That "(x).fx:⊃:fa" is a tautology shows that fa follows from (x) ,fx, etc. etc.
6.1202 It is clear that we could have used for this purpose contradictions instead of tautologies.
6.1203 In order to recognize a tautology as such, we can, in cases in which no sign of generality occurs in the tautology, make use of the following intuitive method: I write instead of "p", "q", "r", etc., "TpF", "TqF", "TrF", etc. The truth-combinations I express by brackets, e.g.:
and the co-ordination of the truth or falsity of the whole proposition with the truth-combinations of the truth-arguments by lines in the following way:
This sign, for example, would therefore present the proposition "p⊃q". Now I will proceed to inquire whether such a proposition as ~(p. ~p) (The Law of Contradiction) is a tautology. The form "~ ξ" is written in our notation
