Introduction : discovering practical reason -- Gyges' choice : rationality and visibility -- Glaucon's dilemma : origins of social order -- Deioces' ultimatum : how to choose a ruler -- Solon's bargain : self-enforcing constitutional order -- Melos' prospect : limits of inter-state rationality -- Socrates' critique : problems for democratic rationality -- Cephalus' expertise : economic rationality -- Conclusions : utility and eudaimonia -- Appendix : probability, risk, and likelihood.