Clavius's Law

Theorem
If, from the negation of a proposition $p$ we can derive $p$, we may conclude $p$:

Also known as
Clavius's Law is also known as consequentia mirabilis.

Also see

 * Reductio ad Absurdum, of which this is a particular case.

The name consequentia mirabilis is Latin for "marvellous (or admirable) consequence".