This dissertation takes up two related questions: What is a constitution and why have one? It seeks answers in one of the earliest and most comprehensive works of modern constitutionalism: The Federalist. In fine, the dissertation argues that for Publius—the pseudonymous author of The Federalist—a constitution is a political arrangement designed to constrain public power for the purpose of defeating the problem of judging in one’s own case. In light of the Roman law maxim nemo iudex in sua causa, the dissertation refers to this problem as the “nemo iudex problem.” Popular governments like republics are always at risk of the nemo iudex problem. And the problem is often fatal. In popular governments, every decision made by the people is simultaneously a private one and a public one. The temptation to create and enforce rules in a self-serving manner proves too much for ordinary citizens or their representatives. Public institutions come to serve private ends, not public ones, and the republic unravels. The Federalist betrays a deep concern with the nemo iudex problem. For starters, the nemo iudex problem underlies the problem of faction in Number 10, which few commentators have emphasized. But Publius’ attention to the nemo iudex problem also drives his critiques of other structural forms like the confederation of republics, the mixed regime, and a pure separation of powers. His analysis of each of these political structures points to the same conclusion: Of their own force, none are able to secure republican stability in the face of the nemo iudex problem. For Publius, then, nascent America faces a dilemma: On one hand, the new American regime must be strictly republican; on the other, no known forms of republican rule have successfully protected a republic from the nemo iudex problem. After grounding this dilemma in the text, the dissertation argues that The Federalist presents a novel theory of constitutionalism to navigate the dilemma. According to Publius, a constitution is a form of government that combines three related but distinct features. I call these features “founding,” “horizontal monism,” and “vertical dualism.” Founding provides a point in time at which all legitimate political authority comes into existence. Horizontal monism channels all legitimate political authority into a legal system. And vertical dualism provides a structure to the legal system according to which there are two levels of law. When these three features coincide, a constitution obtains, and the constitution can take certain actions for popular majorities and officers off the table. Moreover, the division between higher law and lower law entails, at least in the republican context, that only supermajorities should be empowered to make higher law. Decisions made by supermajorities are less likely to aim at private concerns, making higher law less likely to be destructively manipulated. The dissertation concludes by discussing Publius’ theory of constitutional enforcement, according to which independent judges must be the guardians of the constitution, and examining several limitations recognized in Publius’ theory of constitutionalism.