Cosmopolitanism is the moral and political idea that all human beings belong to a single community, so every person, not only our fellow citizens, holds a moral claim on us — though cosmopolitans differ sharply over how far that claim reaches. It runs from the Greek Cynics and Stoics through Kant's plan for perpetual peace to today's arguments over global justice, migration, and human rights.
Asked where he came from, the Greek philosopher Diogenes is said to have answered that he was a citizen of the world. In an age that took the city-state to be the whole horizon of loyalty, it was close to saying he belonged nowhere. Twenty-three centuries later, the same three words sit on protest signs and passports of the mind — still promising everything, still accused of meaning nothing.