Descartes, Discourse on Method

            The first rule was that I would not accept anything as true which I did not clearly know to be true. That is to say, I would carefully avoid being over hasty or prejudiced, and I would understand nothing by my judgments beyond what presented itself so clearly and distinctly to my mind that I had no occasion to doubt it.

            The second was to divide each difficulty which I examined into as many parts as possible and as might be necessary to resolve it better.

            The third was to conduct my thoughts in an orderly way, beginning with the simplest objects, the ones easiest to know, so that little by little I could gradually climb right up to the knowledge of the most complex, by assuming the same order, even among those things which do not naturally come one after the other.

            And the last was to make my calculations throughout so complete and my examinations so general that I would be confident of not omitting anything.

(…)

Thus, because our senses deceive us sometimes, I was willing to assume that there was nothing which existed the way our senses present it to us. And because there are men who make mistakes in reasoning, even concerning the most simple matters of geometry, and who create para-logisms, and because I judged that I was subject to error just as much as anyone else, I rejected as false all the reasons which I had taken earlier as proofs. Finally, considering that all the same thoughts which we have when awake can also come to us when we are asleep, without there being truth in any of them at the time, I determined to pretend that everything which had ever entered my mind was no more true than the illusions of my dreams.

            But immediately afterwards I noticed that, while I wished in this way to think everything was false, it was necessary that I—who was doing the thinking—had to be something. Noticing that this truth—I think, therefore I am—was so firm and so sure that all the most extravagant assumptions of the sceptics would not be able to weaken it, I judged that I could accept it without scruple as the first principle of the philosophy I was looking for.