A calculus of the absurd

17.2.2 Law of the excluded middle

This one seems “obvious”, but it’s an important axiom (something we assume, without proof, to be true). In propositional logic, it is always the case that

\begin{equation} A \lor \lnot A = T \end{equation}

That is, it is always true that either \(A\) or \(\lnot A\) is true.