What is the difference between 「とは限らない」and 「とも限らない」


When looking it up on the internet I find that とは限らない means
「イメージと違って、100％～と言えないという意味」. But the problem is I also get similar meanings for
とも限らない.

I've asked a Japanese speaker about it and they gave the answer below:

> 「Xとは限らない」は、Xが成立しないことがある、と言う意味。  
>  「Xとも限らない」は、Xが成立しないことがあるし、同類の事柄があることを暗示している。
>
> たとえば、  
>  「お金持ちになれたらいいのに。」  
>  A:「お金持ちが幸せだとは限らないよ。」  
>  B:「お金持ちが幸せだとも限らないよ。（お金があるとトラブルになることもあるから。）」
>
> 「早く日本に行って日本語を勉強したい。」  
>  A:「早く日本に行くことが良いとは限らないよ。」  
>  B:「早く日本に行くことが良いとも限らないよ。（いまここでしっかり基礎を学ぶことも大切かもしれないし、
> 日本で生活するための準備をきちんとすることも重要だよ。）」

Based on the answer he gave me, I still don't get the difference. They both
seem the same but I know there has to be a difference.




とはいえない and ともいえない are interchangeable, but the latter sounds a little milder
and indirect. This usage of も is described on 明鏡国語辞典 as the twelfth
definition:

> (12) さりげなくとりたてて、文意をやわらげる。  
>  (ア) 《多く、文末表現に使って》一歩引く気持ちで、その判断をやわらげる。
>
> 「自由に発言すること **も** 可能だ」「もう帰って **も** いいんだぜ」「彼だって素直になること **も** ある」「その件なら知らなく
> **も** ない」「そんな感じがしないで **も** ない」「乞われれば応じ **も** しよう」「やむをえないと考え **も** するが…」

一歩引く気持ち (literally "take-a-step-back feeling") is a confusing explanation, but
anyway this も adds an extra nuance of admitting, concession, euphemism, etc.
Perhaps an English word that has a similar role is an interjection 'well' as
in " _Well_ , you can say that, but..." or "They were, well, not quite good."
They are perfectly grammatical without 'well', but they sound a little milder,
"nuanced" or "deep" with it.

I think the native speaker you asked explained like that because they wanted
to say this も has such a "nuance". But this role of も may be difficult to
grasp if you try to analyze it as an extension of 'also' too much. In this
case, I think this も in とも限らない can be translated as "you know" or "well" (if
it needs to be translated).

By the way, this usage of も is explained in the dictionary next to [this
も](https://japanese.stackexchange.com/q/42048/5010). And [this
question](https://japanese.stackexchange.com/q/32886/5010) is also loosely
related.




とはいえない and ともいえない are interchangeable, but the latter sounds a little milder
and indirect. This usage of も is described on 明鏡国語辞典 as the twelfth
definition:

> (12) さりげなくとりたてて、文意をやわらげる。  
>  (ア) 《多く、文末表現に使って》一歩引く気持ちで、その判断をやわらげる。
>
> 「自由に発言すること **も** 可能だ」「もう帰って **も** いいんだぜ」「彼だって素直になること **も** ある」「その件なら知らなく
> **も** ない」「そんな感じがしないで **も** ない」「乞われれば応じ **も** しよう」「やむをえないと考え **も** するが…」

一歩引く気持ち (literally "take-a-step-back feeling") is a confusing explanation, but
anyway this も adds an extra nuance of admitting, concession, euphemism, etc.
Perhaps an English word that has a similar role is an interjection 'well' as
in " _Well_ , you can say that, but..." or "They were, well, not quite good."
They are perfectly grammatical without 'well', but they sound a little milder,
"nuanced" or "deep" with it.

I think the native speaker you asked explained like that because they wanted
to say this も has such a "nuance". But this role of も may be difficult to
grasp if you try to analyze it as an extension of 'also' too much. In this
case, I think this も in とも限らない can be translated as "you know" or "well" (if
it needs to be translated).

By the way, this usage of も is explained in the dictionary next to [this
も](https://japanese.stackexchange.com/q/42048/5010). And [this
question](https://japanese.stackexchange.com/q/32886/5010) is also loosely
related.

