For those of you who have studied Descartes then you may know that he is the author of this famous quote: "Cogito, ergo sum" (I think, therefore I am). Descartes determined that whatever he could bring himself to doubt, he would, until he saw reason to no longer doubt. He came to realize that they only thing he could be sure of was his own existence. Doubting his own existence was proof that he did, in fact, exist.

Descartes was sure of his own existence, but could not be sure of others. As far as he could prove (or not prove), nothing outside of his mind existed. With this in mind (pun intended), how does that make you feel? It's a fundamental philosophical problem which really can't be proved. I can't prove that the world outside of my mind really exists. For the sakes of simplicity I accept that it does, perhaps for the sake of my sanity too. To think that the world outside of my body is simply created by my brain is quite disturbing.