There is nothing more ancient than the truth.

Although my knowledge grows more and more, nevertheless I do not for that reason believe that it can ever be actually infinite, since it can never reach a point so high that it will be unable to attain any greater increase.

In order to improve the mind, we ought less to learn, than to contemplate.

Everybody thinks himself so well supplied with common sense that even those most difficult to please. . . never desire more of it than they already have.

For to be possessed of a vigorous mind is not enough; the prime requisite is rightly to apply it.

Divide each difficulty into as many parts as is feasible and necessary to resolve it.

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.