Potential form of non-volitional verbs

Question

I have recently learned that apparently non-volitional verbs cannot have potential forms.

Non-volitional verbs cannot have potential forms. This includes verbs of natural phenomenon like 降る, 光る, 流れる, and 凍る, those concerning human emotion and physiology (痛む, 痺れる (to be paralyzed), 羨む (to be jealous), any verbs that end in ある (as they have no volition), and any pattern that has no control involved like phrases with つく and いく such as 想像がつく (one can imagine) and 納得がいく (to accept as valid).

So 光れる and 凍れる for example are not valid conjugations? Is this invalidity strictly a structural limitation or is it semantic?

For example, I feel like "able to shine" or "able to be frozen" are reasonable concepts with semantic meaning. What about rephrasing as 光ることができる or 凍ることができる? Are these ungrammatical as well? If not, I don't see the problem with the potential form.

Answer 1

凍れる is (almost always) invalid not because it's syntactically prohibited as a rule but because it's (almost always) nonsense. The use of the れる form is limited to one's ability, potential or right; you cannot use it to describe the possibility of something happening.

• Anything can happen.
  どんなことも起こりうる。
  (×どんなことも起これる。)
• That can be true.
  本当かもしれない。/ それはありえる。
  (×本当であれる。)
• This lake can freeze in winter.
  この湖は冬に凍ることがあります。
  (×この湖は冬に凍れます。)

できる is also for ability, so you cannot say この湖は冬に凍ることができます, either. As a rule of thumb, whenever you want to translate "can", think if you can replace it with "be able to" or "be allowed to". Since "This lake is able to freeze" is strange, so is 凍れる.

However, you can use 凍れる when you are actually talking about someone's "ability to be frozen". For example, if a character like Iceman lost his ability to turn into an ice form, he might say もう凍れないんだ or もう凍ることはできないんだ. The same is true with 光れる. In this song, there is a metaphorical phrase もう光れない ("We can't shine any longer") as if he were a star in the sky.

Answer 2

光れる and 凍れる for example are not valid conjugations? Is this invalidity strictly a structural limitation or is it semantic?

I think it is more natural to say they are not idiomatic. Structurally, there is nothing wrong with them. Everybody will conjugate 光れる as 光れない etc if forced to.

光ることができる or 凍ることができる?

These are not idiomatic either. As you understand, the potential forms sound like subjects do the shining or freezing at their will. In reality, there are no such things (光れる does not apply to fireflies either). At the same time, it is easy enough to imagine that there are anime or game characters that 光れる or 凍れる. (Note 凍れる is able to freeze oneself rather than able to be frozen.) Even if these do not sound too natural, they make sense in some contexts (I guess 光を出せる or (自分を)凍らせられる may be used for such characters).

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Cf.

• 発光する can mean to glow (in the dark).
• 冷凍できる means able to be frozen/freezable (for food).