I think therefore I am.
Je pense donc je suis. Cogito ergo sum. These are the words which Descartes used to slay total nihilism. Nihilism is the philosophical denial of existence, either of anything at all or more specific portions of existence. Everyone, at some point in their philosophical musings, wonders whether anything at all exists. Descartes was clever enough to see that pondering, doubting, existence was sufficient to prove that at least one thing exists; the thinker. This has given philosophy something to build on as we can now be certain that one thing exists. There have been criticisms of Descartes argument as a tautology (I think therefore I am) but the basic principle stands as a buttress against the void of nihilism.
The writer Milan Kundera has joked “‘I think, therefore I am’ is the statement of an intellectual who underrates toothaches.”