The problem is that of what makes the identity of the single person at a time or through time. To take the latter first, we can each imagine ourselves as having been rather different, or as becoming rather different, as indeed we will in the normal course of life. What makes it the case that I survive a change, that it is still me at the end of it? It does not seem necessary that I should retain the body I now have, since I can imagine my brain transplanted into another body, and I can imagine another person taking over my body, as in multiple personality cases. But I can also imagine my brain changing either in its matter or its function while it goes on being me that is thinking and experiencing, perhaps less well or better than before. My psychology might change, so its continuity seems only contingently connected with my own survival. So, from the inside, there seems nothing tangible making it I myself who survives some sequence of changes. The problem of identity at a time is similar: it seems possible that more than one person (or personality) should share the same body and brain, so what makes up the unity of experience and thought that we each enjoy in normal living? The problems of personal identity were first highlighted in the modern era by Locke, who recognized that the idea that the sameness of a person might consist in the sameness of underlying mental substance, the solution proposed by Descartes, was incapable of providing any criterion for use in the ordinary empirical world, for instance in connection with the just attribution of responsibility for past action. Locke’s own solution lay in the unity of consciousness, and in particular in the presence of memory of past actions; this account has been criticized as either circular, since memory presupposes identity, or insufficiently consonant with normal practice, since people forget things that they themselves did. The unity of the self failed to survive the scrutiny of Hume, whose own theory that the unity consisted in a kind of fiction (perhaps like that of a nation or a club, whose existence through time is not an all-or-nothing affair) was one of the few parts of his philosophy with which he declared himself dissatisfied. The organizing principle behind the unity of consciousness was a central element in Kant’s reaction to the sensational atomism of the empiricists. Contemporary philosophy contains successors of all these attacks on the problem.