Descartes believes that knowledge comes from within the mind, a single indisputable fact to build on that can be gained through individual reflection. While seeking true knowledge, Descartes writes his Six 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. He casts doubt upon all his previous beliefs, including "matters which are not entirely certain and indubitable [and] those which appear to be manifestly false." (Descartes, p.75, par.3) Once Descartes clears away all beliefs that can be called into doubt, he can then build a strong base for all true knowledge to stand upon. Descartes attacks all his previous beliefs by going to the root of their origin, the senses and intellect. He then supposes to say that everything he presumed to be absolutely true, such as simple arithmetic, was created by an evil demon.
Descartes starts the first argument by attacking the very basis of his beliefs, human senses. People learn their beliefs through their external and internal senses. "All that...I have accepted as most true and certain I have learned either from the senses or through the senses." (Descartes, p. 75, par. 3) By means of the five external senses -- sight, sound, touch, taste, smell -- you learn various ideas about the world around you. Yet, how reliable are these external senses, these sources of beliefs? Everyone will admit 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. (Descartes, p.75, par.3) For instance, imagine that you spot a person from across the street that looks like your friend. You run all the way down the street and tap the person on the shoulder, only to find out that this is not your friend but a person who looks somewhat like her. According to your sense of sight you believed this person to be your friend, but your sense deceived you. To build a foundation of knowledge upon beliefs derived from external senses is foolish since those senses are deceptive.
Perhaps true beliefs come from your internal senses. Internal senses include an awareness of oneself, such as believing you have a stomach and a heartbeat, without seeing them. An example of this internal sense is seen when Descartes says "There is the fact that I am here, seated by the fire, having the paper in my hands...and how can I deny that these hands and this body is mine." (Descartes, p.75, par.4) His internal sense makes him aware of his hands and body. Though, Descartes must explore all doubts involving this internal sense if he wants to use it as his foundation for knowledge.
Descartes brings up the possibility that perhaps at this point, right now, he is dreaming. A person who is dreaming may have difficulty differentiating between the dream and reality. Descartes says "How often has it happened to me that in the night I dreamt that I found myself in this particular place, that I was dressed and seated near the fire, whilst in reality I was lying undressed in bed!" (Descartes, p.76, par.1) According to this idea, I may believe, even now, I am dreaming, this not my body, and I am not writing this paper for philosophy but I am really lying in bed somewhere sleeping. This dream hypothesis would invalidate the beliefs that are based on internal sense; for if you are dreaming then what you believe to be your awareness of self is truly false. You may say that everyday life exhibits a smoothness and understanding, which dreams do not. Dreams have little rhyme or reason; while life experience is orderly and controlled. However, this scale of measuring the differences of coherence between dreams and reality is unreliable. Sometimes dreams are incoherent...