This book defends the relational approach to law. It begins by exploring how law is constituted in the context of either rational or reasonable conduct. Having identified reasonableness as the unifying theme of natural law theories, it argues that the form and authority of law originate in the practical resolution of a moral antinomy that these theories leave unaddressed. The reasonableness of law lies in the structure and principles of relations in which rights are correlated with obligations and powers with liabilities. Rather than descending upon us from above in the form of directives, law emerges from our interactive efforts to cope with persistent moral disagreements. Ultimately, the relational approach views the legal rules governing our interactions as expressions of a common will. The book concludes that, unsurprisingly, modern constitutionalism represents a thoroughly pragmatic and most defensible conception of the authority of law.