reductio ad absurdum; proof by contradiction
Technical term in logic and mathematics. Cross-referenced with 背理法, which is a more common Japanese term for the same concept.
See also: 背理法
Reductio ad absurdum is a method of proving a proposition by assuming its negation and deriving a contradiction.
From 帰 (return, result in) + 謬 (mistake, fallacy) + 法 (method), literally 'method of reducing to absurdity'. The term is a direct translation of the Latin 'reductio ad absurdum'.