Its all conjecture, like belief in a god. Except that in my world, he or you only exist only in my journey, not separate from my journey (a scenario which is non-existent). I read the book, and so he exists in my world- I probably don't exist in his world unless he reads philly speaks. It's another variation on the scientific theory of multiple universes, which some physicists feel they can prove is the case, via mathematics.