This result could have been achieved either by his [God] endowing my intellect with a clear and distinct perception of everything about which I would ever deliberate, or simply by impressing the following rule so firmly upon my memory that I could never forget it: I should never judge anything that I do not clearly and distinctly understand.

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

It is not enough to have a good mind. The main thing is to use it well.

These long chains of perfectly simple and easy reasonings by means of which geometers are accustomed to carry out their most difficult demonstrations had led me to fancy that everything that can fall under human knowledge forms a similar sequence; and that so long as we avoid accepting as true what is not so, and always preserve the right order of deduction of one thing from another, there can be nothing too remote to be reached in the end, or to well hidden to be discovered.

Common sense is the most widely shared commodity in the world, for every man is convinced that he is well supplied with it.

There is nothing so strange and so unbelievable that it has not been said by one philosopher or another.

The entire method consists in the order and arrangement of the things to which the mind's eye must turn so that we can discover some truth.