Rene Descartes used doubt to prove his beliefs to be true. In his most famous work, Meditations on First Philosophy, Descartes enters into his most radical phase of methodological doubt when he introduces his evil genius hypothesis, also known as the evil demon argument. Descartes' demon hypothesis is that an extremely powerful, malicious spirit, who strives to deceive him, is creating a reality for him. This hypothesis operates not only in Meditation I and II, where it is fully and explicitly discussed, and Meditation III where it is technically disproved, but also in the background throughout the rest of the six meditations. It is only truly relaxed in Meditation VI after the proofs for the existence of God have been done in both Meditations III and V. I feel that Descartes rushed his proof that the evil genius does not exist and inadequately explained it away with the circular logic he used to prove God.
[...] This means, in agreement with Descartes' idea of innate thoughts because if Descartes did not think up the evil genius and God did not create it therefore did not put thoughts of it into Descartes' head , the evil genius created itself thereby disproving Descartes idea of deception stemming from defect or disproving Descartes' idea of different levels of perfection. According to Descartes, a being must originate from a being more perfect then itself and the only way a being can create itself is if it is perfect. [...]
[...] Descartes proves the existence of God by deciding that everything he clearly and distinctly perceives as true must be certain. This is the case because having ascertained that he exists and that he is a thinking thing, Descartes tries to determine how he can know these things and whether he might come to know other things as well by similar means. He concludes that his knowledge of the cogito and the sum res cogitans are clear and distinct perceptions. Thus, he concludes, all clear and distinct perceptions (also referred to as "the natural light") must be true. [...]
[...] Descartes can be fooled into believing that there is a table in front of him but not be fooled by the proposition that he sees a table before him or that he thinks there is a table before him. So the evil demon can trick Descartes into thinking certain things but cannot fool Descartes about thinking. Therefore the demon has no true rule over Descartes' introspective thoughts. The demon cannot cause Descartes to think or fool him into believing that he is thinking when he is not. [...]
[...] However, even before his circular proof of God in Meditation III and his subsequent ruling out of the deceiver, Descartes sets up a weakening logical impossibility. There are some propositions that he can know like am thinking” without ruling out the evil genius hypothesis. Even if there is an evil demon with supreme power and cunning trying to constantly and deliberately deceive him, the demon cannot fool Descartes into thinking when he is not. For this to be possible, a situation in which Descartes thinks he is thinking when he is actually not thinking must arise. [...]
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