This book defends the relational approach to law from the perspective of human action. The book begins by exploring how what we take to be law is constituted through the lens of either rational or reasonable conduct. Having examined reasonableness as the unifying theme of natural law theories, it then argues that the form and authority of law originate from resolving a moral antinomy that these theories failed to address. The reasonableness of law resides in the resultant structure and principles of relations in which rights are matched with obligations and powers with liabilities. Rather than descending upon us from above in the form of directives, the law emerges from interactive efforts to cope with persistent moral disagreements. Ultimately, the relational approach views the legal rules governing our interactions as based on some common will. The book concludes that, unsurprisingly, modern constitutionalism is to be regarded as a thoroughly pragmatic and most defensible conception of the authority of law.