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.