A law of logic, in the form of a proposition, that describes a universal truth; no matter what values are assigned to the variables in the proposition the result is always true. An example from the propositional calculus is
where ∨ and ∧ are the
or and
and operators and
P′ is the negation of
P. In the truth table for a tautology the final result column contains only the value true. If the final column contains only the value false, then a
contradiction has been identified.