The separation of powers is not a theory of mechanical checks and balances or counterforce. Any sufficiently complex organization will have competing interests or sub-units; most do not have a separation of powers. This chapter identifies the conceptual and normative core of the separation of powers as a particular kind of institutionalization of the rule of law. It is an attempt to guarantee a separation of general rules from applications to particular persons by keeping them apart not only in time but also in personnel and institutional space. The chapter further argues that the idea of the separation of powers as articulated by Montesquieu joined that understanding of the rule of law to bodies and estates of the mixed constitution, relying in particular on independent and high-status nobles to defend the law against the political demands of the executive monarch equipped with coercive force. The democratization of the separation of powers in the American founding stripped away that social independence, and left the separation of powers weaker than has generally been noticed. The chapter concludes with considerations of the modern executive branch, and suggests that separation of powers reasoning might need to be applied internally to it.