Abstract: Kang Youwei develops a theory of world political order in his Book of Great Unity in an age of competing among nation-states. He advocates a world state with great unity to go beyond the separation and conflict among nation-states. Zhao Tingyang echoes Kang Youwei to some degree in his project of tianxia system. A dialogue among Kang Youwei, Zhao Tingyang, Kant, Habermas and Thomas Pogge leads us to have a new understanding of the significance of a world state, which is “a discarded choice” in Kant.