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the law of traditional logic that holds that if from a certain proposition A there follows the proposition B, then from the negation of the proposition B follows the negation of the proposition A (or in the language of conditional propositions: from the truth of the proposition “If A, then B” follows the truth of the proposition “If not B ?, then not- A”).
REFERENCESChurch, A. Vvedenie v matematicheskuiu logiku. Moscow, 1960. (Translated from English.)
Tarski, A. Vvedenie v logiku i metodologiiu deduktivnykh nauk. Moscow, 1948. (Translated from English.)