Reductio ad Absurdum

Comment
It can be seen that this result depends on the rule of Double Negation Elimination.

As this depends on the Law of the Excluded Middle, it invalidates the Reductio Ad Absurdum from the intuitionist system.

Also see

 * Consequentia Mirabilis, otherwise known as Clavius's Law.


 * Proof by Contradiction, often treated as another aspect of the same thing.

From the point of view of purely classical logic, this is acceptable. However, in the context of intuitionistic logic, it is essential to bear in mind that only the proof by contradiction is valid.

Linguistic Note
Reductio ad absurdum is Latin for reduction to absurdity.