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 same 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 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 philosophical 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 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. 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 (the human brain) possessing electrochemical properties. A related problem is to explain how propositional attitudes (e.g. beliefs and desires) 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. (The reader is supposed to substitute their own case for Hellie's.) 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
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