 In some areas of medieval thought, probability-related terms were important for the regulation of decision-making and conduct. In these contexts, the term “probable” (probabilis) took on a specific significance. Whether a merchant wanted to follow an opinion concerning the moral legitimacy of a contract or a lawyer formulated a presumption, it had to be probabilis to possess moral or juridical weight. It is now necessary to discuss the most important medieval understandings of probabilis, grounding them in ancient definitions or precedent. 