It is only prudent never to place complete confidence in that by which we have even once been deceived.
The only secure knowledge is that I exist
Divide each difficulty into as many parts as is feasible and necessary to resolve it.
One cannot conceive anything so strange and so implausible that it has not already been said by one philosopher or another.
Whenever anyone has offended me, I try to raise my soul so high that the offense cannot reach it.
Each problem that I solved became a rule which served afterwards to solve other problems.
The two operations of our understanding, intuition and deduction, on which alone we have said we must rely in the acquisition of knowledge.
It is easy to hate and it is difficult to love. This is how the whole scheme of things works. All good things are difficult to achieve; and bad things are very easy to get.