I study humans as a matter of survival. I need to understand how humans work, because I live in a world full of them. I notice things about them that they don’t recognize themselves. For example, humans define boundaries. It’s how they make sense of the world, how they organize it, how they pigeonhole all of its myriad bits.
A boundary surrounds a concept. What’s included is different in kind from everything beyond it. Without boundaries, the universe would be an indistinct blur, lacking mental handholds. Boundaries are clumsy organizational tools, though. They are seldom as well-defined as you might assume. The closer you look at the boundary, the more difficult it is to fix. As essential as boundaries are, they tend to be imprecise.
Consider the concept of North America. Anyone (I hope) can look at a map of the world and instantly point to that continent. When I start asking questions, though, the concept of North America, and the borders that define it, becomes murky.