This article closely examines the way in which Thomas Aquinas understood the relationship between the various forms of human community. The article focuses on Aquinas's theory of law and politics and, in particular, on his use of political categories, such as city, province and empire, together with the associated concepts of kingdom and nation, as well as various social groupings, such as household, clan and village, alongside of the distinctly ecclesiastical categories of parish, diocese and universal church. The analysis of (...) these categories is used in the article to help explain Aquinas's role in the development of theories about subsidiarity, federalism and mixed constitutionalism. In the first place, it is argued that a close inquiry into Aquinas's discussion of the many and various forms of human community sheds light on the origins and development of the idea of subsidiarity within Catholic social teaching. Second, while Aquinas certainly did not advance a theory of federalism as that idea is presently understood, it is argued that recovering what Aquinas had to say about the categories of human community helps us to understand the origin and later development of federal ideas. Finally, it is argued that far from endorsing a system of absolute monarchy as is sometimes alleged, when understood in this way, Aquinas supported a particular kind of mixed constitution in which monarchy is tempered by a variety of constitutional constraints founded upon a conception of the body politic as itself constructed out of a plurality of smaller, intermediate corporations and communities of a political, ecclesiastical and social character. Keywords: Thomas Aquinas, political theory, subsidiarity, federalism, mixed constitution, absolute monarchy, civitas, provincia, imperium, regnum, gens, natural law . (shrink)