We treat those inside differently than we do those outside. This is necessary and good. The disaster at the southern border is an illustration of what happens when a community refuses to police its boundaries. A nation simply can’t exist without them. But neither can any human organization, whether it’s the family, a neighborhood, a corporation, or a school of thought. Knowing who’s “in” and who’s “out” is essential for allocating resources, building consensus, and organizing effectively to pursue goals.