It has been often claimed that one of the main differences between the medieval conception of nature and the modern one lies in their account of powers and laws of nature. Medieval philosophers — generally inclined to accept the core theses of Aristotle’s physics — explain the regularities of natural processes by appealing to causal powers inhering in material substances and do not attribute a primitive explanatory role to the laws of nature.

Contrastingly, the dominant tendency of natural philosophers (including the founders of modern physics) in the early Modern period is to explain the physical behavior of concrete things by laws of nature which determine their intrinsic properties. In other words, whereas medieval philosophers tended to ground the laws of nature on powers inhering in concrete substances, early modern philosophers sought to eliminate causal powers by reducing them to mere aspects of nomological relations. Although this complete reversal is widely acknowledged, the story of this shift in the philosophical and scientific approach of natural processes remains to be told.

This presentation will sketch the history of the notion of ‘power’ in the late Middle Ages and in early Modern thought (from the 14th century to Galileo), focusing on one major factor of its evolution. It will be shown that the late medieval reflections on the laws of motions and the properties of physical reactions explain why philosophers increasingly conceived causal powers as actual quantities and, ceasing to define them as dispositional properties, were led to emphasize the explanatory role of laws of nature.