"...a theorem of propositional logic if and only if f(p1, p2 ,..., pn) is a tautology. … He (Emil L.Post) uses the word to discuss the adequacy of a system of functions to express all the possible truth tables (this is nowadays called truth-functional completeness). In this way he shows not only that through the connectives of Principia (∼ and ∨) one can generate all possible truth tables but also that there are only two connectives which can, singly generate all the truth tables."