Since the beginning of the 1980’s, the hypothesis of hysteresis of the unemployment rate has met a great success. However, what this hypothesis means is not very clear. After having distinguished hysteresis and persistence, we propose an interpretation of the use of this physician’s term in economics. This project requires, on the one hand, to give up the popperian prohibition to study, from an epistemological point of view, the context of discovering ; it implies, on the other hand, to focus attention on the a priori of the economic discourse. We associate the importation of the hysteresis term with the recognition by the mainstream of a presupposion which has for a long time remained characteristic of the heterodox thought: the historical dependance of equilibrium.