The physical world knows no borders, and it is precisely its indifference to political frontiers that reveals the deepest truth of the ancient Tianxia: the principle of "no-outside." Today, the challenges that confront us—from climate to algorithms, from pandemics to artificial consciousness—are inherently "without an outside." They exist beyond every wall. Perhaps, then, a peaceful world order will not arise from a treaty, but from the humble acknowledgment of a necessity already written into the physics of our time. Coexistence ceases to be an ethical ideal and becomes the only rational strategy for inhabiting problems that no sovereignty, alone, can contain. The "heaven" of ancient philosophy is now the shared roof of a global vulnerability, and its law is interdependence.
