David Boucher uses the ideas of western philosophy's most significant thinkers to trace the history of political theory in international relations. He examines current conceptions, offering an alternative thematic interpretation of how the most significant thinkers in the Western tradition perceived relations between communities, nations, states, and the discovery of the new world. His organizing principle centres on the idea that the great philosophers were searching for a criterion of state conduct associated with different theories of human nature and which were used for justificatory, appraisive, and injunctive purposes. The author asserts that great thinkers from Thucydides to Marx formulated and applied these criteria to interpret the changing international system and concludes by showing how contemporary theories compare with and extend the themes addressed by their predecessors.