I'm not sure what the profound insight is supposed to be here. Everyone knows that borders are established by convention and agreement. That doesn't mean they d

I'm not sure what the profound insight is supposed to be here. Everyone knows that borders are established by convention and agreement. That doesn't mean they don't matter. Borders exist for the sake of the common good, in a way that's analogous to private property, which also exists for the sake of the common good. I realize this must be a strange notion for those of us raised in a culture that's been shot through with liberalism and liberal/radically individualist ideas (I have here in mind the philosophical stance, not liberal institutions), where private property is understood as prior to the common good and where the common good is conceived as something that's grudgingly ceded from private property. But traditionally, the common good was taken to be prior to private property, and private property to be something the exists for the sake of the common good because it enables human flourishing. (Incidentally, this understanding resolves ethical problems concerning the use of what would otherwise be taken to be private property during times of crisis, such as whether the taking of food from a privately owned stockpile, by starving people, during a famine would constitute theft.) Now, how particular borders are enforced or respected or treated is a matter of prudential political judgement. Schengen is not borderless. It is itself defined by a border that is enforced by Frontex! It is precisely the Schengen border that enables free movement within Schengen. Member states have determined that free movement between them and them only is for the common good of these member states. Furthermore, just because free movement is now possible doesn't mean borders have ceased to exist. Administrative borders still exist because they circumscribe legal jurisdictions, and these may or may not coincide with ethnic boundaries or cultural regions. States have simply become more permissive about enforcing movement because of their membership in Schengen. But if a crisis were to occur, such as a flood of migrants that threatens the good of a nation's population, you can be sure that member states would control the flow of people over their borders again, if only temporarily, to contain the crisis and protect the common good of the population. Protecting the common good is, after all, the job of the state.