those who claim that they know things. Descartes discusses two sources of knowledge, the senses and the intellect. The common-sense thinker would say that the senses are a reliable source of knowledge. To them, sense-perception is a reliable way to know things. But to the skeptic, sense-perception is not error-proof. For example, a straight stick looks bent from a distance when half immersed in water. Beyond that the skeptic would say that senses are not reliable because we could be dreaming or simply be insane. Next, we will look at intellect, which the common-sense thinker would say is a source of reliable knowledge even if senses fail. Whether someone is dreaming or not, 2+2=4 must be a self-evident truth since the a priori truths of mathematics are certain. Descartes uses the skeptics point of view to challenge that thought with the idea that we could be actively being deceived by a demon. It could be possible that an evil demon has the will, power and knowledge to make us the constant victim of deception. So even if we think something is self-evidently true, it is not. Here Descartes goes beyond historical skepticism and engages in exaggerated or hyperbolic doubt. Descartes thinks that since he cannot be sure that there is no such evil spirit, he cannot rightly claim to know anything about which such a spirit might be deceiving him. At this point in the Mediations, it seems that the only certainty Descartes can find is that nothing is certain.
Things change though, when Descartes finds one belief that he can know with certainty: the belief in his own existence – “I think, I exist.” This belief in his own existence is the foundational belief Descartes was seeking. From this stage onward, Descartes is able to construct a series of known truths, indeed a complete metaphysics. Descartes argues that, even assuming there is an evil spirit who constantly deceives him, it is certain that his own self exists. For the very notion of an evil spirit assumes that the spirit deceives someone, namely Descartes. So even if he is constantly deceived, he can’t doubt his existence. Descartes holds that the sentence “I exist” must be true whenever he thinks it to himself. He may be utterly deceived as to what he believes, but even the most radical doubt of all – doubting his own existence – implies that he exists. Descartes shows an internal contradiction in the position of skeptic who asserts that by doubting everything (i.e. thinking about everything) he is uncommitted to all beliefs. But even a skeptic believes that he can think.
Descartes uses the “wax argument” to further understand his existence. According to Descartes, when he melts wax, all the sensible qualities change (i.e. the smell, hardness) Yet he judges that the wax remains the same. He knows that it is still the same piece of wax, through his intellect. Although the sensible properties have changed, his intellect can know that the wax (whether melted or solid) is extended (material), flexible and changeable. So, when he carefully examines the way in which he comes to make claims about the world, he realizes that the claims are based on mental powers which he possesses (judgment, inference, etc.), rather than his senses. This confirms his own existence as (at least) a thinking being.