ArgumentThis paper discusses the theory of motion of the philosopher Honoré Fabri (1608–1688), a senior representative of early modern Jesuit scientists. It argues that the consensus prevailing among historians – according to which Fabri's theory of impetus is diametrically opposed to Galileo's or Descartes' concept of inertia – is false. It shows: that Fabri carefully constructed his concept of impetus in order to easily incorporate the principle of linear conservation of motion (designated here as “limited inertia”), by adopting formal (rather (...) than efficient) causality between impetus and motion and by allowing impetus to act only along straight lines; that he explicitly deemed Aristotle's famous “inertial paradox” not as a paradox at all; and that linear conservation of motion was not limited to “counterfactual” circumstances but played a significant part in his discrete theory of free fall and constituted an integral component of his general physical (and even mathematical) philosophy. However, the paper also shows that Fabri's notion of inertia was restricted to one dimension only, and therefore should indeed be considered “limited.” In his analysis of horizontal projectiles, Fabri explicitly rejected Galileo's important principle of superposition, and thus revealed significant constraints to his “inertial” thinking. (shrink)

In an effort to widen the discussions about internal resistance in 14thcentury theories of motion, this article examines the evolution of the concept in the work of the Parisian scholar Nicole Oresme. In hisPhysics, he presents a position that is not fully developed; as well, he raises some questions regarding the motive qualities of the elements constituting mobile objects. This ambiguity becomes even more evident when his position changes later in his discussions in the treatiseOn the Heavens. Oresme is then (...) even less inclined to accept the idea of internal resistance in motion. (shrink)