That's prolly not gonna be a shock to anyone, but I'd have to say.... balance.
Balance doesn't mean "in the middle" more like "in between" and "one can't exist without the other".
In reality nothing is ever just white or just black. It's always in between, leaning towards one side or the other.
One of my fav poem fragments, by Ursula K. Le Guin from "A Wizard of Earthsea:
“Only in silence the word,
Only in dark the light,
Only in dying life:
Bright the hawk's flight
On the empty sky.
—The Creation of Éa”
That's also one of the reasons my character is a druid and my font is gray...
edit: grammar