Many theorists of justice aim to guide action. They attempt to describe what a society with just institutions and interpersonal relationships would look like. In this work, I pursue two principal tasks. Part I examines the epistemic limits that theorists and members of political societies face when trying to identify and articulate the demands of justice. I argue that action-guiding theorists of justice are in epistemic positions similar to those of economic central planners and political representatives in large, complex democracies. I also advance a challenge to so-called ideal theory and develop a positive vision of the important epistemic contributions made by justice theorists. Part II shows how experimental and traditional social prac...