Abstract: Professor Nicholas (Nick) Barber’s learned and intelligent book on The Principles of Constitutionalism is best understood as an exercise in Aristotelian naturalism and moral and constitutional realism, of a sort much more characteristic of the classical law than of modern positivism. In view of the book’s implicitly classical approach, it would have benefited, at a number of key junctures, from drawing upon the rich and enduring tradition of the ius commune. Barber often speaks in the register of the classical law, perhaps without knowing it. Doing so more often, and more explicitly, would have improved an already impressive work.