In the past three centuries, states have replaced empires and tribes as the dominant form of political organization. But one clear lesson of the twentieth century is that the vast powers of states can be put to disastrous as well as beneficent ends.

Philosophical reflection about states often begins with Thomas Hobbes and the rational justification of social order as mutually advantageous. Many more contemporary philosophers have ignored the state, however, focusing instead on justice and the rights and liberties that states should respect. Indeed, the most important work in political philosophy in the twentieth century (Rawls 1972) does not discuss the state—it lacks even one entry for "the state" in its index.

In recent years there has, however, been a renewed interest in the state that has developed along several lines. Some have used modern game theory to pursue Hobbes's question of the possibility of...