In the middle of fourteenth century, besides evidence and certitude, the probable occupies an increasingly important place in the evaluation of argumentation and the epistemic analysis of our concepts. This is particularly the case for Peter of Ailly. In his Commentary on the Sentences, the concept of the probable is as important as, or not more important than, the concept of evidence in order to qualify our knowledge. This contribution examines the place and function of the probable in the works of Peter of Ailly. The importance of the probable modifies the whole picture of our argumentation in natural philosophy as well as in theology. It characterizes the whole field of the "natural light" and "natural reason."