Someone else's soul - darkness
That's what they say when you try to understand what's going through another person's mind and you realize it's impossible. No matter how you peek or investigate, the darkness remains.
The word "dark" is key here. It's not just "darkness", but dull and impenetrable darkness - like in a cave or underground.
The proverb reminds: do not judge strictly, do not draw hasty conclusions. You see the action, but you don't see what's behind it: fear, pain, hope or just plain tiredness.
Sometimes it's better to ask than to guess in the dark.
Proverbs
๐ฟ Russian proverbs