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Transposition (logic) : ウィキペディア英語版
Transposition (logic)

In propositional logic, transposition〔Moore and Parker〕 is a valid rule of replacement that permits one to switch the antecedent with the consequent of a conditional statement in a logical proof if they are also both negated. It is the inference from the truth of "''A'' implies ''B''" the truth of "Not-''B'' implies not-''A''", and conversely.〔Brody, Bobuch A. "Glossary of Logical Terms". ''Encyclopedia of Philosophy''. Vol. 5–6, p. 76. Macmillan, 1973.〕〔Copi, Irving M. ''Symbolic Logic''. 5th ed. Macmillan, 1979. See the Rules of Replacement, pp. 39-40.〕 It is very closely related to the rule of inference modus tollens. It is the rule that:
(P \to Q) \Leftrightarrow (\neg Q \to \neg P)
Where "\Leftrightarrow" is a metalogical symbol representing "can be replaced in a proof with."
== Formal notation ==
The ''transposition'' rule may be expressed as a sequent:
:(P \to Q) \vdash (\neg Q \to \neg P)
where \vdash is a metalogical symbol meaning that (\neg Q \to \neg P) is a syntactic consequence of (P \to Q) in some logical system;
or as a rule of inference:
:\frac
where the rule is that wherever an instance of "P \to Q" appears on a line of a proof, it can be replaced with "\neg Q \to \neg P";
or as the statement of a truth-functional tautology or theorem of propositional logic. The principle was stated as a theorem of propositional logic by Russell and Whitehead in ''Principia Mathematica'' as:
:(P \to Q) \to (\neg Q \to \neg P)
where P and Q are propositions expressed in some formal system.

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