The separation of powers became the ambivalent, largely rhetorical tradition it is today as a result of the plasticity of its core concepts, power, authority, and liberty. Throughout the Progressive and New Deal eras its basic meaning was complicated by an experimentalist reinvention of its traditional institutional faces. This experimentalism resulted in new institutional forms and new theories of legitimate legal rulemaking. But these eras built from the doctrine's past more than they departed from it. Moreover, the experimentalist attitude was perhaps most important for the reaction it engendered: a legalist attitude of critique and opposition to the very premises of experimentalism. The tensions between these two, in fact, are what set the stage for the specification of administrative law's core construct separating powers/authorities and protecting liberty in the administrative state - the APA. This is most evident in its creation of notice and comment rulemaking. I argue that these competing arrangements of the separation of powers' pieces - legalism and experimentalism - lie behind the instability within the tradition today. My claim is that, if we are to actively shape any of the broad institutional changes that result from this instability, we must first grasp the fissures set out in the historical and analytical reconstructions of this study.