 Evidence. How do we find out who is who? What evidence bears on the question of whether the person here now is the one who was here yesterday? One source of evidence is first-person memory: if you remember doing some particular action (or seem to), and someone really did do it, this supports the claim that that person is you. Another source is physical continuity: if the person who did it looks just like you, or even better if she is in some sense physically or spatio-temporally continuous with you, that too is reason to think she is you. Which of these sources is more fundamental? Does first-person memory count as evidence all by itself, for instance, or only insofar as we can check it against publicly available physical facts? What should we do when they support opposing verdicts? Suppose Charlie’s memories are erased and replaced with accurate memories (or apparent memories) of the life of someone long dead—Guy Fawkes, say (Williams 1956–7). Ought we to conclude, on the basis of memory evidence, that the resulting person is not Charlie but Guy Fawkes brought back to life, or should we instead infer on the basis of physical continuity that he is simply Charlie with different memories? What principle would answer this question?