Reductio ad Absurdum/Proof Rule

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

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

As a proof rule it is expressed in the form:
 * If, by making an assumption $\neg \phi$, we can infer a contradiction as a consequence, then we may infer $\phi$.


 * The conclusion $\phi$ does not depend upon the assumption $\neg \phi$.

It can be written:
 * $\ds {\begin{array}{|c|} \hline \neg \phi \\ \vdots \\ \bot \\ \hline \end{array} \over \phi} \textrm{RAA}$