Descartes begins his project with the decision to sweep away all his dubious beliefs and build a new groundwork for the sciences. Unlike the previous questionable foundation, the new foundation is intended to indubitable. In order to reach his goal, he intends to reject any belief that is not completely reliable.
Realizing that an attempt to examine every belief would be an impossible task, he elects to instead examine the foundational beliefs on the supposition that if they fall, all the rest will fall with them.
After settling on his methodology, he turns to the senses. Though he once trusted them, he realizes they can deceive. Following his method, he decides to no longer put faith in them.
He pauses for a moment and considers that although the senses might deceive in certain cases, to deny their general evidence would be rather insane. But, as he notes, he has dreamed that he was awake when he was actually asleep-so he could be sleeping now and thus be deceived. He considers that perhaps the dream world is not as vivid as the world he experiences when he is ‘awake’ and hence distinguishable, but then realizes that there is no sure standard to distinguish the ‘real’ world from the ‘dream’ world. Thus, he decides to assume that though he thinks he is awake, he is instead dreaming.
Though well on his skeptical journey, Descartes pauses again. He considers that what he experiences in his dreams are like paintings of things such as satyrs: even if the composite beings are unreal, surely the simpler parts, like the head and legs, are real things. Further, even if the being is a complete fiction, at least the colors that compose it must be real. By analogy, he considers that the same is true of the ‘real’ world and hence the general things, such as body, extension, shape, quantity, number, and spatial location, must be real. Because of this view, he considers the sciences that deal with complex entities, such as physics and medical science, lack certainty. In contrast, since the mathematical sciences are not concerned with matters of existence, he considers them as certain-at least in some respects. After all, he reasons, whether he is awake or slumbering, adding two and three yields five and squares are four sided figures. At this point it seems as if Descartes’ project has come to and end-he considers that mathematical sciences cannot be doubted. However, this is not the case-he takes his project to another level by considering God and the evil ‘demon.’
While Descartes believes in God, he does not know whether God is causing him to perceive a world that, in fact, does not exist. He also considers that given the fact that other people are self-deceptive in matters they believe they know extremely well, he could also be mistaken when doing math or geometry.
He pauses for a moment and reflects that since God is alleged to be good, perhaps He does not want Descartes to be deceived. But he rejects this-after all, if a good God allows him to be deceived sometimes, then a good God could allow him to be deceived all the time-and he is deceived at least some of the time.
In the face of this difficulty, he decides that he will reject any claim that is not certain. By doing this, he hopes to make it possible to create a foundation of certainty for the sciences. Ironically, to reach this goal he must consider a situation of the most extreme skepticism.
For the sake of his project, he decides to consider the possibility that he is the victim of an evil ‘demon’ who has ‘created’ an illusory world to deceive him. Though Descartes decides to regard himself as lacking a physical body, he does draw a limit to the power of the demon. Though the demon is supposed to be very powerful and deceitful, it cannot force Descartes to believe-he retains the ability to suspend judgment even in the world of illusion.