This paper explores the way the Scholastic argument against usury, which culminated in the 13th century with Thomas Aquinas’s question on interest loans in the Summa Theologiae, found an end with Hugo Grotius’s introduction of economic issues, in De Jure Belli ac Pacis (1625). Whereas Grotius inherited at least part of his predecessors’ repugnance of interest lending, he found in his questioning of categories from Roman law the source of both a criticism of the main features of the Scholastic argument and an alternative analysis of interest loans in which the income received by the lender is explained and legitimate.