The cogito refers to Rene Desacrtes’ famous phrase, “I think therefore I am,” which is “cogito ergo sum” in the original Latin. The phrase is the answer to the question of how we know that we exist. Descartes is following a skeptical line of thinking which seeks to discover what we can really be sure of. Do we really exist? How can we be sure? What if the world is not real? What if we cannot rely on our senses? What if we are being tricked into thinking that the world is real? What if we are in the matrix? A skeptic will have doubts about all these questions. But, the one thing we cannot doubt is that we have doubts. And since there must be someone thinking these doubts, then that person must exist. So the very fact of doubting everything else is the proof of existence. I think, therefore I am.