Reductio ad absurdum, reducing to an absurdity, is a method of proof in polemics, logic and mathematics, whereby assuming that a proposition is true leads to absurdity; a proposition is assumed to be true and this is used to deduce a proposition known to be false, so the original proposition must have been false. It is also an argumentation style in polemics, whereby a position is demonstrated to be false, or "absurd", by assuming it and reasoning to reach something known to be believed as false or to violate common sense; it is used by Plato to argue against other philosophical positions.