There’s a scene in the movie Memphis Belle which I think really sums up the rational behind the philosophical doctrine of cosmopolitanism. If you don’t know, cosmopolitanism is the idea that all people, just by virtue of being human, possess some intrinsic “something,” and this binds us (or should bind us) all together into a single community.
Memphis Belle is about the crew of an American bomber during WWII. On their last bombing run before being sent home, and after much danger, they finally drop their bombs on the enemy target. They now have to return safely to base though. The captain, to motivate his men, says something to the effect of “OK boys, we’ve done our job for Uncle Sam. Now we fly for ourselves!”
This, it seems to me, gets at the heart of cosmopolitan thought. The captain here is positing that at a certain point our social identity– those values and responsibilities and ways of being that are imposed on us by our community– fall away. After dropping their bombs, he and his men are no longer “Americans” on a mission from “Uncle Sam.” Instead, they are simply humans, existing in their most base animal form. They are now obeying a calling higher than community– that of survival.
I think that if cosmopolitanism is going to make any sense, we have to think of it in just these terms. Where, after all, does this magical “something” that ties all humans together come from? If you’re a theist, you can say that it is “God given.” If you don’t believe in an all-powerful creator though, this doesn’t work.
Alternatively, you could say that this shared bond arises from some sort of social contract. We all agree that all humans are family and that therefore makes it so. This is a nice idea, and theoretically feasible, but it just doesn’t account for facts on the ground. In the worst incidents of oppression (when we most strongly need cosmopolitan theory), the oppressors simply disregard the humanity of their victims. Hutus or Israelis or drunken frat boys agree among themselves (make a social contract, you could say) that their enemies lie outside the bonds of community. In short, if we take a contractarian approach, it seems that cosmopolitanism abandons us when we need it most.
So, to make cosmopolitanism work– I.E. to validate the idea that we are all, despite our cultural differences, members of the same community– we need some sort of external, non-negotiable justification. As a pragmatist, I’m not big on universal rules. It seems though that if there is any bond which connects all humans it is necessarily a physical one. We all have bodies. We all eat and sleep and (except for Macduff) come from women. Stripped of our socially-generated material trappings (clothes, cars, houses) and our socially-constructed values (love, honor, belief) we are all basically the same.
The captain of the Memphis Belle knows this. He locates his crew’s most basic and therefore most communal desire– bodily survival– and references it as a motivational point. The body is something they can all rally around. Could such rhetoric be scaled up though? Could one make the claim that we all have certain rights not because of God or the constitution, but simply by virtue of existing within a human body? Don’t hurt my body, this argument would go, because my body is the same as yours. Perhaps. Perhaps advocates of cosmopolitanism should watch Memphis Belle.
MATTHEW OVERSTREET 
I like you suggestion that, “to make cosmopolitanism work […] we need some sort of external, non-negotiable justification.” But I doubt the human body provides that sort of justification, if only because human bodies differ, and bodily survival is important to non-humans as well. I think the scene you describe expresses a-politism rather than cosmopolitanism: “Instead, they are simply humans, existing in their most base animal form. They are now obeying a calling higher than community– that of survival.” They’re obeying a call that requires no other men, just the individual.
Thanks for the thoughtful response. A couple thoughts. #1 The claim here would be that human bodies, despite differences in superficial things like size, color, etc., are fundamentally the same. I think you can argue this. I’ve never met a human who doesn’t eat, sleep, breath, etc. #2 The point would be to mobilize this discourse for political ends. In the Memphis Belle example, they have to work together to survive, so the captain’s statement has that political component. He uses this shared interest (bodily survival) to tie the community together to achieve the shared end (getting the plane back to base).