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Revision theory is a subfield of philosophical logic that consists of a general theory of definitions, including, but not limited to, circular and interdependent concepts. A circular definition is one in which the concept being defined, the ''definiendum'', occurs in the statement defining it, the ''definiens''. An example is the following: being a is defined as being both blue and to the left of a . Revision theory provides a formal semantics for defined expressions, and there are formal proof systems for studying the logic of circular expressions. Definitions are important in philosophy and logic. Traditionally, circular definitions have been regarded as logically incorrect or even incoherent, but revision theory shows that they are meaningful and can be studied using formal tools from mathematical and philosophical logic. Revision theory can be, and has been, used to provide circular analyses of philosophical and logical concepts. This article will focus on ''predicates'' with circular definitions, but it is possible to provide circular definitions for other types of expressions. ==History== Revision theory is a generalization of the revision theories of truth developed Anil Gupta, Hans Herzberger, and Nuel Belnap.〔See, respectively, Gupta (1982), Herzberger (1982), and Belnap (1982).〕 In the revision theories of Gupta and Herzberger, revision is supposed to reflect intuitive evaluations of sentences that use the truth predicate. Some sentences are stable in their evaluations, such as the truth-teller sentence, :The truth-teller is true. Assuming the truth-teller is true, it is true, and assuming that it is false, it is false. Neither status will change. On the other hand, some sentences oscillate, such as the liar, :The liar sentence is not true. On the assumption that the liar is true, one can show that it is false, and on the assumption that it is false, one can show that it is true. This instability is reflected in revision sequences for the liar. The generalization to circular definitions was developed by Gupta, in collaboration with Belnap. Their book, ''The Revision Theory of Truth'', presents an in-depth development of the theory of circular definitions, as well as an overview and critical discussion of philosophical views on truth and the relation between truth and definition. 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「Revision theory」の詳細全文を読む スポンサード リンク
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