Reductio ad Absurdum

Also see

 * 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.