Personal identity
Personal identity is the unique identity of a person over time. Discussions regarding personal identity typically aim to determine the necessary and sufficient conditions under which a person at one time and a person at another time can be said to be the person, persisting through time.
In philosophy, the problem of personal identity is concerned with how one is able to identify a single person over a time interval, dealing with such questions as, "What makes it true that a person at one time is the same thing as a person at another time?" or "What kinds of things are we persons?"
In contemporary metaphysics, the matter of personal identity is referred to as the diachronic problem of personal identity. The synchronic problem concerns the question of what features and traits characterize a person at a given time. Analytic philosophy and continental philosophy both inquire about the nature of identity. Continental philosophy deals with conceptually maintaining identity when confronted by different philosophic propositions, postulates, and presuppositions about the world and its nature.
Continuity of substance
Bodily substance
One way to explain how persons persist over time is to say that identity consists in physical or bodily continuity. However, there are problems with this view. As the Ship of Theseus thought experiment 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 most of the matter they once consisted of. It is thus problematic to ground the persistence of personal identity over time in the continuous existence of our bodies. Nevertheless, this approach has its supporters, who define humans as biological organisms. They assert the proposition that a psychological relation is not necessary for personal continuity. This personal identity ontology assumes the relational theory of life-sustaining processes instead of bodily continuity.The teletransportation problem proposed by Derek Parfit is designed to bring out intuitions about corporeal continuity. The thought experiment discusses cases in which a person is teleported from Earth to Mars. Ultimately, the inability to specify where on a spectrum the transmitted person stops being identical to the initial person on Earth appears to show that having a numerically identical physical body is not the criterion for personal identity.
Christian List has argued against physicalist conceptions of consciousness and personal identity on the basis of how first-personal facts relate to third-personal facts. He argues that first-personal facts cannot supervene on third-personal facts, and that this refutes not only physicalism, but also most forms of dualism with purely third-personal metaphysics. List also argues that there exists a "quadrilemma" for theories of consciousness. He claims that at most three of the following metaphysical claims can be true: 'first-person realism', 'non-solipsism', 'non-fragmentation', and 'one world' – and thus at least one of these four must be false. List has proposed a model he calls the "many-worlds theory of consciousness" in order to reconcile the subjective nature of consciousness without lapsing into solipsism.
Mental substance
In another concept of mind, the set of cognitive faculties are considered to consist of an immaterial substance, separate from and independent of the body. If a person is then identified with their mind, rather than their body—if a person is considered to 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.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 non-material mind can influence a material body and vice versa.
This is controversial and problematic, and adopting it as a solution raises questions. Perceptual experiences depend on stimuli which arrive at various sensory organs from the external world and these stimuli cause changes in mental states; ultimately causing sensation. A desire for food, for example, will tend to cause a person to move their body in a manner and in a direction to obtain food. The question, then, is how it can be possible for conscious experiences to arise out of an organ possessing electrochemical properties. A related problem is to explain how propositional attitudes can cause neurons of the brain to fire and muscles to contract in the correct manner. These comprise some of the puzzles that have confronted epistemologists and philosophers of mind from at least the 16th century.
Some philosophers have used a form of indexicality as a way of defining the self. The philosopher Benj Hellie coined the phrase "the vertiginous question" to describe the question of why, of all the subjects of experience out there, this one—the one corresponding to the human being referred to as Benj Hellie—is the one whose experiences are live. Other philosophers have described similar phenomena. Tim S. Roberts refers to the question of why a particular organism out of all the organisms that happen to exist happens to be you as the "Even Harder Problem of Consciousness". Herbert Spiegelberg has referred to it as the "I-am-me experience", and it has been called the "Ich-Erlebnis" by German psychologists. Japanese philosopher Hitoshi Nagai defines the self as the "one who directly experiences the consciousness of oneself".
Continuity of consciousness
Locke's conception
considered personal identity to be founded on consciousness, and not on the substance of either the soul or the body. Chapter 27 of Book II of his Essay Concerning Human Understanding, entitled "On Identity and Diversity", 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 "depends on consciousness, not on substance" nor on the soul. We are the same person to the extent that we are conscious of the past and future thoughts and actions in the same way as we are conscious of present thoughts and actions. If consciousness is this "thought" which "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. 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.
This border case leads to this problematic thought that since personal identity is based on consciousness, and only oneself can be aware of one's consciousness, exterior human judges may never know if they are really judging—and punishing—the same person, or simply the same body. In other words, Locke argues that one may be judged only for the acts of the body, as this is what is apparent to all but God. We are only responsible for the acts of which we are conscious. This forms the basis of the insanity defense—one cannot be held accountable for acts of which one was unconscious—and therefore leads to philosophical questions:
Or again:
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. He creates a third term between the soul and the body. For Locke, the body may change, while consciousness remains the same. Therefore, personal identity, for Locke, is not in the body but in consciousness.
Philosophical intuition
presents a thought experiment appealing to the 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 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 one is 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 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 that since the person who expresses the psychological characteristics of person A to be person A, then intuition is that psychological continuity is the criterion for personal identity.
The second approach is to suppose that someone is told that one will have memories erased and then one will be tortured. Does one need to be afraid of being tortured? The intuition is that people will be afraid of being tortured, since it will still be one despite not having one's memories. Next, Williams asked one to consider several similar scenarios.
Intuition is that in all the scenarios one is to be afraid of being tortured, that it is still one's self despite having one's memories erased and receiving new memories. The last scenario is identical to the first.
In the first approach, intuition is to show that one's psychological continuity is the criterion for personal identity, but in second approach, intuition is that it is one's bodily continuity that is the criterion for personal identity. To resolve this conflict Williams feels one's 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.