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Wikipedia claims that Japanese verbs are a closed class and that loanwords
from Chinese always use する. 信じる, 感じる seems to be an exception. Why aren't they
信をする and 感をする? Maybe because one kanji is too short?

Also, what is the origin of the ending じる used with these two borrowed verbs?
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2013-03-13T00:24:49.180
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2014-06-16T16:48:25.083
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kanji etymology
Origin of 信じる, 感じる, etc?
-ziru is from -zuru, which in turn is verb -suru voiced due to compounding. zuru becomes ziru during the push to normalize verbs to 一段 class.
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2013-03-13T00:29:19.373
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2013-03-13T00:29:19.373
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### Japanese verbs as a class

The [**Word class system** section of the **Japanese grammar** Wikipedia
article](https://en.wikipedia.org/wiki/Japanese_grammar#Word_class_system)
does currently state that Japanese verbs and adjectives are [closed
classes](https://en.wikipedia.org/wiki/Closed_class). Strictly speaking, I
don't think this is quite correct -- closed classes don't have new members,
and generally don't have many members. Japanese verbs are quite numerous
indeed, and it's not uncommon for new ones to pop up -- witness recent
neologisms like [ググる "to
Google"](http://www.weblio.jp/content/%E3%82%B0%E3%82%B0%E3%82%8B), or [スタバる
"to go to
Starbucks"](http://kotobank.jp/word/%E3%82%B9%E3%82%BF%E3%83%90%E3%82%8B).
Sure, these are slang, but they're also new verbs. It's also not unheard of
for folks to coin new verbs by just tacking a -る on the end, such as [クッキングる
"to cooking"](http://twicsy.com/i/TFW4Yc) > humorous for "to cook", or
[ドライビングる "to
driving"](https://www.google.com/search?q=%22%E3%83%89%E3%83%A9%E3%82%A4%E3%83%93%E3%83%B3%E3%82%B0%E3%82%8B%22)
> humorous for "to drive [a vehicle]".

That said, I think it would be accurate to say that _-jiru_ verbs are a closed
class. These all seem to have arisen through regular sound changes from
`[single kanji]` + transitive verb suffix す.

### _Shinjiru_ and other _-jiru_ verbs

As an addendum to Dono's answer, Shogakukan's entries suggest that these
`[single kanji]` + じる terms were originally used as verbs in Japanese by
simply appending す, the transitive auxiliary verb (superseded by modern する).
This す then became ず due to [rendaku](http://en.wikipedia.org/wiki/Rendaku)
caused by the final ん in 信{しん}, and then became ずる when the attributive and
terminal forms for verbs fused, and finally became じる due to the shift to
_ichidan_ verbs that Dono mentions.

You can track some of this back via online resources by starting at [the
Daijirin entry for
信じる](http://www.weblio.jp/content/%E4%BF%A1%E3%81%98%E3%82%8B). Note how it
says:

  * 「 信ずる 」に同じ。  
Same as _shinzuru_.

Click on 信ずる and you'll jump straight to the 信ず entry. The second line here
says:

  * 「信ず」の口語形としては、サ行変格活用の動詞「信ずる」が対応する。  
The colloquial form of _shinzu_ is the _sa-hen_ conjugation verb _shinzuru_.

So we have a clear historical progression:

  * `[single kanji ending in ん]` + す → 
  * `[single kanji]` + ず → 
  * `[single kanji]` + ずる → 
  * `[single kanji]` + じる

Other examples include:

案{あん}じる 演{えん}じる 応{おう}じる 禁{きん}じる 準{じゅん}じる

生{しょう}じる 談{だん}じる 転{てん}じる 封{ふう}じる 論{ろん}じる

Other terms borrowed from Chinese and used as verbs in Japanese historically
took the form `[kanji term]` + す, with す again changing to する when the
terminal and attributive verb forms merged. Rendaku generally didn't happen
for multi-character words ending in ん, which is why you'll only see these -じる
forms for single-character on'yomi terms, and mostly (but not always) where
the on'yomi ends in ん.
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