Version: v1, Published online: 1998
Retrieved May 21, 2019, from https://www.rep.routledge.com/articles/thematic/personal-identity/v-1
1. Criteria of personal identity
What is it to be a person? What is it for a person at one time to be identical to some person at a later time? Although the two questions are obviously related, my concern in this entry will be with the second question. (For more on the first question, see Persons; Mind, bundle theory of.) However, I assume this much about what it is to be a person: a person is a rational and self-conscious being, with a (more or less) unified mental life. There are indeed cases (multiple personality, split-brain patients, and so on) in which the apparent lack of mental unity casts doubt on whether a single person occupies a given body (see Split brains). But such cases are exceptional. A normal person is a mentally unified individual. The central question of personal identity is the question of what distinguishes the sorts of changes we mentally unified individuals can survive from the sorts of changes which constitute our death.
On one very familiar view (associated with Plato, Descartes and the Christian tradition) a person can survive bodily death. Bodily death is not the sort of change which constitutes personal death. On this view, a person is an immaterial (that is, non-spatial) soul, only contingently attached to a physical body (see Soul, nature and immortality of the; Soul in Islamic philosophy). This view has few philosophical adherents today. It is fraught with metaphysical and epistemological difficulties. (For example: how can an immaterial soul interact with the material world? How can I know that you have a soul?) In what follows I simply assume, without further argument, that our continued existence is not the continued existence of an immaterial soul.
I do not have to deny that in some possible worlds, there are persons who are immaterial souls; but ours is not such a world. And our concern here is with the conditions for the identity over time of actual (human) persons. Once we have given up the immaterialist view of ourselves, we can say the following. A person is a psychophysical entity, which is essentially physically embodied. That is a person (a typical adult human, for example) consists of a biological organism (a human body), with a control centre (the brain) that supports their mental life. Persons are essentially mental, and essentially physically embodied. But this is not the end of puzzles about personal identity; it is just the beginning.
When we judge that a friend before us now is identical to the friend we saw yesterday, we typically make this judgment of personal identity under optimal conditions. In such a case, our friend today is physically continuous with our friend yesterday (they possess the very same brain and body). And our friend today is also psychologically continuous with our friend yesterday (they possess the very same beliefs, character, desires, memories, and so on, with only very slight changes). In this case, our identity judgment is true in virtue of the obtaining of both physical and psychological continuities. The puzzle of personal identity is: which continuity (if any) is the more important or central to our concept of personal identity? Evidently, reflection on the paradigm case just described will not help us to answer that question. We need to consider thought experiments where the continuities come apart.
There are three broad accounts or criteria of personal identity over time: the physical criterion, the psychological criterion, and the mixed criterion. These criteria do not purport just to offer quite general ways of telling or of finding out who is who. They also purport to specify what the identity of persons over time consists in: what it is to be the same person over time. According to the physical criterion, the identity of a person over time consists in the obtaining of some relation of physical continuity (typically either bodily continuity or brain continuity). On this view, to be the same person is to be the same living biological object (whether body or brain).
According to the psychological criterion, the identity of a person over time consists in the obtaining of relations of psychological continuity (overlapping memory chains, or memory together with the retention of other psychological features such as well-entrenched beliefs, character, basic desires, and so on). The psychological criterion splits into a narrow version and a wide version. According to the narrow version, the cause of the psychological continuity must be normal (that is, the continued existence of one’s brain) if it is to preserve personal identity; according to the wide version, any cause will suffice (normal or abnormal). (See Parfit 1984 ch. 10 for more on this distinction.) Sub-versions of the wide and narrow versions differ over the question of whether any one psychological relation is privileged with respect to identity preservation. (For example, John Locke thought that memory was such a privileged relation.)
Each of the physical and psychological criteria divides into many different versions. The distinctive claim of the mixed criterion is that no version of either the physical or psychological criterion is correct. The best account of a person’s identity over time will make reference to both physical and psychological continuities.
I now want to examine in more detail different versions of the physical and psychological criteria. My conclusion will be that all familiar versions of these criteria are open to objection, and that we should accept the mixed criterion. The mixed criterion best captures our core (that is, minimally controversial) beliefs about personal identity.
We can begin with the physical criterion. As noted, this criterion divides into two criteria: the bodily criterion and the brain criterion.
Garrett, Brian. Criteria of personal identity. Personal identity, 1998, doi:10.4324/9780415249126-V024-1. Routledge Encyclopedia of Philosophy, Taylor and Francis, https://www.rep.routledge.com/articles/thematic/personal-identity/v-1/sections/criteria-of-personal-identity.
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