If only one of us were real, who would it be, you or me? Would you be a figment of my imagination or would I be a figment of yours? Let's say I can prove the latter is the case. You would have to have dreamed up this person, me, who is calling into question your very existence, because if I am right that only you exist, your existence depends upon my being right. And if I don't really exist, that is a very tenuous platform to base your existence on. And if I happen to be wrong, then I exist and you don't. Therefore, you are probably safer to regard yourself as a figment of my imagination, because you stand just as much of a chance of my being wrong (and therefore your being real) as if I were a figment of yours. And this way, there's not as much chance for unpleasant surprises if you start out with no expectations of having any real existence.