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  • In propositional logic, modus tollens is a valid argument form and a rule of inference. The first to explicitly state the argument form modus tollens were the Stoics. The inference rule modus tollens, also known as the law of contrapositive, validates the inference from implies and the contradictory of, to the contradictory of . The modus tollens rule can be stated formally as: where stands for "P implies Q", stands for "it is not the case that Q". Then, whenever "" and "" each appear by themselves as a line of a proof, "" can validly be placed on a subsequent line. The history of the inference rule modus tollens goes back to antiquity. Modus tollens is closely related to modus ponens. There are two similar, but invalid, forms of argument: affirming the consequent and denying the antecedent.

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