Reductio ad Absurdum/Proof Rule

Proof Rule
The reductio ad absurdum is a valid deduction sequent in 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 does not depend upon the assumption $\neg \phi$.

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