The first precept was never to accept a thing as true until I knew it as such without a single doubt. - René Descartes

Je donnerais tout ce que je sais pour la moitié de ce que je ne sais pas.

I am like a prisoner who happens on enjoy an imaginary freedom in his dreams and who subsequently begins to suspect that he is asleep and, afraid of being awakened, conspires silently with his agreeable illusions.

while I wanted thus to think that everything was false, it necessarily had to be the case that I, who was thinking this, was something. And noticing that this truth — I think, therefore I am — was so firm and so assured that all the most extravagant suppositions of the skeptics were incapable of shaking it, I judged that I could accept it without scruple as the first principle of the philosophy I was seeking.