Classical Athens provides a historical case study of effective joint action by a democratic community, at scale, over time, and across a socially and epistemically diverse population. Athens was concerned both with aggregating diverse knowledge for decision-making and with building common knowledge for coordinated joint action. A preserved prosecution speech delivered in an Athenian treason trial reveals how common knowledge was generated by democratic institutions and employed in legal arguments. Common knowledge facilitated eff ective coordination among citizens through productive alignment (...) cascades. Yet Athenian lawcourt procedures rendered cascading difficult, pointing to an institutionalized awareness that coordination among jurors might violate fairness as equality before the law; such violations threatened democratic values and the equilibrium between elite and non-elite citizens. Common knowledge was promoted by repeated public rituals, public monuments, and public architecture that allowed for the interpresence of large numbers of deliberative and intervisible citizens. (shrink)