To understand the growth of Western constitutional thought, we need to consider both ecclesiology and political theory, ideas about the Church as well as ideas about the state. In this book Professor Tierney traces the interplay between ecclesiastical and secular theories of government from the twelfth century to the seventeenth. He shows how ideas revived from the ancient past - Roman law, Aristotelian political philosophy, teachings of Church fathers - interacted with the realities of medieval society to produce distinctively new doctrines of constitutional government in Church and state. The study moves from the Roman and canon lawyers of the twelfth century to various thirteenth-century theories of consent; later sections consider fifteenth-century conciliarism and aspects of seventeenth-century constitutional thought. Fresh approaches are suggested to the work of several figures of central importance in the history of Western political theory. Among the authors considered are Thomas Aquinas, Marsilius of Padua, Jean Gerson, Nicholas of Cues and Althusius, along with many lesser-known authors who contributed significantly to the growth of the Western constitutional tradition.