 A consistency proof requires (i) an axiomatic system (ii) a demonstration that it is not the case that both the formula "p" and its negation "~p" can be derived in the system.But by whatever method one goes about it, all consistency proofs would "seem" to necessitate the primitive notion of "contradiction"; moreover, it "seems" as if this notion would simultaneously have to be "outside" the formal system in the definition of tautology.When Emil Post, in his 1921 "Introduction to a general theory of elementary propositions", extended his proof of the consistency of the propositional calculus (i.e.