While many areas of philosophy are concerned with issues of personal identity, the investigation most usually referred to as ‘the problem of personal identity’ within analytic philosophy centers on the question of what makes individuals at different times the same person. This is a complex and difficult question because we change a great deal over the course of our lives. A woman of 50, for instance, is made up of largely different matter from her ten-year-old self, and looks quite different. Her beliefs, desires, and values have probably changed a great deal; she has a host of memories and relationships that her ten-year-old self did not have, and she fills quite different social roles. Despite all of this we might unequivocally judge that the woman before us is the same person as the ten-year-old. Philosophers of personal identity seek to describe what it is that constitutes the identity of the fifty-year-old and the ten-year-old (if they are indeed identical).
As it is usually conceived, the question of personal identity is a metaphysical question and not an epistemological question. Rather than asking how we know when someone at one time is identical to someone at another time, it asks what it is that actually makes it the case that they are the same. This question is also a question of numerical identity rather than qualitative or psychological identity; it is about the relation that makes something the self-same entity over time rather than about what makes entities indistinguishably similar to one another (see Identity). This last distinction is important to make because in everyday speech talk of personal identity is often connected to questions about what someone truly believes or desires, or what is fundamentally important to them, and not about what makes them a single entity. Everyday talk of identity is thus connected to judgments about similarity of character or personality.
Historically, there have been three main approaches to addressing the metaphysical question about the numerical identity of persons over time. 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 bodily or biological continuity, although there have been several other approaches offered as well. All of these accounts have had their adherents, and all have their difficulties.
The bulk of philosophical discussion of personal identity during the late twentieth and early twenty-first centuries has focused on the relative merits of psychological and biological approaches. For most of this period psychological accounts were dominant. These views, inspired by John Locke, 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.
Toward the end of the twentieth century, biological accounts of identity 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. A metaphysical view in which persons are constituted by human animals has also been offered to show a way in which a psychological account of identity can avoid the difficulties with explaining the relation of persons to human animals uncovered by animalists.
As the debate between animalists and psychological theorists has continued, a variety of other views have been put forward, including narrative accounts of identity and minimalist accounts which place identity in the continuation of bare sentience. Over time a number of interesting general questions.
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