This chapter discusses the nomos-phusis debate of the fifth and fourth century BCE and introduces the book’s main argument: that the Epicureans defend a sophisticated middle position (vis-à-vis Plato and Aristotle, on the one hand, and some sophists, on the other) in this debate when it comes to justice. On the Epicurean view, justice is neither fully natural nor conventional; there is a robust virtue of justice and it is always better to be just than to be unjust, but it is not always better to obey the laws.