Hedley Bull's major idea about the international system was one that contradicted the major premise of realism. Realism holds that the international order is anarchical. It says that no entity can force states to do anything and so states aggressively pursue power and security. In this way, the international order is something of a Hobbesian war of each against all.
Bull sees the world in more Lockean terms. He agrees that the international order is anarchical, but he argues that this drives states to cooperate. He says that states will interact enough to form a system where they feel bound to obey rules to at least some degree. The states will form a sort of social contract to avoid the Hobbesian state that the realists envision.