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

All is to be doubted.

I desire to live in peace and to continue the life I have begun under the motto 'to live well you must live unseen

Perfect numbers like perfect men are very rare.

Each problem that I solved became a rule, which served afterwards to solve other problems.

Archimedes, that he might transport the entire globe ... demanded only a point that was firm and immovable; so also, I shall be entitled to entertain the highest expectations, if I am fortunate enough to discover only one thing that is certain and indubitable.

If we possessed a thorough knowledge of all the parts of the seed of any animal (e.g. man), we could from that alone, be reasons entirely mathematical and certain, deduce the whole conformation and figure of each of its members, and, conversely if we knew several peculiarities of this conformation, we would from those deduce the nature of its seed.