Book excerpt: From the Introduction: We need not be surprised, then, that in the Middle Ages also there were rulers who aspired to supreme political and temporal power. The truly exceptional thing is that in medieval times there were always at least two claimants to the role, each commanding a formidable apparatus of government, and that for century after century neither was able to dominate the other completely, so that the duality persisted, was eventually rationalized in works of political theory and ultimately built into the structure of European society. This situation profoundly influenced the development of Western constitutionalism.