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:''This article uses forms of logical notation. For a concise description of the symbols used in this notation, see List of logic symbols.'' In logic, the law of identity is the first of the three classical laws of thought. It states that "each thing is the same with itself and different from another". By this it is meant that each thing (be it a universal or a particular) is composed of its own unique set of characteristic qualities or features, which the ancient Greeks called its essence. Consequently, things that have the same essence are the same thing, while things that have different essences are different things.〔"Two things are called one, when the definition which states the essence of one is indivisible from another definition which shows us the other (though in itself every definition is divisible).” (Metaphysics, Book VI, Part 4 (c) - Translated by W. D. Ross )〕 In its symbolic representation, "A is A", the first element of the proposition represents the subject (thing) and the second element represents the predicate (its essence), with the copula "is" signifying the relation of "identity".〔"Each thing itself, then, and its essence are one and the same in no merely accidental way, as is evident both from the preceding arguments and because to know each thing, at least, is just to know its essence, so that even by the exhibition of instances it becomes clear that both must be one.” (Metaphysics, Book VII, Part 6 - Translated by W. D. Ross )〕 Further, since a definition is an expression of the essence of that thing with which the linguistic term is associated, it follows that it is through its definition that the identity of a thing is established.〔“For if a definition is an expression signifying the essence of the thing and the predicates contained therein ought also to be the only ones which are predicated of the thing in the category of essence; and genera and differentiae are so predicated in that category: it is obvious that if one were to get an admission that so and so are the only attributes predicated in that category, the expression containing so and so would of necessity be a definition; for it is impossible that anything else should be a definition, seeing that there is not anything else predicated of the thing in the category of essence.”(Topics, Book VII, Part 1 - Translated by W. A. Pickard-Cambridge )〕 For example, in the definitive proposition:〔A definitive proposition is that wherein one term is the definition of the other, e.g., "Hope is the looking with pleasure into the future."〕"A lawyer is a person qualified and authorized to practice law", the subject (lawyer) and the predicate (person qualified and authorized to practice law) are declared to be one and the same thing (identical). Consequently, the Law of Identity prohibits us from rightfully calling anything other than "a person qualified and authorized to practice law" a "lawyer". In logical discourse, violations of the Law of Identity (LOI) result in the informal logical fallacy known as equivocation.〔Things are said to be named 'equivocally' when, though they have a common name, the definition corresponding with the name differs for each.〕 That is to say, we cannot use the same term in the same discourse while having it signify different senses or meanings – even though the different meanings are conventionally prescribed to that term. In everyday language, violations of the LOI introduce ambiguity into the discourse, making it difficult to form an interpretation at the desired level of specificity. == History == The earliest recorded use of the law appears to occur in Plato's dialogue Theaetetus (185a), wherein Socrates attempts to establish that what we call "sounds" and "colours" are two different classes of thing: Aristotle takes recourse to the law of identity—though he does not identify it as such—in an attempt to negatively demonstrate the law of non-contradiction. However, in doing so, he shows that the law of non-contradiction is not the more fundamental of the two: Both Thomas Aquinas (''Met.'' IV., lect. 6) and Duns Scotus (''Quaest. sup. Met.'' IV., Q. 3) follow Aristotle. Antonius Andreas, the Spanish disciple of Scotus (d. 1320), argues that the first place should belong to the law "Every Being is a Being" (''Omne Ens est Ens'', Qq. in Met. IV., Q. 4), but the late scholastic writer Francisco Suarez (''Disp. Met.'' III., § 3) disagreed, also preferring to follow Aristotle. Another possible allusion to the same principle may be found in the writings of Nicholas of Cusa (1431-1464) where he says: Gottfried Wilhelm Leibniz claimed that the law of Identity, which he expresses as "Everything is what it is", is the first primitive truth of reason which is affirmative, and the law of noncontradiction, is the first negative truth (''Nouv. Ess.'' IV., 2, § i), arguing that "the statement that a thing is what it is, is prior to the statement that it is not another thing" (Nouv. Ess. IV., 7, § 9). Wilhelm Wundt credits Gottfried Leibniz with the symbolic formulation, "A is A".〔(La philosophie éternelle ou traditionnelle, la métaphysique, la logique, la raison et l'intelligence )〕 George Boole, in the introduction to his treatise ''The Laws of Thought'' made the following observation with respect to the nature of language and those principles that must inhere naturally within them, if they are to be intelligible: John Locke (''Essay Concerning Human Understanding'' IV. vii. iv. ("Of Maxims") says: African Spir proclaims the law of identity as the fundamental law of knowledge, which is opposed to the changing appearance of the empirical reality.〔''Forschung nach der Gewissheit in der Erkenntniss der Wirklichkeit'', Leipzig, J.G. Findel, 1869 and ''Denken und Wirklichkeit: Versuch einer Erneuerung der kritischen Philosophie'', Leipzig, J. G. Findel, 1873.〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「law of identity」の詳細全文を読む スポンサード リンク
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