In philosophy, personal identity refers to the essence of a self-conscious person, that which makes him or her unique. It persists: though a person may change in socially important aspects, such as religious belief, these modifications happen through one single identity.
The question regarding personal identity has addressed the conditions under which a person at one time is the same person at another time, known as personal continuity. This sort of analysis of personal identity provides a set of necessary and sufficient conditions for the identity of the person over 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.
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 non-material 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 or unpleasant. 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 (viz. Memory), and not on the substance of either the soul or the body. Chapter XXVII "On Identity and Diversity" in An Essay Concerning Human Understanding (1689) has been said to be one of the first modern conceptualization 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, Socrate 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."
"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.
David Hume undertook looking at the mind/body problem (and Mind/brain identity). 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".
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.
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' exist 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
In psychology (which historically is philosophically concerned with dualism), 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 quality of the mind are consistent from moment to the next, generally regarded to comprise qualities such as self-awareness, sentience, sapience, and the ability to perceive the relationship between oneself and one's environment. 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, or the method 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.
See main article: Id, ego, and super-ego. Ego integrity is 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 and of the other nuclear conflicts in order to face the fear of ego loss in situations which call for self-abandon.
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.
Other criticisms state that the intuitive concept of self is an evolutionary artifact. In the monkey-riding-a-tiger 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.
According to this line of criticism, the sense of self is an evolutionary artifact, which saves time in the circumstances it evolved for. But sense of self breaks down when considering some rare events such as memory loss, split personality disorder, brain damage, brainwashing, and various thought experiments.  When presented with these imperfections in the intuitive sense of self and the consequences to this important concept which rely partly on the strict concept of self, people tend to try to mend the concept, possibly because of cognitive dissonance. Critics of personal continuity believe that this leads to extending the concept of self beyond its practical application and justification.