The first precept was never to accept a thing as true until I knew it as such without a single doubt.
Rene Descartes... regard this body as a machine which, having been made by the hand of God, is incomparably better ordered than any machine that can be devised by man, and contains in itself movements more wonderful than those in any machine. ... it is for all practical purposes impossible for a machine to have enough organs to make it act in all the contingencies of life in the way in which our reason makes us act.
Rene DescartesIf you would be a real seeker after truth, it is necessary that at least once in your life you doubt, as far as possible, all things.
Rene DescartesI am accustomed to sleep and in my dreams to imagine the same things that lunatics imagine when awake.
Rene DescartesThis result could have been achieved either by his [God] endowing my intellect with a clear and distinct perception of everything about which I would ever deliberate, or simply by impressing the following rule so firmly upon my memory that I could never forget it: I should never judge anything that I do not clearly and distinctly understand.
Rene Descartes