Modern human rights instruments reflect earlier transformations of natural rights into constitutional rights. The effect of this transformation was most apparent in the intertwining of natural rights with emerging conceptions of the separation of governmental powers. For this to take place, early modern natural law theory needed to abandon its defence of absolutist forms of government and embrace ideas developed within the common law. This chapter traces the progress of this surprising marriage. It shows how the concern of common lawyers to secure freedom under law by separating governmental powers came to be justified increasingly in terms of natural law, rather than by reference to English constitutional history. This discursive shift was given political expression in the American revolution and finally adopted into Immanuel Kant’s natural law theory as a requirement of practical reason. The essentially collaborative understanding of the relationship between legislature and judiciary which emerged is still of value in the debate between modern-day natural law theorists over the role of judicial power in the protection of human rights.