I think, therefore I am.

Source: http://en.wikipedia.org/wiki/Methodic_Doubt

René Descartes : I think, therefore I am.

In Meditation I, Descartes stated that if one were mad, even briefly, and the insanity might have driven man into believing that what we thought were true, could be merely our minds deceiving us. He also brought another reasoning in which there was no reliable way of distinguishing when one was awake or was dreaming. This raises the question, 'How can you tell what is real when you cannot even tell when you are awake or asleep?' His third reasoning stated that there could be 'some malicious, powerful, cunning demon' that had conceived us, preventing us from judging correctly.

However, while methodic doubt has a nature, one need not hold that knowledge is impossible in order to apply the method of doubt. Indeed, Descartes applied methodic doubt to everything from God, to the external world, and even to himself, but ultimately concluded that he could be certain of each. For example, Descartes' attempt to apply the method of doubt to the existence of himself spawned the proof of his famous saying, "Cogito ergo sum" (I think, therefore I am). That is, Descartes tried to doubt his own existence, but found that even his doubting showed that he existed, since he could not doubt if he did not exist.