If a proposition follows from another, then the latter says more than the former, the former less than the latter.

5.141    If p follows from q and q from p then they are one and the same proposition.

5.142    A tautology follows from all propositions: it says nothing.

5.143    Contradiction is something shared by propositions, which no proposition has in common with another. Tautology is that which is shared by all propositions, which have nothing in common with one another.

Contradiction vanishes so to speak outside, tautology inside all propositions.