Reductio ad Absurdum

Reductio ad absurdum (Latin: "reduction to absurdity") is a common form of argument which seeks to demonstrate that a statement is true by showing that a false, untenable, or absurd result follows from its denial, or in turn to demonstrate that a statement is false by showing that a false, untenable, or absurd result follows from its acceptance. First appearing in classical Greek philosophy (the Latin term derives from the Greek ἡ εἰς ἀτοπον ἀπαγωγη or he eis atopon apagoge, "reduction to the impossible", for example in Aristotle's Prior Analytics), this technique has been used throughout history in both formal mathematical and philosophical reasoning, as well as informal debate.

The "absurd" conclusion of a reductio ad absurdum argument can take a range of forms:

Rocks have weight, otherwise we would see them floating in the air.
Society must have laws, otherwise there would be chaos.
There is no smallest positive rational number, because if there were, it could be divided by two to get a smaller one.

-Wikipedia