Reductio ad absurdum

Reductio ad absurdum is a Latin phrase which means "reduction to the absurd". The phrase describes a kind of indirect proof. It is a proof by contradiction, and is a common form of argument. It shows that a statement is true because its denial leads to a contradiction, or a false or absurd result. It is a way of reasoning that has been used throughout the history of mathematics and philosophy from classical antiquity onwards.

The ridiculous or "absurdum" conclusion of a reductio ad absurdum argument can have many forms. For example,

• 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.

History

The phrase can be traced back to the Greek η εις άτοπον απαγωγή (hê eis átopon apagogê). This phrase means "reduction to the impossible". It was often used by Aristotle. The method is used a number of times in Euclid's Elements.

Method

Reduction ad absurdum can be a tool of discovery.

The method of proving something works by first assuming something about it. Then other things are deduced from that. If there is a contradiction, it shows that the first something cannot be correct. For example,

To prove A is true, correct, valid, credible ....

Assume the opposite -- that "not-A" is true....

Assume that if "not-A" is true, then it must mean or imply B.

Show that B is false, incorrect, invalid, incredible ...

Therefore, A must be true after all.

Related pages

• Mathematical proof
• Proof by induction
• Straw man
• Boltzmann brain

Other websites

• Conditional Proof and Reductio ad Absurdum