<philosophical terminology> tautologies that express the logical equivalence of pairs of elementary statement forms, each of whose substitution instances may be used to replace those of the other wherever they occur within a formal proof of the validity of a deductive argument. The rules of replacement that we employ here include: De Morgan's Theorems, Commutation, Association, Distribution, Double Negation, Transposition, Implication, Equivalence, Exportation, and Tautology. These, taken together with the nine rules of inference, adequately secure the completeness of the propositional calculus.
[A Dictionary of Philosophical Terms and Names]
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