Thus, because our senses sometimes deceive us, I decided to suppose that nothing was such as they led us to imagine. And since there are people who make mistakes in reasoning, committing logical fallacies concerning the simplest questions in geometry, and because I judged that I was as prone to error as anyone else, I rejected as unsound all the arguments I had previously taken as demonstrative proofs. Lastly, considering that the very thoughts we have while awake may also occur while we sleep without any of them being at that time true, I resolved to pretend that all the things that had ever entered my
mind were no more true than the illusions of my dreams. But immediately I noticed that even while I was endeavoring in this way to think that everything was false, it was necessary that I, who was thinking this, was something. And observing this truth “I am thinking, therefore I exist” was so firm and sure that all the most extravagant suppositions of the skeptics were incapable of shaking it, I decided that I could accept it without scruple as the first principle of philosophy I was seeking.