WORD OF THE DAY

reductio ad absurdum

Main Entry: re·duc·tio ad ab·sur·dum Pronunciation: ri-'d&k-tE-"O-"ad-&b-'s&r-d&m, -'d&k-sE-O-, -shE-, -'z&r-
http://www.m-w.com/cgi-bin/audio.pl?reduct03.wav=reductio+ad+absurdum
Function: noun
Etymology: Late Latin,
literally, reduction to the absurd
1 : disproof of a proposition by showing an absurdity to which it leads when carried to its logical conclusion
2 : the carrying of something to an absurd extreme

From Wikipedia, the free encyclopedia

Reductio ad absurdum (Latin for "reduction to the absurd", traceable back to the Greek ἡ εις άτοπον απαγωγη (hi eis átopon apagogi), "reduction to the impossible", often used by Aristotle), also known as an apagogical argument or reductio ad impossibile, is a type of logical argument where we assume a claim for the sake of argument, arrive at an absurd result, and then conclude the original assumption must have been wrong, since it gave us this absurd result. This is also known as proof by contradiction. It makes use of the law of non-contradiction — a statement cannot be both true and false. In some cases it may also make use of the law of excluded middle — a statement which cannot be false, must then be true.

In philosophy

The following dialogue is an example of reductio ad absurdum:

A — You should respect C's belief, for all beliefs are of equal validity and cannot be denied.

B — Isn't it right to deny D's belief? (where D believes something that is considered to be wrong by most people, such the earth's being flat)

A — I agree it is right to deny D's belief.

B — If it is right to deny D's belief, it is not true that no belief can be denied. Therefore, I can deny C's belief if I can give reasons that suggest it too is incorrect.

The following is a trickier reduction, but one which is stronger from the philosophical point of view because it does not rely on A's accepting that D's opinion is wrong:

A — You should respect C's belief, for all beliefs are of equal validity and cannot be denied.
B —
I deny that belief of yours and believe it to be invalid.
According to your statement, this belief of mine (1) is valid, like all other beliefs.
However, your statement also contradicts and invalidates mine, being the exact opposite of it.
The conclusions of 2 and 3 are incompatible and contradictory, so your statement is logically absurd.
In each case, B has used a reduction to the absurd to argue his or her point.


As a figure of speech

Among some people, there is a misconception that reductio ad absurdum just means "a silly argument".
In general practice, a reductio ad absurdum is a tactic in which the logic of an argument is challenged by reducing the concept to its most absurd extreme. It is thus often similar in nature to the slippery slope argument.
For example:
A — I don't think the police should arrest teenagers for soft drug possession.
B — So, you are basically arguing the police should not enforce the law and we should live in a society of violent anarchy.
This version of reductio ad absurdum is often used in Dilbert, where Dilbert often uses this strategy to disprove the beliefs and arguments of his boss, his coworkers, and, most often, those of his dates.
See also appeal to ridicule, which is another type of logical fallacy.