There is no good human life outside of a state, and the good state enables us to live well together – so says Constitutivism, the theory developed in this book. Reinvigorating Aristotelian ideas, the author asks in what sense citizens of modern, populous and pluralistic societies share a common good. While we can easily find examples of cooperation that benefit each member, such as insurances, the idea that persons could share a common good became puzzling with modernity – a puzzlement epitomised in Margaret Thatcher’s ‘What is society? There is no such thing!’ This puzzlement, the author argues, results from our profoundly modern understanding of rational actions, which we see as means toward outcomes. If we allow that not only outcomes but also histories and identities can be good reasons for actions, then it makes sense to see a person’s good and the common good of their political community as constitutive of one another, as Aristotle thought. Building on this idea, the author argues that in designing our institutions, we also give ourselves an identity – in other words, we constitute ourselves as persons.