  Perhaps medieval scholars did not arrive at inertia because of their reluctance to accept and then analyze at length motion in a vacuum. One of Aristotle’s arguments for the impossibility of a vacuum was the lack of resistance of the medium. If velocity was a function of the proportion of force to the resistance of the medium, then with no resistance there would be motion in an instant, something usually considered an impossibility. In  book II, q. 16, a. 5  of his Sentences commentary, however, Marchia has to wonder why an angel, which is not a “corpus quantum,” or bodily mass, cannot move instantaneously, i.e. from one place to another without any temporal duration. Part of the answer is simply that it is a contradiction to be two places at once, but Marchia adds that there must be some sort of internal resistance in angels that makes instantaneous motion impossible. The notion of internal resistance unrelated to natural place (even though no corpus quantum is involved) and the concept of impetus (Buridan’s making the virtus derelicta permanent) appear to be primitive versions of the ingredients of a theory of inertial mass. Since Marchia’s writings predate those of his more famous successors, the Oxford Calculators and Buridan and Oresme at Paris, and some of his ideas at least resemble Galileo’s in some way, Marchia’s possible impact on later scientists is a good topic for future research.