Law of excluded middle lem

holds. It asserts that every statement is either true or false, with no middle ground.

$$A \lor \neg A$$

Remark. Classical logic accepts this as an Axiom. Intuitionistic/Constructive logic rejects it: asserting $A \lor \neg A$ requires either a proof of $A$ or a proof of $\neg A$, and for many statements we have neither.