 For example, that there is an effective method for determining whether or not any given formula of the propositional calculus is a tautology (such as the truth table method) is expressed in function-speak by saying that there is an effective method for obtaining the values of a function, call it T, whose domain is the set of formulae of the propositional calculus and whose value for any given formula x, written \(T(x)\), is 1 or 0 according to whether x is, or is not, a tautology.