The two operations of our understanding, intuition and deduction, on which alone we have said we must rely in the acquisition of knowledge.

I did not imitate the skeptics who doubt only for doubting's sake, and pretend to be always undecided; on the contrary, my whole intention was to arrive at a certainty, and to dig away the drift and the sand until I reached the rock or the clay beneath.

Neither the true nor the false roots are always real; sometimes they are imaginary; that is, while we can always imagine as many roots for each equation as I have assigned, yet there is not always a definite quantity corresponding to each root we have imagined.

Perfect numbers like perfect men are very rare.

Let whoever can do so deceive me, he will never bring it about that I am nothing, so long as I continue to think I am something.

In the matter of a difficult question it is more likely that the truth should have been discovered by the few than by the many.

I think; therefore I am.