In Book III, Part 2 of the Treatise, Hume presents a natural history of justice. Self-interest clearly plays a central role in his account; our ancestors invented justice conventions, he maintains, for the sake of reciprocal advantage. But this is not what makes his approach so novel and attractive. Hume recognizes that prudential considerations are not sufficient to explain how human beings – with our propensities towards temporal discounting and free-riding – could have established conventions for social exchange and collective action in commercial societies. This leads him to develop an innovative account of the role that emotional aversions play in establishing trust between strategically rational agents.