The problem of the unity of the mind falls within the general area of metaphysics and ontology. Problems concerning identity over time have attracted most attention. Consider the case of Theseus's ship which has been sailing for some time, planks having been gradually replaced, until finally not a single piece of wood remains of all those used to construct it in the builder's yard. Does Theseus have the same ship? Yes: after all, our cells change every seven years. But then what if Professor X reconstructs the original ship from the rotten planks that Theseus has discarded? Or, again, a skilled neurosurgeon many years in the future clears all the memories from the brain of a condemned man and replaces them with the memories I have now. Has the condemned man paid the ultimate penalty? Have I travelled in time?