This book presents an answer to the question of why modern legal institutions and the idea of citizenship are important for leading a free life. The majority of views in political and legal philosophy regard the law merely as a useful instrument, employed to render our lives more secure and to enable us to engage in cooperate activities more efficiently. The view developed here defends a non-instrumentalist alternative of why the law matters. It identifies the law as a constitutive feature of our identities as citizens of modern states. The constitutivist argument rests on the (Kantian) assumption that a person's practical identity (its normative self-conception as an agent) is the result of its actions. The law co-constitutes these identities because it maintains the external conditions that are necessary for the actions performed under its authority. Modern legal institutions provide these external prerequisites for achieving a high degree of individual self-constitution and freedom. Only public principles can establish our status as individuals who pursue their life plans and actions as a matter of right and not because others contingently happen to let us do so. The book thereby provides resources for a reply to anarchist challenges to the necessity of legal ordering.