The problem of determining whether a formula is a tautology is fundamental in propositional logic. If there are "n" variables occurring in a formula then there are 2"n" distinct valuations for the formula. Therefore, the task of determining whether or not the formula is a tautology is a finite and mechanical one: one needs only to evaluate the truth value of the formula under each of its possible valuations. One algorithmic method for verifying that every valuation makes the formula to be true is to make a truth table that includes every possible valuation.