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On <http://www.guidetojapanese.org/learn/grammar/should#part5>, the author
doesn't explain the etymology of べからず. Can anyone explain?
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2015-02-24T05:33:03.813
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2015-02-26T09:08:20.593
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grammar etymology
Why is the negative form of [べき] べからず?
# Conjugation of べき

べき, or rather べし (kanji 可し), is an auxiliary adjective* just like for example
熱い.

However, it retains the classical conjugation of adjectives. 熱い had once been
熱【あつ】き, and there existed an additional form 熱【あつ】し. 熱き is used for modifying
nouns (熱き炎), 熱し for ending sentences (炎は熱し).

In the classical conjugation, there existed an あり-form for each adjective: The
verb ある ( _to be_ ) joined with the adjective (in its 連用形-base/masu-form)**.

  * 熱き → 熱く+ある → 熱かる***
  * べき → べく+ある → べかる

Literally it meant _to be [adjective]_ , and it could now be modified further
like any other verb, and be joined with some helper verbs etc. that required a
verb.

Other than the adjective ない for negation, in classical Japanese you could
negate verbs by adding the helper verb ず/ぬ. Some traces of this remain in
modern Japanese in あらぬ疑い or in the shortened form せん (negation of する).

Like with ない, ず/ぬ is added to the 未然形-base:

  * 行く→行かない→行かず
  * ある→[あらない](https://japanese.stackexchange.com/questions/12644)→あらず

Thus:

  * 熱かる→熱からず
  * べかる→べからず

Finally, ず is both the 終止形 (for ending sentences) and 連用形 (for joining
words/actions), while ぬ is the 連体形 (for modifying nouns).

Note that you can add ある again to form べからざる. This is often used to modify
nouns (instead of べからぬ).

# Examples:

  * 無用の者立ち入る **べからず**
  * 働かざる者食う **べからず**
  * 許す **べからざる** 行為
  * この庭のいたづらに広きこと、あさましく、 **あるべからぬ** 事なり。 (from 徒然草, 第２２４段, modern: この家の庭はいたずらに広くて、みっともないものである。)

* * *

# Further reading:

  * [How to negate べき](https://japanese.stackexchange.com/a/18646/1016)
  * [The function of あり in i-adjectives](https://japanese.stackexchange.com/questions/11135)
  * [An introduction to classical Japanese conjugation](https://kafkafuura.wordpress.com/classical-japanese/) See especially the paragraph that starts with (use ctrl+f) `All く形容 have an accompaning あり form`
  * [An historical grammar of Japanese, auxiliary adjectives](https://archive.org/stream/historicalgramma00sansuoft#page/110/mode/2up) by Sir George Bailey Sansom

* * *

# Notes

* As far as its conjugation is concerned, べし mostly behaves like any other i-adjective. However, note that its usage is rather odd, especially what remains in modern Japanese: you would expect adjectives to join to the 連用形 (such as 分かりやすい) - or perhaps to the 未然形 like ない. But in reality we have got forms such as 来るべき時.

** Sources for this etymology:

  * [An historical grammar of Japanese, p. 105](https://archive.org/stream/historicalgramma00sansuoft#page/104/mode/2up)

  * 全訳古語辞典・第三版、旺文社、Miyakoshi・Sakurai・Ishii・Oda 2003, かり

> 〔形容詞、または形容詞型に活用する助動詞「べし」「まじ」「ごとし」などの連用形語尾「く」に、動詞「あり」が付いた「くあり」のつづまったもの〕

  * A history of the Japanese language, by Bjarke Frellesvig (Cambridge University Press, 2010), p. 233, chapter _8.2.1 Secondary conjugations: ar- extended forms_

> In OJ the existential verb _ar-_ was used with the infinitive of the
> adjectival copula (-ku), the regular copula ( _ni_ , _to_ ) and the negative
> auxiliary ( _-zu_ ) to form analytic forms, and these combinations sometimes
> fused phonologically: ADJ- _ku ar_ => ADJ- _kar_ , [...] In EMJ [Early
> Middle Japanese] the fused forms gave rise to secondary conjugations, all of
> which belong to the _r_ -irr conjugation and have the basic paradigm in
> Table 8.7. [eg. for adjectives: -kar, kara, kaQ-, kari, kari, karu, kare,
> kare]

*** This plain form is rarely used, because as mentioned, it is used mainly
for more complex conjugations. See this question, [the function of あり in
i-adjectives](https://japanese.stackexchange.com/questions/11135).
Occasionally, the plain form gets used: 「はしたなめ、煩はせ給ふ時もおほかり」(from 源氏物語, modern:
（桐壺の更衣を）きまり悪い目にあわせ、苦しめ困らせなさるときも多い。)
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