reductio ad absurdura
Preserved but no longer current.
A proof of inference arising from the demonstration that every other hypothesis involves an absurdity. Thus, suppose I want to prove that the direct road from two given places is the shortest, I should say, "It must either be the shortest or not the shortest. If not the shortest, then some other road is the direct road; but there cannot be two shortest roads, therefore the direct road must be the shortest.
Era: ca. 1898 (Victorian)
Sources & notes
Reproduced from Brewer’s Dictionary of Phrase and Fable (1898 edition, public domain; archive.org, retrieved 2026-07-31; OCR text, minor errors possible). Reflects the scholarship and attitudes of its era.
Common questions
What does “reductio ad absurdura” mean?
A proof of inference arising from the demonstration that every other hypothesis involves an absurdity. Thus, suppose I want to prove that the direct road from two given places is the shortest, I should say, "It must either be the shortest or not the shortest. If not the shortest, then some other road is the direct road; but there cannot be two shortest roads, therefore the direct road must be the shortest.
Is “reductio ad absurdura” formal or informal?
No register label has been reviewed yet — treat it as neutral and check the usage notes above.