The idea that a simple unilateral promise, until it is accepted, is not binding according to natural law is defended by Grotius in his major work with an argumentation drawn directly from Lessius, an important source of inspiration for the Dutch jurist, who in turn solves the dispute rooted in the tradition of ius commune. This article aims to reconstruct, in its essential stages, an itinerary through the main positions of medieval and early modern civil and canon lawyers about this controversial issue. These sources constitute the background of early modern scholastics and Grotius as well. The paper analyses some of the principal texts of both bodies of law, highlighting arguments and adding new findings. Notably it is shown that Lessius’s and Grotius’s statements represent a turning point, as far as they react against the resumption of the theory of the binding force of simple unilateral promises in the sixteenth century. With Lessius and Grotius, on the other hand, acceptance became a necessary requirement for every transfer of rights and duties to be enforceable.