Personal identity is the unique numerical identity of persons through time. 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. In the modern philosophy of mind, this concept of personal identity is sometimes referred to as the diachronic problem of personal identity. The synchronic problem 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 key question in continental philosophy is in what sense we can maintain the modern conception of identity, while realizing many of our prior assumptions about the world are incorrect.
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One basic concept of personal persistence over time is simply to have continuous bodily existence. However, as the Ship of Theseus problem illustrates, even for inanimate objects there are difficulties in determining whether one physical body at one time is the same thing as a physical body at another time. With humans, over time our bodies age and grow, losing and gaining matter, and over sufficient years will not consist of any of the matter they once consisted of. It is thus problematic to ground persistence of personal identity over time in the continuous existence of our bodies.
Nevertheless, this approach has its supporters. Eric Olson gives a definition of a human as a biological organism and asserts that a psychological relation is not necessary for personal continuity. Olson's personal identity lies in life-sustaining processes instead of bodily continuity. This biological approach squares with many other psychological accounts of personal identity but does not fall into common metaphysical traps.
Derek Parfit presents a thought experiment designed to bring out our intuitions about the corporeal continuity. This thought experiment discusses cases in which a person is teletransported from Earth to Mars. In the one case, the person enters the teletransporter and has each molecule of his body disassembled, teletransported to Mars, and then reassembled. In another case the person enters the teletransporter where that person’s body is destroyed while all the exact states of that person’s cells are recorded. This information is then teletransported to Mars, where another machine uses organic material to produce a perfect copy of that person’s body. The question is whether in either of these cases the person on Mars is identical to the person on Earth. Suppose that these two cases are just the furthest opposite points on a spectrum. In-between these two cases there are more cases in which an increase amount of the person on Mars is constituted of the numerically identical matter as the person on Earth. The question for that the criterion for personal identity becomes where on this spectrum does the person on Mars stop being identical to the person on Earth. Is it at 1, 51, or 99.9 percent? It appears that we are not able to draw a line. This inability appears to show that having a numerically identical physical body is not the criterion for personal identity.
In a dualist concept of mind, a person's mind is considered to consist of an immaterial substance, separate from and independent from the body. If a person is then identified with their mind, rather than their body — if a person is considered to be their mind — and their mind is such a non-physical substance, then personal identity over time may be grounded in the persistence of this non-physical substance, despite the continuous change in the substance of the body it is associated with. However, dualism is far from uncontroversial or unproblematic, and adopting it as a solution raises a host of other questions. The mind-body problem concerns the explanation of the relationship, if any, that exists between minds, or mental processes, and bodily states or processes. One of the aims of philosophers who work in this area is to explain how a supposedly immaterial mind can influence a material body and vice-versa.
Our perceptual experiences depend on stimuli which arrive at our various sensory organs from the external world and these stimuli cause changes in our mental states; ultimately causing us to feel a sensation, which may be pleasant, unpleasant, or neutral. Someone's desire for a slice of pizza, for example, will tend to cause that person to move their body in a specific manner and in a specific direction to obtain said pizza. The question, then, is how it can be possible for conscious experiences to arise out of a lump of grey matter endowed with nothing but electrochemical properties. A related problem is to explain how someone's propositional attitudes (e.g. beliefs and desires) can cause that individual's neurons to fire and his muscles to contract in exactly the correct manner. These comprise some of the puzzles that have confronted epistemologists and philosophers of mind from at least the time of René Descartes.
John Locke considered personal identity (or the self) to be founded on consciousness, and not on the substance of either the soul or the body. Book II Chapter XXVII entitled "On Identity and Diversity" in An Essay Concerning Human Understanding (1689) has been said to be one of the first modern conceptualizations of consciousness as the repeated self-identification of oneself. Through this identification, moral responsibility could be attributed to the subject and punishment and guilt could be justified, as critics such as Nietzsche would point out.
According to Locke, personal identity (the self) "depends on consciousness, not on substance" nor on the soul. We are the same person to the extent that we are conscious of our past and future thoughts and actions in the same way as we are conscious of our present thoughts and actions. If consciousness is this "thought" which "that goes along with the substance ... which makes the same person", then personal identity is only founded on the repeated act of consciousness: "This may show us wherein personal identity consists: not in the identity of substance, but... in the identity of consciousness". For example, one may claim to be a reincarnation of Plato, therefore having the same soul substance. However, one would be the same person as Plato only if one had the same consciousness of Plato's thoughts and actions that he himself did. Therefore, self-identity is not based on the soul. One soul may have various personalities.
Neither is self-identity founded on the body substance, argues Locke, as the body may change while the person remains the same. Even the identity of animals is not founded on their body: "animal identity is preserved in identity of life, and not of substance", as the body of the animal grows and changes during its life. On the other hand, identity of humans is based on their consciousness. Take for example a prince's mind which enters the body of a cobbler: to all exterior eyes, the cobbler would remain a cobbler. But to the prince himself, the cobbler would be himself, as he would be conscious of the prince's thoughts and acts, and not those of the cobbler. A prince's consciousness in a cobbler's body: thus the cobbler is, in fact, a prince.
