理 vs 由 - what's the difference?

So if both mean “reason”, and the two of them together end up making the vocabulary for “reason”, 理由, what is the difference between them? Aside from their shape, of course.

Are they perhaps different kinds of “reason”, i.e. “reasoning / sense of reason” vs “the reason for x”?

This is fairly confusing, as if they are entirely the same, having two Kanji with different readings refer to the exact same concept seems… unreasonable.



I use my native translations instead of the English ones and those Kanji are translated as follows:

理: reason as in reasonable or logic
由: This is really the reason for sth. like “I fell because the ladder collapsed”

I hope this helps.


Thank you, that’s the kind of distinction I was looking for!


This is an extremely common way of constructing kanji compounds.

Spoilers, every possible concept has like 5 kanji or more.


Thank you, I was struggling with this too! :sweat_smile:

Are the two together a different kind of reason too?

理: logic (reasoning)
由: cause (the reason)
理由: ?


From my understanding, the different kinds of reason are what the kanji mean on their own, but aren’t actual words. When you see them used in a word, this is the kind of meaning you can expect them to have in it, as with other Kanji.

理由, on the other hand, is the actual vocabulary for when you want to say the word “reason”.

In the example sentences for it, it is used for both cause and reasoning, so I’m guessing the vocabulary word works the same as the English “reason” does and encompasses both possible applications.


Ah, I figured that was the case. But I wasn’t too sure. Thank you.

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They actually have kunyomi readings, though they’re not beginner words anyway.

理 is ことわり
由 is よし

You won’t encounter them often.


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