Definition
Tautology is a sentence which is always true, not due to its word meanings but to its logical form. A tautology is also called a logical truth or a necessary truth. A sentence of propositional logic is a tautology when it is true for every possible assignment of truth values to the propositional letters of that sentence. The formula p v Neg p is a tautology of propositional logic. A sentence of predicate logic is a tautology when it is true for every possible denotation of the variables and individual and predicate constants that it contains. The formula All(x) [ P(x) v Neg P(x) ] is a tautology of predicate logic.
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References
 Gamut, L.T.F. 1991. Logic, language, and meaning, Univ. of Chicago Press, Chicago.
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