Law of Excluded Middle

Proof Rule
The law of (the) excluded middle is a valid argument in certain types of logic dealing with disjunction $\lor$ and negation $\neg$.

This includes classical propositional logic and predicate logic, and in particular natural deduction, but for example not intuitionistic propositional logic.

Also see

 * Definition:Disjunction
 * Definition:Negation


 * Principle of Non-Contradiction