But this interesting border-case leads to this problematic thought that since personal identity is based on consciousness, and that only oneself can be aware of his consciousness, exterior human judges may never know if they really are judging - and punishing - the same person, or simply the same body. In other words, Locke argues that you may be judged only for the acts of your body, as this is what is apparent to all but God; however, you are in truth only responsible for the acts for which you are conscious. This forms the basis of the insanity defense: one cannot be held accountable for acts from which one was unconscious - and therefore leads to interesting philosophical questions:
personal identity consists [not in the identity of substance] but in the identity of consciousness, wherein if Socrates and the present mayor of Queenborough agree, they are the same person: if the same Socrates waking and sleeping do not partake of the same consciousness, Socrates waking and sleeping is not the same person. And to punish Socrates waking for what sleeping Socrates thought, and waking Socrates was never conscious of, would be no more right, than to punish one twin for what his brother-twin did, whereof he knew nothing, because their outsides were so like, that they could not be distinguished; for such twins have been seen.
Or again:
PERSON, as I take it, is the name for this self. Wherever a man finds what he calls himself, there, I think, another may say is the same person. It is a forensic term, appropriating actions and their merit; and so belong only to intelligent agents, capable of a law, and happiness, and misery. This personality extends itself beyond present existence to what is past, only by consciousness, --whereby it becomes concerned and accountable; owns and imputes to itself past actions, just upon the same ground and for the same reason as it does the present. All which is founded in a concern for happiness, the unavoidable concomitant of consciousness; that which is conscious of pleasure and pain, desiring that that self that is conscious should be happy. And therefore whatever past actions it cannot reconcile or APPROPRIATE to that present self by consciousness, it can be no more concerned in it than if they had never been done: and to receive pleasure or pain, i.e. reward or punishment, on the account of any such action, is all one as to be made happy or miserable in its first being, without any demerit at all. For, supposing a MAN punished now for what he had done in another life, whereof he could be made to have no consciousness at all, what difference is there between that punishment and being CREATED miserable? And therefore, conformable to this, the apostle tells us, that, at the great day, when every one shall 'receive according to his doings, the secrets of all hearts shall be laid open.' The sentence shall be justified by the consciousness all person shall have, that THEY THEMSELVES, in what bodies soever they appear, or what substances soever that consciousness adheres to, are the SAME that committed those actions, and deserve that punishment for them.
Henceforth, Locke's conception of personal identity founds it not on the substance or the body, but in the "same continued consciousness", which is also distinct from the soul since the soul may have no consciousness of itself (as in reincarnation). He creates a third term between the soul and the body - and Locke's thought may certainly be meditated by those who, following a scientist ideology, would identify too quickly the brain to consciousness. For the brain, as the body and as any substance, may change, while consciousness remains the same. Therefore personal identity is not in the brain, but in consciousness. However, Locke's theory also reveals his debt to theology and to Apocalyptic "great day", which by advance excuse any failings of human justice and therefore humanity's miserable state. The problem of personal identity is at the center of discussions about life after death, and immortality. In order to exist after death, there has to be a person after death who is the same person as the person who died.
Bernard Williams presents a thought experiment appealing to our intuitions about what it is to be the same person in the future. The thought experiment consists of two approaches to the same experiment. For the first approach Williams suggests that we suppose that there is some process by which subjecting two persons to it can result in the two persons have “exchanged” bodies. The process has put into the body of person B the memories, behavioral dispositions, and psychological characteristics of the person who prior to undergoing the process belonged to person A; and conversely with person B. To show this we are to suppose that before undergoing the process person A and B are asked to which resulting person, A-Body-Person or B-Body-Person, they wish to receive a punishment and which a reward. Upon undergoing the process and receiving either the punishment or reward, it appears to us that A-Body-Person expresses the memories of choosing who gets which treatment as if that person was person B; conversely with B-Body-Person. This sort of approach to the thought experiment appears to show us that since we take the person who expresses the psychological characteristics of person A to be person A, then our intuition is that psychological continuity is the criterion for personal identity.
The second approach is to suppose that you are told that you will have your memories erased and then you will be tortured. Are you to be afraid of being tortured? The intuition is that you probably are afraid of being tortured, since it will still be you despite not having your memories. Next, we are asked to consider several similar scenarios. You have your memories erased, you are given new “fake” memories, and then you are to be tortured; You have your memories erased, you are given copies of another person's memories, and then you are to be tortured; You have your memories erased, you are given another person's genuine memories, and then you are to be tortured; You have your memories erased, you are given another person’s genuine memories, that person is given your memories, and then you are to be tortured. Our intuition is that in all these cases we are to be afraid of being tortured, that is it is still us despite having our memories erased and receiving new memories. However, the last scenario is an identical scenario to the one in the first approach. In the first case, our intuition shows us that our psychological continuity is the criterion for personal identity, but in second case, our intuition is that it is our bodily continuity that is the criterion for personal identity. To resolve this conflict Williams feels our intuition in the second approach is stronger and if he was given the choice of distributing a punishment and a reward he would want his body-person to receive the reward and the other body-person to receive the punishment, even if that other body-person has his memories.
