What Does the "Volitional" Really Mean?

In everyday language and for early learners, when the verb ends in おう or よう, it is conventionally called the "volitional" form, for example:

1. 今夜はもうねよう (I'll just go to sleep for tonight).

It can also be used as sort of a hortative of sorts, such as:

2. 早く、学校に行こう (Hurry up, let's go to school).

So far, there aren't really problems with calling this form "volitional"; it expresses a will of some sort. However, in a more literary, less conversational context, the form can also be used to show a sort of certainty or "the way things are" (I'm not really sure how to explain this, so hopefully my example will help).

3. 光を持つ者に道は[拓]{ひら}こう (The road shall open for the one who bears the light). (Imagine this being written on a signpost in a video game or something).

This is where we start to run into problems: How does this relate to the meanings above, exactly? We begin to see that calling it "volitional" is more of a work-around we're using to attempt to translate something absent in our language. There are other phrases where this categorization fails as well, such as:

4. 彼が[足掻]{あが}こうが足掻くまいがどうにもならない (Whether or not he struggles, nothing will come of it).

This also introduces the issue of the negative "volitional," which, although rarely used, does not quite carry the meaning one would expect from a form known as the "volitional." I could mention other issues, like why we usually translate だろう as "probably," "surely," "most likely," etc., but this question is getting pretty long-winded as it is, so I'll ask my main question now.

What is the actual "idea" that this form is trying to express, and how does that lead to the interpretations of it we use in these situations? Also, if any of my examples are incorrect or using the form wrong, please tell me (I'm aware they don't all sound like something someone would actually say, though).

Answer

Same form, different meanings

What's referred to as the Volitional Form is also used to express what the speaker imagines "will happen".

The Volitional Form usually expresses 意思 (willingness) or 勧誘 (an invitation) but can also be used to mean 推測・推量 (a guess or expected result), especially in some kind of literary context.

Underlying Idea

I think the common thread might be, "expected to happen" or "expected to be the case".

In each expression there is an expectation of something happening or being the case based on either the intention or the guess of the speaker/writer; and I think that that is what this construction is meant to allude to.

English Comparison

"will", "shall"  

It's a little older English grammar, but it still checks out as indicating expectation.

• 
  

  今夜はもう寝よう
  (I'll be off to sleep for tonight).

  
  



• 
  

  早く、学校に行こう
  (Hurry up, won't (we) go to school?) = "let's go"

  
  


• 
  

  彼が足掻こうが足掻くまいが、どうにもならない
  (Whether he will struggle or not, nothing will come of it).

  
  


• 
  

  光を持つ者に道は拓こう
  
  (The road shall open for the one who bears the light).

  
  

Subjunctive（仮定）usage of the Volitional Form

• 
  

  雨が降ろうが降るまいが、試合が行われる
  (We will have the game, whether it will rain or not).

  
  


• 
  
  

  雨が降ろうが降るまいが、試合が行われる
  (We will have the game, whether it will rain or not).

  
  

Objective Supposition + Conditional Phrase (whether...or...)
= Conditional Phrase (Subjunctive Mood)

客観的推量の意 ＋「と・が」＝ 仮定表現（仮定法）