Cogito, ergo sum
Usually translated from the Latin as 'I think, therefore I am'.
Possibly the best known of all philosophical quotations; this is from the French philosopher René Descartes in Discourse on Method, 1637, where he attempted to prove his existence as a thinking being, by thinking. 'I think, therefore I am' comes to us in English via two translations.
I think therefore I am is the only philosophical quotation that many people know.
Descartes' original statement in French was "Je pense, donc je suis".
This is such a well-known line that it has spawned humorous alternatives, not least:
"I'm pink, therefore I'm spam"
"René Descartes was a drunken fart - I drink therefore I am".