It's not rocket science. You encounter them just about every day. They're everywhere you look. Not complex. Not complicated. Just incomplete.
I'm writing about "gray boxes" today. The term isn't in vogue, like "black swans" or "pink flamingoes" a few years back. But I think it ought to be a the top of mind from time to time.
I think about "boxes" based on two criteria: what you can see and what you can know about a process or thing.