  The Physics commentary contains an array of examples of innovative and damagingly critical commentary. One of the most celebrated achievements is the theory of impetus, which is commonly regarded as a decisive step away from an Aristotelian dynamics towards a modern theory based on the notion of inertia. Concepts akin to those deployed in Philoponus’ impetus theory appear in earlier writers such as Hipparchus (2nd c. BCE) and Synesius (4th c. CE), but Philoponus nowhere intimates that he was influenced by any one of them. As far as one can tell from the text In Phys. 639–42, he takes his point of departure from an unsatisfactory Aristotelian answer to a problem that was to puzzle scientists for centuries: Why does an arrow continue to fly after it has left the bow-string, or a stone after it has ceased to be in contact with the hand that throws it? Since Aristotle supposed that a) whenever there is motion there must be something which imparts the motion, and b) mover and moved must be in contact, he was led to conclude that the air displaced in front of the projectile somehow rushes round it and pushes from behind, thus propelling the projectile along. To be sure, the rapid motion of an object through a medium creates turbulence, but this turbulence does not sustain any movement but contributes to its deceleration. Aristotle’s theory was still in vogue among Peripatetic scholars of the sixteenth century, despite the fact that a thousand years earlier Philoponus had thoroughly dismantled it. He proposed instead, much more plausibly but still erroneously, that a projectile moves on account of a kinetic force which is impressed on it by the mover and which exhausts itself in the course of the movement.  Philoponus compares this impetus or ‘incorporeal motive enérgeia’, as he calls it, to the activity earlier attributed to light.