Lucien Febvre’s interwar texts in defense of a history of science reveal the Annales School’s interest in prospecting and founding new fields of study and how an effort was in course to establish a new definition of what history itself should be. Parallel to those efforts, Alexandre Koyré was writing his first works, later to be considered foundation stones of the modern history of scientific thought. Albeit stemming from distinct intellectual ranks, both were formulating the same kind of problem and taking a stance in opposition to the same concepts and historiographic tradition. At the heart of their delineation of the problem was a critique of how history had defined temporality and their finding that it needed to be reformed so that history itself should not be falsified. Febvre and Koyré’s arrival at a history of science via a history of religions or of mysticism shows us how necessary it was to articulate the relationship between credulity and rationality in a new way and to define the kind of historical temporality that could accommodate this new mode of articulating them.