6.1201 That, e.g., the propositions “p” and “~p” in the combination “~(p. ~p)” give a tautology, shows that they contradict one another. That the propositions “p → q”, “p” and “q” combined with one another 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.

I'm using the arrow for "if ... then."