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I always get a bit confused with `だろう`, which I've [asked about
before](https://japanese.stackexchange.com/questions/2049/what-is-the-proper-
use-of-%E3%81%A0%E3%82%8D%E3%81%86-is-it-masculine-and-how-is-it-different-
from-%E3%81%A8). In my other question, I got the impression that `たろう` meant a
certain amount of assertion, and `だろうか` left some room for uncertainty.

This sentence is at the end of a longer piece of text in my JLPT practise
book:

> 教師{きょうし}のアドバイスに耳{みみ}を傾{かたむ}ける必要{ひつよう}もあるのではないだろうか。

I took it to mean something like, "Should we necessarily listen to the advice
of teachers?" The rest of the text, too long to reproduce here, explores two
sides of an issue, which the teachers are on one side of. So I thought it
reasonable that the author is leaving it to the reader to decide if they want
to agree with the teachers or not.

However, from the content of the questions, it seems that the book is telling
me that the person who wrote the above sentence definitely sides with the
teachers.

So, is `だろうか` more assertive than I thought it was?

Would it be more accurate to translate it as: "We should definitely listen to
the advice of teachers."
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grammar word-choice jlpt
Does this use of だろうか indicate assertion of an opinion?
It's a tentative statement, not an assertion of fact, but the writer's opinion
clearly shows through.

I like "Perhaps (there is a case to be made that)..."
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The use of のではないだろうか I think you understand as 'isn't it the case' or 'it
might be the case'.

What may be confusing is that だろうか can used for casting strong, questioning
doubt on the preceding text. So this use is actually fairly different and
almost opposite of だろう and のではないだろうか.

For my source, surprisingly I don't see that many online examples, so I'll
copy out of an old unicom 実力アップ jlpt 2 kyuu book.

だろうか -

文の形は疑問文だが、否定が強い。（反語）

1.お金で愛情買うなんて、そんなことできるだろうか。

2.あんなうそつきの人の言うことを信じていいだろうか。

3.彼のように無能な人間が、いったい社長になれるだろうか。

It seems like right before だろうか there often comes a potential form of the
verb, but that is not always the case, as seen in example 2.

And in fact, when I looked だろうか up in the index it was listed as だろうか（できるだろうか）
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I don't think that comparing だろうか to だろう in terms of assertion strength is the
right approach. Try tearing the sentence down and rebuilding it...

  * (X する) 必要もある。 = There is also a need to X.
  * (X する) 必要もあるのではないか。 = Isn't there also a need to X? 
  * (X する) 必要もあるのではないだろうか。 = Isn't there perhaps also a need to X?

The first is a bald statement. The second is a rhetorical question, which
might be more literally translated "Is it not the case that there is also a
need to X?" It is TECHNICALLY possible that listeners/readers might respond
"Indeed, that is not the case. There is no need to X whatsoever," but that
would not be expected, in English or Japanese.

The third, the version used in your target sentence, is the same sort of
rhetorical question, but ない has changed to ないだろう. (You could also use なかろう [a
bit old-fashioned/informal] or ないでしょう [more polite].) In other words, the
"not" in "is it not the case" has changed from straight assertion to pseudo-
tentative/hedged assertion. And I say "pseudo" because here the
"tentativeness" is just a politeness strategy, exactly like the "perhaps" in
my translation.

Compare this to the version using just だろう:

  * (X する) 必要もある。 = There is also a need to X.
  * (X する) 必要もあるのではない。 = There is no need to X, either.
  * (X する) 必要もあるのではないだろう。 = (I put it to you that) there is no need to X, either.

(That last one might also be translated as a rhetorical question -- "There is
no need to X, either, right?" -- but the implied answer is completely opposite
from the ないだろうか version as you can see.)

So you can see that the だろう and だろうか versions are as different as the ない and
ないか versions. This is what I meant when I said, at the top, that comparing だろう
and だろうか as if they worked the same way but indicated different levels of
certainty is the wrong approach.
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