In fifteenth-century Florence, friendship—or the client–patron relationship, as contemporaries termed it—was often associated with uncertainties and risks. An investigation of diaries, notebooks, and letter correspondences of the time, from the perspective of game theory and decision theory, reveals how Florentines reasoned about the uncertainties of friendship, deploying an array of knowledge-constructing practices, under the rubric of “commonplacing,” to understand it. The preventive techniques that Florentines applied to cope with the conflicting testimonies of the contemporary information culture (the increase in the variety and the availability of vernacular texts, the expansion of literacy, etc.) only served to intensify their predicament. The fact that clients and patrons largely viewed their relationship in the same way challenges the traditional notion of that relationship as asymmetrical.