# 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

$$A \lor \lnot A = T$$

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