Proof by Contradiction

Proof Rule
Proof by contradiction is a valid argument in types of logic dealing with negation $\neg$ and contradiction $\bot$.

This includes classical propositional logic and predicate logic, and in particular natural deduction.

Variants
The following forms can be used as variants of this theorem:

Also see

 * Definition:Negation
 * Definition:Contradiction
 * Reductio ad Absurdum
 * Law of Excluded Middle