Some good reading on the etymology of the word here, although it doesn’t answer your question directly:
" The Japanese verb wakaru is most often glossed as to understand . However, wakaru is intransitive, and it takes the thing that is understood, distinguished, or recognized as the subject (usually marked by particle が (ga)), and not the object (usually marked by particle を (o)). Strictly speaking, wakaru is thus closer to English to be understandable , as the verb wakaru describes the thing itself, unlike English to understand , which describes the action of the person doing the understanding, distinguishing, or recognizing."
It never even occurred to me that I conjugate it that way!
My guess is that the English translation “to distinguish” (or more accurately, to be distinguishable!) is probably the closest bridge between the meanings “understand” and “separate”. The root “dis” even means “apart”.
You can distinguish one red marble in a bag from all the other blue marbles. That both carries this mental image for me of the red marble standing out, or being apart from, all the others; you can see it’s borders; in your mind’s eye, you draw a mental border around it’s edges and cut it away from all the other noise in the picture. This sense of “distinguish” seems related to separating, to me.
But “distinguish” also means you’re seeing it clearly-- you can’t distinguish a blurry shape in the bag if you have bad eyesight. To see it clearly and sharply sounds a lot like what your mind would do with a hazy, non-concrete concept. You can distinguish a concept and make it out-- it is distinguishable in your mind. You understand it
PS-- this reminds me of another set of things that never occurred to me were related until I started studying Japanese: The “r” and the “l” sounds seemed like a totally arbitrary group of sounds to confuse for each other at first. But then hearing the English examples “Airy” “Eddy” and “Ellie” you can hear how they’re all very close (and how weirdly enough the Japanese “r” can be replicated with the English “dd”!)