Does the Rule of Law Require the Separation of Powers?

The separation of powers is an enigmatic political ideal, particularly in the context of the modern administrative state, in which administrative officials routinely perform a combination of legislative, judicial, and executive functions. In this article we reexamine this ideal by interrogating Adrian Vermeule's provocative claim that the separation of powers is neither necessary nor desirable for the rule of law. We begin by comparing Vermeule's view with the constitutional models of two theorists who share a similar skepticism of the separation of powers – Thomas Hobbes and Carl Schmitt. We argue that Vermeule's view is best understood as a variation of Schmitt's apology for unitary executive power. By contrast, Hobbes emphasises what we call law's systematicity. We then proceed to elaborate this idea. We claim that at the conceptual level law's systematicity issues into a ‘proto‐separation’ of state institutions that together realise the rule of law. Because law is systematic, it also makes normative sense to combine the rule of law and the separation of powers. The separation of powers allows us to articulate the demands of the rule of law through a process of public deliberation and scrutiny. By contrast, a political order that relinquishes its commitment to the separation of powers for the sake of unleashing the power of the executive degrades its commitment to the rule of law as well.