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What's the difference between 必要 and 必要性 and in what situation should I use
which
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2020-04-03T13:18:35.747
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2020-04-04T05:46:41.750
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word-choice
必要 and 必要性 usages
Dictionaries say 必要性 means そのものが、どれだけ必要であるかの度合い. That is "the degree of
necessity". For example, we can say 日本語を勉強するうえで、この教科書の必要性はとても高い. 必要 can't be
replaced with 必要性 in this sentence.
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2020-04-03T20:06:02.410
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2020-04-03T20:06:02.410
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First of all, 必要 is primarily a **na-adjective** rather than a noun. It means
"necessary".

  * ✅必要な量 ❌必要性な量  
necessary amount

  * ✅それは必要だ。 ❌それは必要性だ。  
That is necessary.

  * ✅必要になった。 ❌必要性になった。  
It became necessary.

If you have _really_ used 必要 only as a pure noun, this is the most important
difference you have to remember. (And I think this is why your question looked
confusing and got downvotes. Your question almost looks to me like "What's the
difference between _necessary_ and _necessity_?", which is too elementary.)

* * *

That being said, there _are_ some cases where 必要 is used as a noun. In some
cases, 必要 and 必要性 are interchangeable, but in some cases, they are not:

  * ✅読む必要がある ✅読む必要性がある 
  * ✅読む必要がない ✅読む必要性がない 
  * ✅必要に応じて読む ❓必要性に応じて読む 
  * ✅読む必要を感じる ✅読む必要性を感じる 
  * ✅必要に駆られる ✅必要性に駆られる 
  * ❌必要を検討する ✅必要性を検討する
  * ❌読む必要が高い ✅読む必要性が高い
  * ❌読む必要が乏しい ✅読む必要性が乏しい
  * ❌ワクチンの必要について ✅ワクチンの必要性について

It's hard to generalize and I may be wrong, but I feel:

  * When the focus is the **degree** of necessity (高い/低い/etc), 必要性 must be used, whereas 必要 and 必要性 are usually interchangeable when the focus is the **existence** of necessity (ある/ない/感じる/etc).
  * 必要 follows a verb, but it tends not to be linked to a noun using の.
  * 必要性 sounds a little more formal or academic.
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2020-04-04T05:46:41.750
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