Why is する's potential form できる?


Godan verbs have the potential form える, while ichidan verbs and 来る use られる for
both the passive and the potential. する also has the passive される, why isn't
that or something like すれる used as its potential, instead of the suppletive
できる?




I can’t answer your question of **why** できる is used as a substitute for the
potential form of する, but let me share what I think is relevant, anyways.

What is now known as the “potential form” started to be used to describe
something’s property, rather than someone’s ability. For example, what would
eventually become 読める meant that a piece of writing was “readable,” or worth
reading, rather than someone was capable of reading something.

Related to this are forms of certain transitive verbs that were used like
intransitive verbs. One example is what would become 切れる, which is still used
as the intransitive version of 切る as in the following example. (In English,
the same verb “to cut” can be used both transitively and intransitively.)

> この包丁はよく切れる。  
>  This kitchen knife cuts well.

知れる is another example. Though “potential” in form, it doesn’t normally
indicate potentiality (可能) but “spontaneity” (自発).
[[1](https://www.nhk.or.jp/bunken/summary/kotoba/term/153.html),
[2](https://japanknowledge.com/articles/blognihongo/entry.html?entryid=380)]
It is now used mostly in certain set phrases in the sense that something is
easily known.

> 彼の気が知れない。  
>  I can’t understand what he is thinking.  
>  (lit. His feelings cannot be known.)

> 彼の能力はたかが知れている。  
>  His skills don’t amount to a hill of beans.  
>  (lit. The upper limit of his skills is known.)

される is similar in that it may indicate “spontaneity” (自発) but not potentiality
(可能).

> 接戦が予想される。  
>  A close contest is expected.

As far as syntax goes, the subjects in these examples are all something, not
someone capable of doing something. I don’t know how and when the potential
form came to be used to describe someone’s ability, but even in that newer
usage, the object of the action is still normally marked with が, as if it is
the subject.

> 漢字が読める。

As for できる, it literally means “to come out.”
[[3](https://www.fleapedia.com/%E4%BA%94%E5%8D%81%E9%9F%B3%E3%82%A4%E3%83%B3%E3%83%87%E3%83%83%E3%82%AF%E3%82%B9/%E3%81%A6/%E5%87%BA%E6%9D%A5%E3%82%8B-%E3%81%A7%E3%81%8D%E3%82%8B%E3%81%A8%E3%81%AF-%E6%84%8F%E5%91%B3/)]
If someone is capable of doing something, that thing comes out in a desirable
state when they do it. Though I couldn't find any reference, I suppose the
focus shifted at some point of time from something’s “spontaneous doability”,
which される would have indicated, to how it comes out as a result of doing it.

Note that what comes out is still marked with が as the syntactic subject.

> 接戦が予想できる。  
>  We can predict a close contest.  
>  (lit. A close contest comes out of our prediction. (?))




I can’t answer your question of **why** できる is used as a substitute for the
potential form of する, but let me share what I think is relevant, anyways.

What is now known as the “potential form” started to be used to describe
something’s property, rather than someone’s ability. For example, what would
eventually become 読める meant that a piece of writing was “readable,” or worth
reading, rather than someone was capable of reading something.

Related to this are forms of certain transitive verbs that were used like
intransitive verbs. One example is what would become 切れる, which is still used
as the intransitive version of 切る as in the following example. (In English,
the same verb “to cut” can be used both transitively and intransitively.)

> この包丁はよく切れる。  
>  This kitchen knife cuts well.

知れる is another example. Though “potential” in form, it doesn’t normally
indicate potentiality (可能) but “spontaneity” (自発).
[[1](https://www.nhk.or.jp/bunken/summary/kotoba/term/153.html),
[2](https://japanknowledge.com/articles/blognihongo/entry.html?entryid=380)]
It is now used mostly in certain set phrases in the sense that something is
easily known.

> 彼の気が知れない。  
>  I can’t understand what he is thinking.  
>  (lit. His feelings cannot be known.)

> 彼の能力はたかが知れている。  
>  His skills don’t amount to a hill of beans.  
>  (lit. The upper limit of his skills is known.)

される is similar in that it may indicate “spontaneity” (自発) but not potentiality
(可能).

> 接戦が予想される。  
>  A close contest is expected.

As far as syntax goes, the subjects in these examples are all something, not
someone capable of doing something. I don’t know how and when the potential
form came to be used to describe someone’s ability, but even in that newer
usage, the object of the action is still normally marked with が, as if it is
the subject.

> 漢字が読める。

As for できる, it literally means “to come out.”
[[3](https://www.fleapedia.com/%E4%BA%94%E5%8D%81%E9%9F%B3%E3%82%A4%E3%83%B3%E3%83%87%E3%83%83%E3%82%AF%E3%82%B9/%E3%81%A6/%E5%87%BA%E6%9D%A5%E3%82%8B-%E3%81%A7%E3%81%8D%E3%82%8B%E3%81%A8%E3%81%AF-%E6%84%8F%E5%91%B3/)]
If someone is capable of doing something, that thing comes out in a desirable
state when they do it. Though I couldn't find any reference, I suppose the
focus shifted at some point of time from something’s “spontaneous doability”,
which される would have indicated, to how it comes out as a result of doing it.

Note that what comes out is still marked with が as the syntactic subject.

> 接戦が予想できる。  
>  We can predict a close contest.  
>  (lit. A close contest comes out of our prediction. (?))

