This chapter examines the history of political-legal reasoning. It suggests that this history begins in the Renaissance with the emergence of a doctrine of ‘reason of state’, a doctrine which was widely debated between the late-sixteenth and early-eighteenth centuries but remained contentious throughout. It argues that reason of state continued to exert an influence in the modern political world, but that that influence is complicated by changes in the nature and forms of government. Most importantly, the modern state presents itself as a constitutional state and once the constitution is established as ‘fundamental law’, whatever remains of reason of state discourse is subsumed under the idea of ‘constitutional legality’. Consequently, those elements of the doctrine that live on in contemporary practice no longer fall into a distinct category of reason of state; they have become a facet of the emergence of the modern ‘state of reason’.