- Identity (philosophy)
In philosophy, identity, from Latin: identitas (“sameness”), is the relation each thing bears just to itself.  According to Leibniz's law two things sharing every attribute are not only similar, but are the same thing. The concept of sameness has given rise to the general concept of identity, as in personal identity and social identity.
An entity can only be fully identical with itself. Any difference gives rise to a separate identity. Thus identity is whatever makes an entity definable and recognizable, in terms of possessing a set of qualities or characteristics that distinguish it from other entities. In layman's terms, identity is whatever makes something the same or different.
Logic of identity
In logic, the identity relation (also called "equality") is normally defined as the binary relation that holds only between a thing and itself. That is, identity is the two-place predicate, "=", such that for all x and y, "x = y" is true if x is the same thing as y. Identity is transitive, symmetric, and reflexive. It is an axiom of most normal modal logics that for all x and y, if x = y then necessarily y = x. That is, identity does not hold contingently, but of necessity.
Put in these terms (above), it can seem rather trivial, but the point, of course, is that the same number, object, or person, can have different "guises", and it is far from trivial if two or more guises refer to the same same number, object, or person. For example, 1+1=2 because '1+1' and '2' are different "guises" for the same number. Similarly, for personal identity over time, "me today" and "me yesterday" are different guises for the same person.
Metaphysics of identity
Metaphysicians, and sometimes philosophers of language and mind, ask other questions:
- What does it mean for an object to be the same as itself?
- If x and y are identical (are the same thing), must they always be identical? Are they necessarily identical?
- What does it mean for an object to be the same, if it changes over time? (Is applet the same as applet+1?)
- If an object's parts are entirely replaced over time, as in the Ship of Theseus example, in what way is it the same?
The Law of identity originates from classical antiquity. The modern formulation of identity is that of Gottfried Leibniz, who held that x is the same as y if and only if every predicate true of x is true of y as well.
Leibniz's ideas have taken root in the philosophy of mathematics, where they have influenced the development of the predicate calculus as Leibniz's law. Mathematicians sometimes distinguish identity from equality. More mundanely, an identity in mathematics may be an equation that holds true for all values of a variable. Hegel argued that things are inherently self-contradictory and that the notion of something being self-identical only made sense if it were not also not-identical or different from itself and did not also imply the latter. In Hegel's words, "Identity is the identity of identity and non-identity." More recent metaphysicians have discussed trans-world identity -- the notion that there can be the same object in different possible worlds. An alternative to trans-world identity is the counterpart relation in Counterpart theory. It is a similarity relation that rejects trans-world individuals and instead defends an objects counterpart - the most similar object.
Qualitative versus numerical identity
Arbitrary objects a and b can be said to be qualitatively identical if a and b are duplicates, that is, if a and b are exactly similar in all respects, that is, if a and b have all qualitative properties in common. Examples of this might be two wine glasses made in the same wine glass factory on the same production line (at least, for a relaxed standard of exact similarity), or a carbon atom in one's left hand and a carbon atom in one's right hand.
Alternatively, a and b can be said to be numerically identical if a and b are one and the same thing, that is, if a is b, that is, if there is only one thing variously called "a" and "b". For example, Clark Kent is numerically identical with Superman in the sense that there is only one person (who happens to wear different clothes at different times). This relationship is expressed in mathematics with the "=" symbol, e.g., a = b, or Clark Kent = Superman.
- Braddon-Mitchell, D.; Miller, K. 2004: How to be a conventional person. Monist, 87(4): 457-474.
- Gallois, A. 1998: Occasions of identity. Oxford: Oxford University Press. ISBN 0198237448 Google books
- Parfit, D. 1984: Reasons and persons. Oxford: Oxford University Press. ISBN 0-19-824908-X Google books
- Robinson, D. 1985: Can amoebae divide without multiplying? Australasian journal of philosophy, 63(3): 299–319. doi:10.1080/00048408512341901
- Sidelle, A. 2000: [Review of Gallois (1998)]. Philosophical review, 109(3): 469-471. JSTOR
- Sider, T. 2001: [Review of Gallois (1998)]. British journal for the philosophy science, 52(2): 401-405. doi:10.1093/bjps/52.2.401
- Zelenak, L. 2009: Tax policy and personal identity over time. Tax law review, 62(3): 333-375.
- General Information
- Stanford Encyclopedia of Philosophy: Identity, First published Wed Dec 15, 2004; substantive revision Sun Oct 1, 2006.
- Stanford Encyclopedia of Philosophy: Identity over time. First published Fri 18 March 2005.
- Stanford Encyclopedia of Philosophy: Personal identity. First published Tue Aug 20, 2002; substantive revision Tue Feb 20, 2007.
- Stanford Encyclopedia of Philosophy: Relative identity. First published Mon 22 April 2002.
- Erich Fromm Interview Excerpt. youtube.com.
- ^ The Cambridge Dictionary of Philosophy, 2nd Edition, CUP: 1995
- ^ Stanford Encyclopedia of Philosophy: Identity, First published Wed Dec 15, 2004; substantive revision Sun Oct 1, 2006.
- ^ T. Thompson (ed.) and S. M. Black (ed.) (2006), Forensic Human Identification: An Introduction. CRC Press. ISBN 0849339545
- ^ This includes operational definition that either yields a yes or no value for whether a thing is present in a field of observation, or that distinguishes the thing from its background, allowing one to determine what is and what is not included in it. Also see pattern recognition.
- Books and publications
- Andrew Bowie, Schelling and modern European philosophy. Routledge. Page 55-90. ISBN 0415103460
- James, W., & Perry, R. B. (2006). Essays in radical empiricism. New York: Longmans, Green, and co. Page 134, 197, 202. (lib.uchicago.edu)
- MacVannel, J. A. (1967). Hegel's doctrine of the will. New York: AMS Press.
- Hegel, G. W. F., & Sterrett, J. M. (1893). The ethics of Hegel; translated selections from his "Rechtsphilosophie,". Boston: Ginn and Co.
- Baldwin, J. M. (1913). History of psychology; a sketch and an interpretation. A history of the sciences. New York and London: G.P. Putnam's Sons.
- Dessoir, M. (1912). Outlines of the history of psychology. New York: The Macmillan company.
- Shaw, C. G. (1908). The precinct of religion in the culture of humanity. London: S. Sonnenschein.
- Alexander, A. B. D. (1907). A short history of philosophy. Glasgow: J. Maclehose and Sons.
- MacVannel, J. A. (1905). The educational theories of Herbart and Froebel. New York: Teachers college, Columbia University.
- Schade, A., & Rocholl, R. (1899). The philosophy of history. Cleveland, O.: A. Schade. Page 140 - 142.
- Külpe, O. (1897). Introduction to philosophy: a handbook for students of psychology, logic, ethics, æsthetics and general philosophy. London: S. Sonnenschein.
- Courtney, W. L. (1895). Constructive ethics, a review of modern moral philosophy in its three stages of interpretation, criticism, and reconstruction. London: Chapman and Hall.
- Manning, Jacob Merrill (1872). Half truths and the truth. Oxford University.
- Paksoy, H.B. (2001) IDENTITIES: How Governed, Who Pays? Florence: European University/Carrie.
- Friedrich Wilhelm Joseph Schelling, Baruch Spinoza, Karl Christian Friedrich Krause, Paul Ricœur, Gottlob Frege
- Being, The Golden Rule, Personal identity, Shunyata, Pseudonymity
- Recognition of human individuals, Cultural identity, Digital identity, Ethnic identity, Social identity, Reputation, Online identity, Identity theft, Counterpart theory
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