Cogito, ergo sum

Meaning

Usually translated from the Latin as 'I think, therefore I am'.

Origin

Possibly the best known of all philososophical 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. Descartes' original statement in French was "Je pense, donc je suis".

This is such a well-known line that it has spawned humourous alternatives, not least:

"I'm pink, therefore I'm spam"
"René Descartes was a drunken fart - I drink therefore I am".