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,[1][2] 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.[3][4] It is a way of reasoning that has been used throughout the history of mathematics and philosophy from classical antiquity onwards.[3]
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.
The phrase can be traced back to the Greek η εις άτοπον απαγωγή (hê eis átopon apagogê). This phrase means "reduction to the impossible".[3] It was often used by Aristotle.[5] The method is used a number of times in Euclid's Elements.
Reduction ad absurdum can be a tool of discovery.[6]
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,
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