Is there an easy way to express the concept of “reductio ad absurdum” in Japanese?

I find that the following dictionary terms tend to be unfamiliar even to college-educated Japanese speakers:



I ask this because I was conversing with a friend in Japanese and using reductio ad absurdum to show how an idea was nonsense. But she said that this sort of argument was extreme and idiotic, to which I responded I that this type of argumentation is quite common in European languages and is explicitly taught by name in schools.

Or, is it maybe the case that there is NO easy way to express this concept in Japanese because the Japanese culture does not have an extensive tradition of discussing logic? It seems so. Wikipedia, for example, has the article on "reductio ad absurdum" in more than 30 languages, but not Japanese.

  • Wikipedia... not Japanese >> 「背理法」の記事ならありました -> ja.wikipedia.org/wiki/%E8%83%8C%E7%90%86%E6%B3%95
    – chocolate
    Jul 1, 2016 at 4:53
  • 1
    I teach critical thinking at a Japanese university as an English class precisely because my students are completely unfamiliar with using this sort of thinking (I don't find math majors any better equipped on this point by the way) and that I think it's integral to understanding Western thought.
    – virmaior
    Jul 1, 2016 at 6:43

2 Answers 2


All adults who graduated from high school should be aware of 背理法, because they all learned it in math classes in order to prove why the square root of 2 is irrational, why there are infinitely many prime numbers, etc. Well, don't ask me how many of them actually remember it.

Anyways, even among people who remember 背理法, most of them consider 背理法 as a purely mathematical technique, but not as a rhetorical device nor a debate technique. And unfortunately, I don't think there is an easy Japanese word for reductio ad absurdum which you can safely use in conversations.

That does not mean Japanese people cannot use 背理法 in daily conversations. Even elementary school kids can make an argument like 「人類は永遠にタイムマシンを発明できないよ。だってもし発明できたら、未来の人が現代にいっぱい来てるはずでしょ?」. We just haven't gotten used to categorizing and naming the types of arguments we use everyday.

To explain reductio ad absurdum in plain words, I would say 「敢えて逆のことを言って矛盾を突くやり方」 or something like that.

  • @chocolate ありがとうございます。高校で覚えた英語忘れてる…w
    – naruto
    Jul 1, 2016 at 5:48

The tradition of logical debate is far more valued in western cultures, so it's not surprising that even though a specific term exists in Japanese, most people wouldn't be familiar with it in that context.

Similarly, there's no easy way to express わびさび in English. Of course, you could use the specific term "wabisabi", but who would understand you? People familiar with Japanese culture and tradition.

However, you can explain wabisabi in English, as "an aesthetic sensibility emphasising quiet simplicity and subdued refinement".

Likewise, you can explain reductio ad absurdum(背理法)in Japanese as「間接証明によって矛盾を指摘する論じ方」.

That being said, as naruto mentions the teaching of 背理法 in math class is part of the official direction from the 文部科学省 as can be seen here:


  • 3
    わび and さび are probably more obscure than 背理法 to most (Japanese) people.
    – user4092
    Jul 1, 2016 at 17:21
  • What makes you say that?
    – sazarando
    Jul 1, 2016 at 22:11
  • 確かに・・「わび」「さび」って何? って聞かれても私は説明できないと思います (日本語でも)。。。 ^^;
    – chocolate
    Jul 2, 2016 at 2:46
  • @chocolate 説明できるほど理解していなくとも、言葉として「わびさび」って聞いたことのある人が「背理法」より断然多いかとは思いますが。。。如何ですか?
    – sazarando
    Jul 2, 2016 at 3:44
  • そですね~ 「背理法」より「わびさび」のほうが認知度高そう(特にお年寄りに) :D
    – chocolate
    Jul 2, 2016 at 5:45

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