The persistence question, the question of what personal identity over time consists in, is literally a question of life and death: answers to it determine, insofar as that is possible, the conditions under which we survive, or cease to exist in the course of, certain adventures. These adventures do not have to be theoretically as fancy as the cases, to be discussed later, of human fission or brain swaps: a theory of personal identity tells us whether we can live through the acquisition of complex cognitive capacities in our development from fetus to person, or whether we have survived car accidents if we find ourselves in a persistent vegetative state. Furthermore, theories of personal identity have ethical and metaphysical implications of considerable magnitude: in conjunction with certain normative premises they may support the justification or condemnation of infanticide or euthanasia, or they could prove or falsify certain aspects of our religious outlook, in deciding the questions of how and whether we can be resurrected and whether we are possessors of souls whose existence conditions are identical with ours. It is not surprising, therefore, that most great philosophers have attempted to solve the problem of personal identity, or have committed themselves to metaphysical systems that have substantial implications with regards to the problem, and that most religious belief systems give explicit answers to the persistence question. Neither is it surprising that virtually everybody holds a pre-theoretical theory of personal identity, if only in the sense of having beliefs about afterlives and the meaning of death. The task of solving the metaphysical problem of personal identity essentially involves answering the question of how the phenomenon or principle in virtue of which "entities like us" persist through time is to be specified, under the widely but not universally accepted premises that there is such a phenomenon or principle and that it can be specified. We are concerned, in other words, with the truth-makers of personal identity statements: what makes it true that our statement that an entity X at time t1 and an entity Y at time t2 are identical, if X and Y are entities like us?