I can prove to myself that I exist (the classic "I think therefore I am"), but I can't prove to you that I exist just as you can't prove to me that you exist. For all I know, I'm God and I created this world for my amusement. Perhaps you exist in that world, but perhaps you don't (at least not as actual entities).
A person can only be sure of his own existance at the present moment, the rest is open to speculation.
Artemis923 said:
If I don't exist, how are you reading my post?
Easy, I wrote it. You don't exist except as my alter ego