Many contemporary logicians seek to account for the notion of class in terms of propositional function. Let f signify, for example, "is red" or "is a dog." Then one has the propositional function f(x) which, x being a variable, has the meaning "x is red" or "x is a dog." By substituting individual terms for the variable, the propositional function gives rise to propositions which, according to the substitutions which are made, are true or false. One then defines the class as the set of individual terms which, substituted for the variable, confer a value of truth upon the propositions derived from the original propositional function by means of these substitutions.