This paper offers an overview of the history of the axiom forma dat esse, which was commonly quoted during the Middle Ages to describe formal causality. The first part of the paper studies the origin of this principle, and recalls how the ambiguity of Boethius’s first formulation of it in the De Trinitate was variously interpreted by the members of the School of Chartres. Then, the paper examines the various declensions of the axiom that existed in the late Middle Ages, (...) and shows how its evolution significantly follows the progressive decline of the Aristotelian model of formal causality. (shrink)