Once upon a time, from 1596 to 1650, there lived a great mathematician and philosopher by the name of Rene Descartes. Of his many discoveries, one is truly revolutionary. He established that he could doubt anything save his existence and the reality of God. He proved his existence in one brilliant statement: "Cogito ergo sum."
Having considered that phrase at length, I find that I doubt my existence nonetheless. This occurs because I am unsure if I think.
Thought has not been adequately defined, and, although impossible to prove or disprove, the necessity of physical existence for thought was never justified. In fact, most faiths deny it, and Descartes was a religious man.
The raison d'etre of Descartes' statement is not apparent; evidently his way of thought is far above ordinary mental monologues. I probably think in the ordinary sense of the word. What else but thought could mental discussions on topics like "Can I prove that nothing changes over time?" be called? This does not suffice, however, to prove to myself that I exist.
Thought is likely the only way in which it is possible to solve this problem. Nobody before or after Descartes thought of a better way to prove one's existence. If only thought, as Descartes used the word, is justification enough to claim existence, two conclusions seem probable:
A worried teenager sitting at his desk, hurriedly writing "... I do not think (the way Descartes did), and thus I do not exist." Just as the ink forming the period at the end of the sentence soaks into the paper, the youth vanishes as if he never was there. The event quickly becomes a sensation as the media spread the details over the whole globe. Then it is quickly forgotten... This finale is hardly satisfactory, for then the youth certainly would not go to the college of his choice.
Another conclusion of the story is possible. Some fifteen years later, for a hobby a computer science professor is sitting at his desk, working on a philosophical treatise on existence...