6.1201
That e.g. the propositions “p” and “~p” in the connexion “~(p.~p)” give a tautology shows that they contradict one another. That the propositions “p⊃q”, “p” and “q” connected together in the form “(p⊃q).(p):⊃:(q)” give a tautology shows that q follows from p and p⊃q. That “(x).fx:⊃:fa” is a tautology shows that fa follows from (x).fx, etc. etc.