Personal identity
Authors: Schechtman Marya 
Published: September 2018

The ‘problem of personal identity’ as it is usually construed in philosophy is a special case of more general questions about the identity of objects over time. There are a variety of puzzles about how a single object can persist through change, and these puzzles run especially deep in the case of persons, due to their complexity and the number of dimensions along which they can alter. More concretely, those working on the problem of personal identity investigate the question of what makes a woman of 50 the same person as the girl she was at 15, despite the fact that she may resemble that girl in very few respects. This question is a metaphysical question about numerical identity, aimed at defining the necessary and sufficient conditions for the literal persistence of one and the same entity through time. It is thus distinguished from epistemological questions which ask what evidence we use to make judgements of personal identity, and also from psychological or sociological questions which ask about identity in the sense of the stability of a unique personality or character.
Historically there have been three main answers to the problem of personal identity. One defines identity in terms of the continuation of a single immaterial substance or soul; one in terms of psychological continuity; and one in terms of biological continuity. All of these views have had their adherents, but it is now widely accepted that sameness-of-soul views have too many difficulties to provide a truly tenable account of personal identity, and most recent discussions of personal identity focus on the relative merits of psychological and biological accounts.
For much of the latter half of the twentieth century psychological accounts represented by far the predominant approach to questions of personal identity. These views, inspired by John Locke’s claim that personal identity should be defined in terms of sameness of consciousness rather than sameness of substance, hold that a person at time t2 is the same as a person at earlier time t1 just in case there is an overlapping chain of psychological connections (memories, beliefs, desires, etc.) between the person at t2 and the person at t1. They have a great deal of intuitive appeal, capturing the widely held sense that if biological and psychological continuity were to diverge, the person would go where the psychological life goes, but they have also been subject to some important objections. Many of these are related to the fact that psychological continuity does not have the same logical form as identity. For instance, a person existing now could in principle be psychologically continuous with two people in the future, but cannot be identical to both of them since they are not identical to each other.
Biological accounts of identity have recently re-emerged with new vigour, mounting a serious challenge to the dominance of psychological accounts. Defenders of the biological approach say that we are, most fundamentally, human animals who persist as long as a single human organism does. The biological approach allows that psychological continuity may be of tremendous importance to us, and that we may ‘identify’ with our psychological states, but insists this continuity is no part of what determines our literal persistence as single entities. Biological theorists point out that if we think of persons as entities distinct from human animals we will be left with a number of awkward questions about the relation between persons and animals, making psychological continuity theories deeply implausible. In response, defenders of the psychological approach have argued that biological accounts suffer from many of the same deficits with which they charge psychological theories. Also, constitution theorists have offered a new kind of psychological account that avoids the problems traditional psychological theorists face in explaining the relation of persons to human animals.
The debate between these psychological and biological approaches to identity is ongoing. In addition, several new kinds of account of personal identity have been defended, including minimalist views that place identity in the continuation of rudimentary psychological capacities and narrative views that see persons as narrative in form and define the unity of a person’s life as a narrative unity. Recently, the problem of personal identity has also played an important role in bioethical debates. This has led to a more general investigation of the relation between metaphysical questions of personal identity and practical judgements.