Descartes believes that knowledge comes from within the mind a fact to build on that can be gained through individual reflection. White seeking true knowledge, Descartes writes his Meditations. In these meditations, Descartes tries to develop a strong foundation, which all knowledge can be built upon. In the First Meditation, Descartes begins developing this foundation through the method of doubt. Descartes observed that many things that he had accepted as true in his early days were in fact false. He realized that everything was based on these erroneous beliefs had to be false as well. He must destroy his existing house of "knowledge" and lay a new foundation on which to construct a indestructible surface. He wants to start at the foundation, and establish what is certainly true. Once Descartes clears away all beliefs that can be doubted, he can then build a strong base for all true knowledge to be tested against. He attacks the principle on which his former beliefs rested because when the foundation is undermined, the entire superstructure will collapse. "The result is a type of rationalism in which the only certainties are discovered by the mind through self-evident insight or reason." (The Quest for Truth page 130 line 14-15) He then supposed to say that everything he presumed to be absolutely true, was created by an evil demon and will go on to believe everything is false. .
Descartes starts the first argument by attacking the basis of his beliefs, human senses. What he has so far accepted as true, he has got from his senses. Since he has been deceived by his senses at times, he knows not to entirely trust them. Everyone can say that their external senses have deceived them on at least one occasion, and according to Descartes, it is a mark of prudence never to place our complete trust in anything that has deceived us even once. For example, we"re walking down the street one day when we think we see a person we know, as we get closer we realize that it's a complete stranger, showing that we"re being deceived by our senses right there.
