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• Personal Identity: the (numerical) identity over time of persons. The question of what personal identity consists in is the question of what it is (what the necessary and sufficient conditions are) for a person existing at one time and a person existing at another time to be one and the same person. Here there is no question of there being any entity that is the “identity” of a person; to say that a person’s identity consists in such and such is just shorthand for saying that facts about personal identity, i.e. facts to the effect that someone existing at one time is the same as someone existing at another time, consist in such and such. (This should not be confused with the usage, common in ordinary speech and in psychology, in which persons are said to have identities and, sometimes, to seek, lose, or regain their identities, where one’s “identity” intimately involves a set of values and goals that structure one’s life.)
• The words ‘identical’ and ‘same’ mean nothing different in judgments about persons than in judgments about other things. The problem of personal identity is therefore not one of defining a special sense of ‘identical,’ and it is at least misleading to characterize it as defining a particular kind of identity. Applying Quine’s slogan “no entity without identity,” one might say that characterizing any sort of entity involves indicating what the identity conditions for entities of that sort are (so, e.g., part of the explanation of the concept of a set is that sets having the same members are identical), and that asking what the identity of persons consists in is just a way of asking what sorts of things persons are. But the main focus in traditional discussions of the topic has been on one kind of identity judgment about persons, namely those asserting “identity over time”; the question has been about what the persistence of persons over time consists in.
• What has made the identity (persistence) of persons of special philosophical interest is partly its epistemology and partly its connections with moral and evaluative matters. The crucial epistemological fact is that persons have, in memory, an access to their own past histories that is unlike the access they have to the histories of other things (including other persons); when one remembers doing or experiencing something, one normally has no need to employ any criterion of identity in order to know that the subject of the remembered action or experience is (i.e., is identical with) oneself. The moral and evaluative matters include moral responsibility (someone can be held responsible for a past action only if he or she is identical to the person who did it) and our concern for our own survival and future well-being (since it seems, although this has been questioned, that what one wants in wanting to survive is that there should exist in the future someone who is identical to oneself).
• The modern history of the topic of personal identity begins with Locke, who held that the identity of a person consists neither in the identity of an immaterial substance (as dualists might be expected to hold) nor in the identity of a material substance or “animal body” (as materialists might be expected to hold), and that it consists instead in “same consciousness.” His view appears to have been that the persistence of a person through time consists in the fact that certain actions, thoughts, experiences, etc., occurring at different times, are somehow united in memory. Modern theories descended from Locke’s take memory continuity to be a special case of something more general, psychological continuity, and hold that personal identity consists in this. This is sometimes put in terms of the notion of a “person-stage,” i.e. a momentary “time slice” of the history of a person. A series of person-stages will be psychologically continuous if the psychological states (including memories) occurring in later members of the series grow out of, in certain characteristic ways, those occurring in earlier members of it; and according to the psychological continuity view of personal identity, person-stages occurring at different times are stages of the same person, provided they belong to a single, non-branching, psychologically continuous series of person-stages.
• Opponents of the Lockean and neo-Lockean (psychological continuity) view tend to fall into two camps. Some, following Butler and Reid, hold that personal identity is indefinable, and that nothing informative can be said about what it consists in. Others hold that the identity of a person consists in some sort of physical continuity – perhaps the identity of a living human organism or the identity of a human brain.
• In the actual cases we know about (putting aside issues about non-bodily survival of death), psychological continuity and physical continuity go together. Much of the debate between psychological continuity theories and physical continuity theories has centered on the interpretation of thought experiments involving brain transplants, brain-state transfers, etc., in which these come apart. Such examples make vivid the question of whether our fundamental criteria of personal identity are psychological, physical, or both.
• Recently, philosophical attention has shifted somewhat from the question of what personal identity consists in to questions about its importance. The consideration of hypothetical cases of “fission” (in which two persons at a later time are psychologically continuous with one person at an earlier time) has suggested to some that we can have survival – or at any rate what matters in survival – without personal identity, and that our self-interested concern for the future is really a concern for whatever future persons are psychologically continuous with us.