The only thing we have power over in the universe is our own thoughts.

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

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.

We do not describe the world we see, we see the world we can describe.

Good sense is the most equitably distributed of all things because no matter how much or little a person has, everyone feels so abundantly provided with good sense that he feels no desire for more than he already possesses.

To live without philosophizing is in truth the same as keeping the eyes closed without attempting to open them.

Here I beg you to observe in passing that the scruples that prevented ancient writers from using arithmetical terms in geometry, and which can only be a consequence of their inability to perceive clearly the relation between these two subjects, introduced much obscurity and confusion into their explanations.