Difference between 原因 and 理由


The posts
[here](https://japanese.stackexchange.com/questions/11877/reason-%E4%BA%8B%E7%94%B1%E3%81%98%E3%82%86%E3%81%86-vs-%E7%90%86%E7%94%B1%E3%82%8A%E3%82%86%E3%81%86)
and [here](https://japanese.stackexchange.com/questions/35904/what-is-the-
difference-
between-%E5%8E%9F%E5%9B%A0-and-%E8%A6%81%E5%9B%A0-%E8%A7%A3%E6%9E%90-and-%E5%88%86%E6%9E%90)
do not address the difference between 原因 and 理由.

So, in short, what are the difference in usage between these words? Both seem
to mean reason.

> 日本に来た理由は見つからない  
>  日本に来た原因は見つからない

その違いは？[中国語](https://chinese.stackexchange.com/questions/33468/usage-
of-%E7%90%86%E7%94%B1-vs-%E5%8E%9F%E5%9B%A0)と同じでしょう？




原因 is _cause_ , whereas 理由 is _reason_. They are similar, but the former
assumes there was an observable but unexpected "event".

When you are describing the reason for something that is always true, you
should use 理由:

  * 素数が無限にある理由  
the reason why there are infinite prime numbers

  * 生きる理由  
the reason to live

In the following example, 原因 sounds like the speaker is asking about the
direct cause (e.g., traffic accident), whereas 理由 may sound like the speaker
is asking about more indirect or ultimate reasons (e.g., fate, grudge).

  * 彼が死んだ原因  
the cause of his death

  * 彼が死んだ理由  
the reason of his death




原因 is _cause_ , whereas 理由 is _reason_. They are similar, but the former
assumes there was an observable but unexpected "event".

When you are describing the reason for something that is always true, you
should use 理由:

  * 素数が無限にある理由  
the reason why there are infinite prime numbers

  * 生きる理由  
the reason to live

In the following example, 原因 sounds like the speaker is asking about the
direct cause (e.g., traffic accident), whereas 理由 may sound like the speaker
is asking about more indirect or ultimate reasons (e.g., fate, grudge).

  * 彼が死んだ原因  
the cause of his death

  * 彼が死んだ理由  
the reason of his death

