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

"The perception of the understanding is sometimes obscure and confused, while that of the will is always clear and distinct."

"It is precisely because I am in the habit of using my senses to think of things that I find it difficult to think of anything else."

"I shall proceed by setting aside all that in which I can suppose there to be the slightest doubt, just as if I had discovered that it was wholly false."

"I have never made any discoveries that were not based on the certainty of mathematics."