In “Fiction,” Robert Nozick suggests the possibility that he, Nozick, is a fictional character and so is everyone else, except for the one who is the author of the piece. My aim here is not to argue with “Fiction” as a whole, but only with aspects of it that are central to Descartes. Nozick says:

Nozick starts with the assumption that Hamlet is a fictional character. Then, of course, no matter what speech Shakespeare puts into Hamlet's mouth, including any proof of Hamlet's existence, it is not going to prove that Hamlet exists. Ex hypothesi, it could not prove to Hamlet that he exists. By analogy, then – so Nozick seems to suggest – if we, like Hamlet, are fictional, no proof of our existence that we can mouth will confirm us in our existence.