Cogito ergo sum. Scholars would recognize this issue of high debates even today, almost five hundred years after the words were uttered; though in the present most people in the world wouldn't understand what it refers to. In 1641, RenÃÂ© Descartes published his "Meditations on First Philosophy," in which his first meditation dismisses everything that he perceives as reality and posits that he is nothing more than a disembodied brain being manipulated by some outside source. In his second meditation, he questions if he is real. And the fact that he is able to ponder his reality leads him to the conclusion that "I think therefore I am": Cogito ergo sum.
His first meditation came to an end with the realization that there was no way to justify his sensual observations. He proceeds to say that in that case he will regard himself "as not having hands, eyes, flesh, blood, and senses- but as having the false belief" that he has all those things.
However, in his second meditation, he questions his knowledge of everything beyond his sensory perception. He begins to argue with himself about if he actually exists. Does he have a body? But his first meditation dictates that he has no body. But if he has no body or vessel, does that mean he doesn't exist? He concludes that he must exist because someone is constantly deceiving him. And if he is constantly being deceived, then there must be something that exists which is being deceived.
I find that Descartes' theories and meditations provide a fallacy of information for the student. If one were to watch the movie Abres Los Ojos, a psychiatrist who helps the protagonist to find out the truth about his own reality. In the end, the movie-watcher finds out...