Indirect demonstration

called also reductio ad absurdum), in which the correct conclusion is an inference from the demonstration that any other hypothesis must be incorrect.
a mode of demonstration in which proof is given by showing that any other supposition involves an absurdity (reductio ad absurdum), or an impossibility; thus, one quantity may be proved equal to another by showing that it can be neither greater nor less.

See also: Demonstration, Indirect