tautology, in logic, a statement so framed that it cannot be denied without inconsistency. Thus, “All men are rational” is held to assert with regard to anything whatsoever that either it is a man or it is not rational. But this universal “truth” follows not from any facts noted about real men but only from the actual use (or one such use) of “man” and “rational” and is thus purely a matter of definition. The statement cannot but be true because it asserts every possible state of affairs: it is true whichsoever of its constituents are true, and it is also true whichsoever are false.
In the propositional calculus, a logic in which whole propositions are related by such connectives as ⊃ (“if . . . then”), · (“and”), ∼ (“not”), and ∨ (“or”), even complicated expressions such as [(A ⊃ B)·(C ⊃ ∼B)] ⊃ (C ⊃ ∼A) can be shown to be tautologies by displaying in a truth table every possible combination of T (true) and F (false) of its arguments A, B, C and after reckoning out by a mechanical process the truth-value of the entire formula, noting that, for every such combination, the formula is T. The test is effective because, in any particular case, the total number of different assignments of truth-values to the variables is finite; and the calculation of the truth-value of the entire formula can be carried out separately for each assignment of truth-values.
tautology, in logic, a statement so framed that it cannot be denied without inconsistency. Thus, “All men are rational” is held to assert with regard to anything whatsoever that either it is a man or it is not rational. But this universal “truth” follows not from any facts noted about real men but only from the actual use (or one such use) of “man” and “rational” and is thus purely a matter of definition. The statement cannot but be true because it asserts every possible state of affairs: it is true whichsoever of its constituents are true, and it is also true whichsoever are false.
In the propositional calculus, a logic in which whole propositions are related by such connectives as ⊃ (“if . . . then”), · (“and”), ∼ (“not”), and ∨ (“or”), even complicated expressions such as [(A ⊃ B)·(C ⊃ ∼B)] ⊃ (C ⊃ ∼A) can be shown to be tautologies by displaying in a truth table every possible combination of T (true) and F (false) of its arguments A, B, C and after reckoning out by a mechanical process the truth-value of the entire formula, noting that, for every such combination, the formula is T. The test is effective because, in any particular case, the total number of different assignments of truth-values to the variables is finite; and the calculation of the truth-value of the entire formula can be carried out separately for each assignment of truth-values.
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