Cogito ergo sum

Cogito ergo sum was a philosophical statement, suggesting that all a person truly knows is that they exist. (AUDIO: Minuet in Hell, Zagreus)

Translated from Latin, it means "I think therefore I am". (AUDIO: Zagreus)

Gideon Crane used it to mock the Eighth Doctor while the latter was in a state of amnesia. (AUDIO: Minuet in Hell)

The Fourth Doctor used an altered version of the statement, "cogitas ergo non sum", which translated to "you exist, therefore I'm long gone", to point out that the Tenth Doctor's very presence was a reminder of his own mortality. (AUDIO: Out of Time)

Behind the scenes
In the real world, the cogito was an argument used by the philosopher René Descartes to argue from a position of total scepticism to knowledge of at least one entity in the world. More completely, the argument took the form, "dubito ergo cogito, cogito ergo sum", or "I doubt therefore I think, I think therefore I am". Descartes, of course, did not stop his philosophical analysis of knowledge here, he continued to attempt to ground knowledge of external entities in pure reason throughout Meditations on First Philosophy.