Definitions
American Heritage® Dictionary of the English Language, Fourth Edition
- n. Disproof of a proposition by showing that it leads to absurd or untenable conclusions.
Century Dictionary and Cyclopedia
- n. A reduction to an absurdity; the proof of a proposition by proving tho falsity of its contradictory opposite: an indirect demonstration. In geometry the reductio ad absurdum consists in drawing a figure whose parts are supposed to have certain relations, and then showing that this leads to a conclusion contrary to a known proposition, whence it follows that the parts of the figure cannot have those relations. Thus, in Euclid's “Elements” the proposition that if a triangle has two angles equal the sides opposite those angles will be equal is proved as follows. In the triangle ABC, let the angles ABC and ACB be equal. Then, suppose AB to be greater than AC. Lay off BD=AC and join DC. Then, comparing the two triangles ACB and DBC, we have in the former the sides AC and BC and their included angle ACB equal in the latter to the sides DB and CB and their included angle DBC. Hence, these two triangles would be equal, or the part would be equal to the whole. This proof is a reductio ad absurdum. This kind of reasoning is considered somewhat objectionable as not showing the principle from which tiie proposition flows; but, it is a perfectly conclusive mode of proof, and, in fact, is in all cases readily converted into a direct proof. Thus, in the above example, we have only to compare the triangle ABC with itself, considering it as two triangles according as the angle B is named before C or vice versa. In the triangle ABC the angles Band C with the included side BC are respectively equal in the triangle ACB to the angles C and B with the included side CB; hence the other parts of the triangles are equal, and tho side AC opposite the first angle B in the first triangle is equal to the side AB opposite the tirst angle C in the second triangle.
Wiktionary
- n. mathematics, logic The method of proving a statement by assuming the statement is false and, with that assumption, arriving at a blatant contradiction.
WordNet 3.0
- n. (reduction to the absurd) a disproof by showing that the consequences of the proposition are absurd; or a proof of a proposition by showing that its negation leads to a contradiction
Etymologies
- Medieval Latin reductiō ad absurdum : Latin reductiō, a bringing back, reduction + Latin ad, to + Latin absurdum, absurdity, from neuter of absurdus, absurd. (American Heritage® Dictionary of the English Language, Fourth Edition)
Examples
“Historic Doubts relative to Napoleon Buonaparte (1819), intended as a reductio ad absurdum of Hume’s contention that no evidence is sufficient to prove a miracle,”
“In St. Anselm's own day this argument was objected to by Gaunilo, who maintained as a reductio ad absurdum that were it valid one could prove by means of it the actual existence somewhere of an ideal island far surpassing in riches and delights the fabled Isles of the Blessed.”
The Catholic Encyclopedia, Volume 6: Fathers of the Church-Gregory XI
“The reconstitution of this particular dream illustrates the reductio ad absurdum of the two previous psycho-analytic "solutions.”
Lists
These user-created lists contain the word ‘reductio ad absurdum’.
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cicatrix
scar tissue
minatory, naira, Cluniac, embracive, prolix, hierophant, timorous, adduce, veracious, dysphoric, sang-froid, vitiate and 505 more...
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Hapax Legomenon
For those special words that we only seem to use once in a lifetime..
hapax legomenon, schadenfreude, malapropism, ipso facto, carte blanche, tautology, pleonasm, reductio ad absurdum, tabula rasa, post hoc ergo pro..., post hoc, cum hoc ergo prop...
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Daily encountered words
Words encountered on daily basis while reading blogs,newspapers,magazines and books.
Euphemism, penny-pinching, thrift, frugal, meticulous, fanciful, convoluted, copious, bogus, corroborate, debunk, diffident and 60 more...
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Latinate
lorem ipsum, citius, altius, fortius, curriculum vitae, bona fide, terra nullius, habeas corpus, quidnunc, voir dire, emeritus, quincunx and 99 more...
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Marginilia
intertextuality, queer, serendipity, eerie, semiotics, schadenfreude, calliope, logophile, marginalia, reductio ad absurdum, dabble, minutia and 141 more...
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Philosophical Jargon
Words philosophical writers use to give the illusion of technical competence, including up-trippingly specialised senses of words that have other jobs during daylight hours.
akrasia, akrates, particularism, particularist, mereology, deontology, cognitivism, naturalism, anti-naturalism, ethics, phenomenology, metaethics and 220 more...
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Nihilarity
Absurdity, dark art, radicalism, anger, tragic irony, and nothingness. Tee-hee!
nihilism, dada, futurism, rayonism, postmodernity, deconstruction, surrealism, existentialism, reductio ad absurdum, discordianism, operation mindfuck, amorality and 127 more...
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latinate
words/phrases/sayings from latin
in medias res, sine qua non, tempus fugit, ne plus ultra, mea culpa, a priori, actus reus, mens rea, vidi aquam, alea iacta est, annus mirabilis, annus horribilis and 67 more...
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What philosophy class taught me
presuppose, concomitant, prevaricate, a priori, a posteriori, fallacious, abjudicate, reductio ad absurdum, phenomenology, quale, ataraxia, peripatetic and 25 more...
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Latin
ad astra per aspera, reductio ad absurdum, peccavi, ad hominem, mea culpa, sapere aude, ante bellum, ad nauseum, tabula rasa, modus operandi, non sequitur, sui generis and 14 more...
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VOCABULARY BUILDER
Words to be added to my vocabulary
subvert, contumacious, mitigate, acquiesce, covet, transgression, juxtapose, ubiquitous, mellifluous, attenuate, futility, loquacious and 48 more...
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ruzuzu's taxonomy
limit cycle, gyre, oscillation, meander, general adaptatio..., barchan dune, soliton, guilloche, obverse, involute, damaskeening, superposition and 62 more...
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Latinese
aliquid amari, res angusta, creta alba, ne plus ultra, sui generis, quotidian ennui, sine qua non, pari passu, ab initio, de novo, ad nauseam, tempus fugit and 11 more...
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R is for Red Clown
My R Words
rabble-rouser, rabblerouser, red clown, rabblerousing, ragamuffin, rama lama ding dong, rapscallion, rascal, ratfink, red herring, red rover, renegade and 39 more...
Tweets
Looking for tweets for reductio ad absurdum.

mcgelligot Reductio ad absurdum is proving something must be true because its negative is impossible. Nov 14, 2008