According to Article 16 of the French Declaration of the Rights of Man and of the Citizen of 1789, “Any society in which the guarantee of rights is not assured, nor the separation of powers determined, has no constitution.” Though widely accepted today, this view was novel at the time, shaped by the experience of the English, American, and French revolutions. The separation of powers developed out of the theory of mixed government during the English Civil Wars of the mid-17th century. In 1642, Charles I belatedly invoked the doctrine of mixed government to defend the joint rule of monarch, lords, and commons as implied by the notion that Parliament meant all three (the doctrine of “King in Parliament” as the sovereign body of the realm). His execution posed the problem of how to control government in a society without distinctions of rank. Dividing the executive, legislative, and judicial functions between three distinct agencies appeared to provide a response to this dilemma. However, it took some time to evolve. Although Book 11, Chapter 6 of Montesquieu’s The Spirit of the Laws has been credited with offering a definitive statement of the doctrine, his account still bore the hallmarks of its origins in the system of mixed government—not least because of its being based on an analysis of the British parliamentary system and the respective roles of monarch, lords, and commons within it. The functional division also remained far from clear-cut, with the judicial branch and function still imperfectly differentiated from the other two. Only with the drafting of the U.S. Constitution and the debates surrounding it—most notably embodied in the Federalist Papers—did the doctrine emerge in its mature form.

The underlying rationale of this separation is that individuals or groups should not be “judges in their own cause.” The division between the three branches aims to ensure that those who formulate the laws are distinct from those entrusted with their interpretation, application, and enforcement. In this way, lawmakers are subject to the same laws and so have an incentive to avoid self-interested legislation and to frame it in general terms that will be equally applicable to all. These laws then guide the decisions of the executive and judiciary, who, because they are similarly under the law, also have good reason to act in an impartial manner. Separating functions also brings the efficiency gains associated with the division of labor. In particular, the activity of the legislature is made less cumbersome through delegating more short-term decisions to an executive branch capable of acting with greater coherence and dispatch.

On its own, it is unclear how effective this separation is. Not only are the four functions hard to distinguish clearly, but unless a different group operates each branch, there is nothing to prevent their acting in concert. However, four other theoretical developments accompanied the shift from mixed government to the separation of powers that changed its character. First, mixed government had been challenged earlier by theorists of sovereignty, such as Jean Bodin and Thomas Hobbes, who regarded the idea of dividing power as incoherent. The separation of powers came into being in a context shaped by the notion that at some level power had to be concentrated, and, in the context of the English, American, and French revolutions, the natural assumption was to shift the sovereign power of the monarch to the people as a whole. Second, the notion of the people as a whole was likewise new. Previously, the “people” had simply meant the “commons” or the “many.” The whole people became the authors of the constitution, which, as the embodiment of their will, became sovereign over the will of any subdivision of the people, including the majority. Third, as a corollary, constitutions became entrenched written documents expressing a “higher” law, which could be amended only by the people as a whole or by some supermajority that could plausibly be said to represent their will. Fourth, notions of rights became key aspects of the constitution. Initially rights were no more an intrinsic part of the separation of powers than they had been of mixed government. The Bill of Rights was an appendix to the U.S. Constitution, which had previously been confined to describing the system of government. Nevertheless, the securing of individual rights gradually became the goal of all constitutional arrangements.

These four developments, but particularly the last two, had a tremendous impact on constitutionalism and proved crucial in moving it in a legal and especially a judicial direction. Within the “pure” theory of the separation of powers, all three branches were coequal. As with the theory of mixed government, the aim was to prevent any one section of society dominating another by obliging each to collaborate with the others. If anything, the legislative power was logically prior to the others—producing in the U.S. scheme federal and bicameral arrangements within the legislature that harked back to the doctrine of mixed government and a clear division between the legislature and executive. As noted earlier, the distinctiveness of judicial functions was weak in the doctrine of mixed government and slow to emerge in the theory of the separation of powers. However, making a legal document sovereign—only challengeable by the sovereignty of the people as a whole—inevitably empowered the judiciary, particularly given the comparative length of judicial appointments and their relative isolation from electoral pressures by contrast to the other branches. The judiciary now decided the competences of the various branches of government, including their own, and set limits not only to the processes of government but also to its goals with regard to individual rights. These features have come to define modern constitutionalism and are reflected in all the constitutional arrangements of postwar democracies. Yet they also coexist with forms of political constitutionalism and mixed government.