The fundamental definition of a tautology is in the context of propositional logic. The definition can be extended, however, to sentences in first-order logic (see Enderton (2002, p. 114) and Kleene (1967 secs. 17–18)). These sentences may contain quantifiers, unlike sentences of propositional logic. In the context of first-order logic, a distinction is maintained between "logical validities", sentences that are true in every model, and "tautologies", which are a proper subset of the first-order logical validities. In the context of propositional logic, these two terms coincide.