El principio de identidad es un principio clásico de la lógica y la filosofía, según el cual toda entidad es idéntica a sí misma. Por ejemplo, Julio César es idéntico a sí mismo (a Julio César), el Sol es idéntico a sí mismo, esta manzana es idéntica a sí misma, etc. El principio de identidad es, junto con el principio de no contradicción y el principio del tercero excluido, una de las leyes clásicas del pensamiento. En lógica de primer orden con identidad, el principio de identidad se expresa: Es decir: para toda entidad x, x es idéntica a sí misma. No se debe confundir al principio de identidad con la siguiente tautología de la lógica proposicional: Esta fórmula expresa que toda proposición es verdadera si y sólo si ella misma es verdadera. Por lo tanto, expresa una verdad acerca de proposiciones y sus valores de verdad, mientras que el principio de identidad expresa una verdad acerca todo tipo de entidades, no sólo proposiciones.

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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. 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". 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 example, in the definitive proposition:"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. 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.

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