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reductio ad absurdum

[ri-duhk-tee-oh ad ab-sur-duh m, -zur-, -shee-oh]
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noun Logic.
  1. a reduction to an absurdity; the refutation of a proposition by demonstrating the inevitably absurd conclusion to which it would logically lead.
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Origin of reductio ad absurdum

First recorded in 1735–45, reductio ad absurdum is from the Latin word reductiō ad absurdum
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018
British Dictionary definitions for reductio ad absurdum

reductio ad absurdum

noun
  1. a method of disproving a proposition by showing that its inevitable consequences would be absurd
  2. a method of indirectly proving a proposition by assuming its negation to be true and showing that this leads to an absurdity
  3. application of a principle or proposed principle to an instance in which it is absurd
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Word Origin

Latin, literally: reduction to the absurd
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Word Origin and History for reductio ad absurdum

Latin, literally "reduction to the absurd." Absurdum is neuter of absurdus. See reduction + absurd. The tactic is useful and unobjectionable in proofs in geometry.

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Online Etymology Dictionary, © 2010 Douglas Harper