In philosophy, the issue of personal identity[1] concerns several loosely related issues, in particular persistence,[note 1] change,[note 2] sameness,[note 3] and time. Personal identity is the distinct personality of an individual and is concerned with the persisting entity particular to a given individual. The personal identity structure[2][note 4] appears to preserve itself from the previous version in time when it is modified. It is the individual characteristics arising from personality by which a person is recognized or known.
b. Personhood
While the formal properties of the concept of identity are necessary constraints on our discussion, the truth of our identity judgments is subject to material conditions of correctness, which these formal properties cannot provide. These material conditions must be supplied by the nature of the relata judged to stand in an identity relation. The obvious suggestion is that, given that we are dealing with personal identity, these relata are person-stages located at different times. This proposal, however, violates the requirement that the persistence question ought to specify its relata without presupposing an answer: should we choose to accept a definition in the vicinity of Locke's characterization of a person as a "thinking, intelligent being, that has reason and reflection, and can consider itself as itself, the same thinking thing in different times and places" (1689, II.xxvii.9), then those criteria of personal identity that sanction the identity of a person at one time with a non-person at another time are categorically ruled out. Fetuses, infants, or human beings in a persistent vegetative state, for example, plainly do not fulfill the criteria envisaged by Locke. As a result, since these beings do not possess cognitive capacities, if they do at all, that qualitatively attain those of thinking beings, couching the persistence question in terms of persons entails that none of us has ever been a fetus or infant or ever will be a human vegetable (Olson 1997a; Mackie 1999). To be sure, these initially baffling claims could be true. However, since these are clearly substantial questions about our persistence, we should not consider ourselves justified to settle the matter by definition. Consequently, we should prefer vagueness over chauvinism and pose the persistence question in terms of the wider notion of human being, postponing the question of whether and in what sense the notions of person and human being ought to be distinguished: for any person X and any human being Y at different times t1 and t0/t2, if X at t1 is numerically identical with Y at t0/t2, what makes this claim necessarily true?
Generally, it is the unique numerical identity of persons through time.[3][4] That is to say, the necessary and sufficient conditions under which a person at one time and a person at another time can be said to be the same person, persisting through time.[note 5] In the modern philosophy of mind, this concept of personal identity is sometimes referred to as the diachronic problem[note 6] of personal identity.[5] The synchronic problem[note 7] is grounded in the question of what features or traits characterize a given person at one time.
Identity is an issue for both continental philosophy and analytic philosophy. A question in continental philosophy is in what sense can the contemporary conception of identity be maintained, while many prior propositions, postulates, and presuppositions about the world are different.
4. Parfit and the Unimportance of Personal Identity
Derek Parfit proposes a theory of the ontological status of persons, which promises to answer the problem of fission and the paradox of personal identity. While this article cannot do justice to the complexities of Parfit's theory, which has been the focal point of debate since 1970, it is worth mentioning its main features.
Although Parfit affirms the existence of persons, their special ontological status as non-separately-existing substances can be expressed by the claim that persons do not have to be listed separately on an inventory of what exists. In particular, persons themselves are distinct from their bodies and psychologies, but the existence of a person consists in nothing over and above the existence of a brain and body and the occurrence of an interrelated series of mental and physical events. These are the foundational claims of Parfit's constitutive reductionism. Consider an analogy: Cellini's Venus is made of bronze. Although the lump of bronze and the statue itself surely exist, these objects have different persistence conditions: if melted down, Venus ceases to exist while the lump of bronze does not. Therefore, they are not identical; rather, so the suggestion, the lump of bronze constitutes the statue. The same is true of persons, who are constituted by, but not identical with, a physiology, a psychology, and the occurrence of an interrelated series of causal and cognitive relations.
Now, how does this relate to the fission case? We must first note that Parfit believes (i) that our persistence consists in physical and/or psychological continuity; (ii) that personal identity is indeterminate in some cases, that is, that sometimes there is no right-or-wrong answer to the question of whether somebody has ceased to exist in the course of a certain adventure (see 3.b.); (iii) that what prudentially matters in survival is psychological continuity; (iv) that personal identity relations must respect the remaining formal properties of identity. This means that in the fission case Y1 and Y2 cannot be identical with X because the transitivity of identity is violated: therefore, X dies in the fission case. It further means, however, that X has two Parfitian survivors, Y1 and Y2, which is, according to Parfit, as good (or even better) than being identical with Y1 and/or Y2. This is the upshot of Parfit's claim that what prudentially matters is psychological continuity: for all we should care, from a purely rational point of view, it is good enough for us to be psychologically continuous with one or more future persons and consequently it would be irrational for us to prefer our own continued existence to death by fission. Generally, according to Parfit, psychological continuity with any reliable cause matters in survival, and since personal identity does not consist merely in psychological continuity with any reliable cause, personal identity is not what matters in survival.