. Attribute of being a single thing or single kind. In ordinary speech two things may be called numerically identical (or one in number: ‘Persia and Iran are identical’), or identical (or one) in type or species (exactly similar, as with ‘identical twins’). Philosophers keep ‘identical’ for the first sense, using ‘indiscernible’ for the second (see LEIBNIZ’S LAW). But what is identity, in this first sense? Is it a relation between a thing and itself? If so, every true statement of identity should be trivial, or else senseless. Hume used time to solve the problem, saying that identity statements state that an object existing at one time is the same as itself existing at another, e.g. ‘This chair is the same as the one here yesterday.’ This could suggest that GENIDENTITY is in question, but anyway this covers only some cases. Suppose Smith is mayor of a certain town. Then ‘Smith is Smith’ is trivial but ‘Smith is the mayor’ is not, even though the words ‘the mayor’ refer to Smith. It was this that made Frege distinguish between sense and reference (see MEANING), saying that what gave content to an identity statement was the different ways in which the object was described.
A distinction exists between two approaches to identity statements. On a conceptualist approach one can only say ‘a is the same so-and-so as b’ where ‘so-and-so’ is a SORTAL term. On a realist approach ‘so-and-so’ can be replaced by a non-sortal term like ‘thing’ or ‘object’. The identity is here given, as it were, in the world itself and does not depend on the concepts we apply. It is a further question, however, whether identity is relative, in the sense that a might be the same so-and-so, but not the same such-and-such, as b, e.g. was Nixon the same official (namely the American president), though not the same man, as Eisenhower?
This distinction between two approaches may be relevant to various problems which arise because things persist in time, for they may persist for different periods if described in different ways. Suppose a gold coin melts. Then it seems that the coin is destroyed but the piece of gold is not. If the coin is the piece of gold, then the same thing seems to be destroyed and not destroyed. If the coin is not the piece of gold, then we seem to have two things in the same place at the same time (though not throughout the same time). Perhaps the gold is not identical with, but ‘constitutes’, the coin (Wiggins; cf. BEING).
Furthermore, a coin which melts is presumably destroyed, but a baby which grows up is not destroyed, though it stops being a baby. But what stops being a baby and lives to be eighty? And how long does the baby last? Eighty years? (Terms like ‘baby’ are called phase terms or phase universals.)
Also can ‘identity statements be contingent? ‘Smith is the mayor’ seems contingent. Yet ‘Smith is Smith’ seems necessary. See MODALITIES.
Sometimes a is called strictly identical with b if whatever can be said of one can be said of the other (INTENSIONALITY apart). Some would say that an adult is identical, but not strictly identical, with the baby he once was, on the grounds that the adult but not the baby could be called, for example, married: one can say, ‘The baby you knew is now married.’ But there are no married babies. ‘Strict’ or ‘perfect identity’ is also sometimes used for identity that cannot be reduced to GENIDENTITY.
Further problems concern the criteria of identity, both for objects and for events, properties, propositions, etc. Is the property red identical with that of reflecting or emitting light of certain wavelengths? Spatiotemporal continuity is an obvious criterion to use for objects, but a suit need not possess it (if trousers and coat are separated), and a sound or toothache can be intermittent. We must also ask, continuity of what? Not of matter, since a body, and still more a flame, are constantly changing their matter; and perhaps they change their shape and other properties too. Furthermore we must be able to individuate places and times themselves, i.e. tell when we have one and when another, if we are to use them to individuate objects. Particular attention has been given to the question of personal identity. What is a PERSON? How are persons, minds and bodies related? What role do things like memory and traits of character play?
Questions of identity are also important in aesthetics. How is Olivier related to Hamlet when it is true to say both that Olivier is now alone on the stage and that Hamlet is now alone on the stage?
On whether identity is intrinsic or extrinsic, and for the ‘ship of Theseus’ problem, see INTRINSIC AND EXTRINSIC.
To identify a with b is simply to claim, or assume, that a and b are identical. To identify a as b (or as a b) is to pick out a by either taking it to be identical with b or attributing b-type characteristics to it. I can identify Smith with a spy only if I already have some spy in mind, but I can identify him as a spy without this.
The law of identity, one of the traditional ‘laws of thought’, says that everything is what it is, or that if something is true, it is true. A proposition that is an instance of this law (e.g. ‘A cat is a cat’), or one that can be transformed into such an instance by applying to it the rules of logic (e.g. ‘If Tiddles is a cat, Tiddles does not fail to be a cat’), can be called identically true, and its negation identically false. See also LEIBNIZ’S LAW, IDENTITY THEORY OF MIND, GENIDENTITY.
D.Hume, Treatise, 1739, book 1, part 4, § 2 (pp. 200–1 in L.A. Selby-Bigge’s edition, Clarendon, 1888 (1946 reprint)).
S.Kripke, Naming and Necessity, Blackwell, 1980, original version 1972. (Discusses, inter alia, both identity across POSSIBLE WORLDS and the nature of identity.)
J.Locke, Essay concerning Human Understanding, 1690, part 2, chapter 27. (Pioneering discussion of personal identity. See discussions by A.Flew, ‘Locke and the problem of personal identity’, Philosophy, 1951, and (more difficult) D.Wiggins, ‘Locke, Butler, and the stream of consciousness: and men as a natural kind’, Philosophy, 1976, both reprinted in Perry, and for elaboration and defence of Locke’s view see C.Rovane, ‘Self-reference: the radicalization of Locke’, Journal of Philosophy, 1993.
M.K.Munitz (ed.), Identity and Individuation, New York UP, 1971. (Essays, including S.Kripke’s ‘Identity and necessity’, which is also in S.P.Schwartz (ed.), Naming, Necessity, and Natural Kinds, Cornell UP, 1977, and (with editorial introduction) in T.Honderich and M.F.Burnyeat (eds), Philosophy As It Is, Penguin, 1979.)
H.Noonan (ed.), Identity and Personal Identity, Dartmouth Publishing Co., 1993. (Two wide-ranging volumes of essays. See also his own Personal Identity, Routledge, 1989, which discusses historical and modern views while developing his own.)
J.Perry (ed.), Personal Identity, California UP, 1975. (Important discussions, historical and modern, though less full than Noonan.)
A.O.Rorty (ed.), The Identity of Persons, Cornell UP, 1976. (Readings.)
M.Schechtman, ‘Personhood and personal identity’, Journal of Philosophy, 1990. (Traits of character, memory, etc., are inadequate as criteria for grounding personal identity.)
S.Shoemaker, Self-Knowledge and Self-Identity, Cornell UP, 1963. (Extended discussion of personal identity. See pp. 36–8 for ‘strict’ or ‘perfect’ identity as not reducible to genidentity (though not so called).)
D.Wiggins, Sameness and Substance, Blackwell, 1980. (Discussion of identity, substance, and personal identity. Conceptualist but claims identity never relative. Cf. his ‘On being in the same place at the same time’, Philosophical Review, 1968 (rather easier), and symposium with M.J.Woods, ‘The individuation of things and places’, Proceedings of the Aristotelian Society, supplementary vol., 1963. On identity and constitution see also H.W.Noonan, ‘Constitution is identity’, Mind, 1993.)
B.A.O.Williams, ‘Personal identity and individuation’, Proceedings of the Aristotelian Society, 1956–7. (Bases personal identity on bodily continuity.)
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