I think; therefore I am.

The only secure knowledge is that I exist.

Let whoever can do so deceive me, he will never bring it about that I am nothing, so long as I continue to think I am something.

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.

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.

Thus each truth discovered was a rule available in the discovery of subsequent ones.

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.