Thomas Aquinas looks at economic activity in a new way. By gradually coming to adopt an approach that is more ontological and economic than anthropological and social, Aquinas marked not only a methodological evolution, but also an epistemological one. By emancipating himself from a framework which concentrated on the pre-established social status of the usurer or merchant, Aquinas broadens his view to include all economic agents and the entire duration of the exchange, since anyone can find himself in a sinful situation in the course of an exchange. He then takes up, but relativises and universalises, the notion of conditioned will stemming from a social approach to usury, according to which the rich lender enslaves the poor borrower, whose will is conditioned by his state of necessity. He thus integrates this social notion with an economic approach based on bargaining power. Moreover, he circumvents the lack of information about agents’ intentions by seeking objective and visible criteria for the justice of exchange. This leads him to focus on the nature of the object exchanged, the quid, which determines the nature and conditions of its exchange, and to which the fair behaviour of the agents must conform.