This article takes as its point of departure the conviction that late medieval science should be studied in its own right, and not merely to determine whether it presaged developments in early modern science. Case in point: Francis of Marchia's theory of virtus derelicta, the theory that the motion of a projectile through the air is due to a force left behind by the original motive force. Certainly, Marchia's view is not a forerunner of inertia. Nevertheless, it is argued that (...) virtus derelicta breaks with two important Aristotelian principles of motion: "everything that has a beginning must necessarily also have an end" and "form is always indivisible." Thus, virtus derelicta is neither an Aristotelian solution to the problem of projectile motion nor a development on the road to early modern science; it belongs to a new (but subsequently undeveloped) understanding of motion. (shrink)