"I think therefore I am" proves nothing to others. It's merely a proof of one's own existence. It's philosophically impossible to prove that anything tangible actually exists outside of the observer's mind. If I were to believe that all of existence is within my own mind, I wouldn't be "wrong" as it is impossible to prove otherwise. So what I'm saying is, I can't prove I'm real. I can't prove anything is real. I can prove that I exist though. Even if I am within a mind or a virtual world, I am forming these words, so "I" exist. What "I" am or whether "I" am something real is not provable even within the mind, things would still exist. For example, if I am not "real" and only within your mind, then I still exist but only within your mind as you can respond to my stimuli and interact with me. Even if I am "fake" in that sense, I still exist even if only in your mind. Do you see what I'm saying? I can't prove that I'm a "real" tangible, independent human being, but by merely interacting with you, I'm proving that I exist. There you go