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...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.
Emil L.Post, in The Adventure of Reason: Interplay Between Philosophy of Mathematics and ..., p.104 | ||