Two decades ago it was widely assumed that liberal democracy and the Open Society had won their century-long struggle against authoritarianism. Although subsequent events have shocked many, F. A. Hayek would not have been surprised that people are in many ways disoriented by the society they have created. For him, the Open Society was a precarious achievement, in many ways at odds with the deepest moral sentiments. He argued that the Open Society runs against humans’ evolved attraction to “tribalism”; that the Open Society is too complex for moral justification; and that its self-organized complexity defies attempts at democratic governance. In this wide-ranging work, Gerald Gaus re-examines Hayek’s analyses. Drawing on work in social and moral science, Gaus argues that Hayek’s program was prescient and sophisticated, always identifying real and pressing problems, though he underestimated the resources of human morality and the Open Society to cope with the challenges he perceived. Gaus marshals formal models and empirical evidence to show that the Open Society is grounded on the moral foundations of human cooperation originating in the distant evolutionary past, but has built upon them a complex and diverse society that requires rethinking both the nature of moral justification and the meaning of democratic self-governance. In these fearful, angry, and inward-looking times, when political philosophy has itself become a hostile exchange between ideological camps, The Open Society and Its Complexities shows how moral and ideological diversity, far from being the enemy of a free and open society, can be its foundation.