This dissertation examines the role of the passions in political life in Thucydides' work, reading Thucydides to a large degree as a political psychologist, focused on what motivates citizens and cities to take the actions that they do. It does so by looking at paradigmatic episodes in Thucydides' narrative, namely those episodes that show a link between passions and the city. It concludes that cities are founded to provide for material security but that, once born, they give rise to certain passions that drive the city onward to its growth and eventual decay. There is something ultimately tragic about human life. Finally, it examines the relationship of the passions to rationality, concluding that a truly capacious rationality acknowledges the ineradicable element of the irrational in human life.