In psychology, personal continuity, also called personal persistence, is the uninterrupted connection concerning a particular person of his or her private life and personality. Personal continuity is the union affecting the facets arising from personality in order to avoid discontinuities from one moment of time to another time. Personal continuity is an important part of identity; this is the process of ensuring that the qualities of the mind, such as self-awareness, sentience, sapience, and the ability to perceive the relationship between oneself and one's environment, are consistent from one moment to the next. Personal continuity is the property of a continuous and connected period of time and is intimately concerned with a person's body or physical being. Associationism, a theory of how ideas combine in the mind, allows events or views to be associated with each other in the mind, thus leading to a form of learning. Associations can result from contiguity, similarity, or contrast. Through contiguity, one associates ideas or events that usually happen to occur at the same time. Some of these events form an autobiographical memory in which each is a personal representation of the general or specific events and personal facts.
Similarly, ego integrity is the psychological concept of the ego's accumulated assurance of its capacity for order and meaning. Ego identity is the accrued confidence that the inner sameness and continuity prepared in the past are matched by the sameness and continuity of one's meaning for others, as evidenced in the promise of a career. Body and ego must be masters of organ modes{[clarify}} and of the other nuclear conflicts in order to face the fear of ego loss in situations which call for self-abandon.
David Hume undertook looking at the mind-body problem. Hume also investigated a person's character, the relationship between human and animal nature, and the nature of agency. Hume pointed out that we tend to think that we are the same person we were five years ago. Though we've changed in many respects, the same person appears present as was present then. We might start thinking about which features can be changed without changing the underlying self. Hume, however, denies that there is a distinction between the various features of a person and the mysterious self that supposedly bears those features. When we start introspecting, "we are never intimately conscious of anything but a particular perception; man is a bundle or collection of different perceptions which succeed one another with an inconceivable rapidity and are in perpetual flux and movement".[1]
It is plain that in the course of our thinking, and in the constant revolution of our ideas, our imagination runs easily from one idea to any other that resembles it, and that this quality alone is to the fancy a sufficient bond and association. It is likewise evident that as the senses, in changing their objects, are necessitated to change them regularly, and take them as they lie contiguous to each other, the imagination must by long custom acquire the same method of thinking, and run along the parts of space and time in conceiving its objects.[2]
Note in particular that, in Hume's view, these perceptions do not belong to anything. Hume, similar to the Buddha, compares the soul to a commonwealth, which retains its identity not by virtue of some enduring core substance, but by being composed of many different, related, and yet constantly changing elements. The question of personal identity then becomes a matter of characterizing the loose cohesion of one's personal experience. (Note that in the Appendix to the Treatise, Hume said mysteriously that he was dissatisfied with his account of the self, yet he never returned to the issue.)
In short, what matters for Hume is not that 'identity' exists but that the relations of causation, contiguity, and resemblances obtain among the perceptions. Critics of Hume might point out that in order for the various states and processes of the mind to seem unified, there must be something which perceives their unity, the existence of which would be no less mysterious than a personal identity.
Another view of personal identity is known as the no-self theory. According to this view the self cannot be reduced to a bundle because the concept of a self is incompatible with the idea of a bundle. This is because the idea of a bundle implies the notion of bodily or psychological relations that do not in fact exist. A principal exponent of this view is James Giles. Giles argues that the no-self or eliminativist theory and the bundle or reductionist theory are in agreement about the non-existence of a substantival self. The reductionist theory, however, makes the mistake of attempting to resurrect the idea of the self in terms of various accounts about psychological relations. The no-self theory, on the other hand, 'lets the self lie where it has fallen'.[3]This is because the no-self theory is a rejection of all theories of the self, even the bundle theory. On Giles' reading, Hume is actually a no-self theorist and it is a mistake to attribute to him a reductionist view like the bundle theory. This reading is supported by Hume's famous assertion that personal identity is a fiction. On this account the Buddhist view of personal identity is also a no-self theory rather than a reductionist theory. This is because the Buddha clearly rejects all attempts to reconstruct the self in terms of consciousness, feelings, or the body.[4]
Other positions state that the intuitive concept of self is an evolutionary artifact. In the free will model of consciousness the brain models its own unconscious processes just as it models other people. This modeling makes the assumption that the model will continue to apply through time, and so assumes they are the same person they were yesterday. This leads to the intuitive sense of self. The sense of ‘self’ has also become part of our language, part of our concept of responsibility, and the basis of self based morality.
The Buddha attacked all attempts to conceive of a fixed self, while stating that holding the view "I have no self" is also mistaken. This is an example of the middle way charted by the Buddha.