Can someone explain the logic of the grammar "とは限らない"

Question

I found this example sentence in "A Dictionary of Intermediate Japanese Grammar" under a grammar point unrelated to "とは限らない":

"運動をよくする人が必ずしも長生きするとは限らない。"

I understand the meaning of the sentence ("People who exercise a lot don't necessarily live a long time.") but I'm struggling to understand the logic behind this usage of "とは限らない". If this dictionary did not have an English translation I would think it meant "Living a long time is not just limited to people who exercise a lot" since "限る" means "to limit" in other contacts.

Without the "とは限らない", the sentence means "People who exercise definitely live a long time." From my understanding of "必ずしも", you can say "長生きしない" instead of "長生きする" and it should also have the meaning "People who exercise a lot don't necessarily live a long time".

So why is "とは限らない" used this way? And bonus question, what is the semantic difference between "運動をよくする人が必ずしも長生きするとは限らない" and "運動をよくする人が必ずしも長生きしない"?

Answer 1

The expression "とは限らない" is used when we negate a general statement.

In your example, the statement we want to negate is:

運動をよくする人は必ず長生きする

You should not interpret the verb "限る" as "limit" in this case. This verb has a meaning "can't be asserted" with a negative word.

"しも" after "必ず" is not necessary but emphasizes the negation.

Here are some examples using "とは限らない":

雷が鳴ったらいつも雨が降るとは限らない。
多数の意見が常に正しいとは限らない。

Answer 2

とは限らない is a fixed expression meaning "not necessarily", and if I were you I would memorize it as such. I'm not sure there's an intuitive way to understand it logically. For example, there's no とは限る.

As for your bonus question, your two sentences mean pretty much the same. 必ずしも is an expression which needs to go with some negative expression after it, and also means "not necessarily". It is different from 必ず, and you cannot use it to mean "necessarily" or "definitely".

Here 必ずしも...とは限らない uses both of the above expressions for extra emphasis.

必ずしも is a bit confusing in the sense that other grammatical elements can come between it and the negative that needs to follow. The below sentences pretty much mean the same:

必ずしも長生きしない
必ずしも長生きするとは限らない
必ずしも長生きするわけではない
必ずしも長生きするとは言えない

For comparison, your sentence "Living a long time is not just limited to people who exercise a lot" could be translated as:

長生きは運動をよくする人に限られない

Answer 3

The と particle turns whatever is before it into a statement... Sort of like how の turns it into a noun phrase... と turns it into a sort of.... adverbial statement.

The は is key

それが正しいとは限らない

それが正しいと　is the statement

は　makes that statement the subject.

限らない is the action performed by the phrase. It is a verb, so something must be doing something, right?

Notice how it is Intransitive... and thereby more or less descriptive of the と phrase

Therefore, it is saying

それが正しいとは... the phrase of それが正しい is a non limiting phrase...

In other words...　それが正しい isn't self evident... It isn't restrictive. It is not the only possible phrase.