The constitution as the form of the city
Constitutional government is the heart of Aristotle's political philosophy, and the platform reads his Politics as the founding work of the empirical study of constitutions. For Aristotle the politeia — the constitution, the arrangement of a city's offices and the distribution of its authority — is what defines a city: change the constitution and you have, in the relevant sense, a different city. The platform reads Aristotle's approach as distinctively empirical where Plato's was idealizing: he and his school collected the constitutions of some 158 Greek cities (one, the Constitution of the Athenians, survives) and generalized from the evidence of how actual cities were governed.
The classification of regimes
The platform reads Aristotle's typology of constitutions as one of the most influential analytical schemes in the history of political thought. He sorts constitutions by who rules (one, few, or many) and for whose benefit (the common good, or the rulers' own), yielding three correct forms — kingship, aristocracy, and "polity" (the rule of the many for the common good) — and their three corruptions — tyranny, oligarchy, and demagogic democracy. The platform reads this scheme, under constitution, as the vocabulary the Western tradition has used to think about regimes ever since — refined by Polybius, carried into Latin by Cicero, and still argued with by the framers of modern constitutions.
The realism of stability
The platform reads Aristotle's deepest contribution as his turn from the question of the best constitution to the question of the stable one. Books IV–VI of the Politics ask not what regime is ideal but how actual, imperfect regimes can be preserved — how to prevent the faction (stasis) that destroys cities, what conditions make each regime type stable or unstable, and which constitution is best for most cities most of the time. His answer — a large middle class, a mixed constitution blending oligarchic and democratic elements, and the supremacy of law — is the founding work of realistic, comparative political science. The platform reads this as Aristotle's great advance over Plato: the study of politics as it is, not only as it should be.
Why it matters for Virtue & Power
Constitutional government is the centre of Aristotle's politics and one of the platform's foundational political themes — the origin of the comparative study of regimes and of the realistic inquiry into political stability. It connects his thought to the platform's whole concern with constitutions and how they survive or fail, and it is read at length in Aristotle and constitutional government.