Man is a political animal, and naturally desirous of forming a political community, both for pleasure and out of necessity. Thus went the neo-Aristotelian convention, based on generations of commentaries on Aristotle's Politics, and one of the premises of this book is that there would be nothing inherently surprising to an early modern reader, at least in the Catholic world, about the idea that groups of people all over the globe, even those most distant and distinct from themselves, could form a properly constituted republic. This remained true even when there were differences of religion, and even when the community's governance did not conform to the most widely recognised form of monarchy. If the political community, which might also be referred to as a republic or commonwealth (Latin res publica), a city (civitas, rather than the built environment of the urbs), a polity (polis), a kingdom (regnum) or the body politic (corpus politicum), terms which were not always strictly synonymous, was in accordance with nature, it was nevertheless also a work of artifice, something that is not given, but has to be built by human agency. To acknowledge its existence was only the beginning, and said very little about how any given political communities might act and interact.

The body politic was a theme which preoccupied humanists and those who professed the scholastic tradition of natural law, but also writers and thinkers of many different hues. Annabel Brett shows how a concern with political community underlay all sorts of problems: some more obvious, such as why and how republics were founded, but others apparently unrelated, such as whether one should allow freedom of movement to beggars, which touched on the permeability of borders, and who falls inside and outside them; how laws bind individual humans; or the meaning of liberty and slavery, all questions which were dealt with at length by the Spanish Scholastics from the mid-sixteenth to mid-seventeenth centuries. Another approach was that of the humanist authors of political treatises, among whom the figure of Niccolò Machiavelli loomed large.