In his book On Aristotle Physics 641, 12; 641, 29; 642, 9 Philoponus first argues explicitly against Aristotle's explanation that a thrown stone, after leaving the hand, cannot be propelled any further by the air behind it. Then he continues: "Instead, some immaterial kinetic force must be imparted to the projectile by the thrower. Whereby the pushed air contributes either nothing or only very little to this motion. But if moving bodies are necessarily moved in this way, it is clear that the same process will take place much more easily if an arrow or a stone is thrown necessarily and against its tendency into empty space, and that nothing is necessary for this except the thrower." This last sentence is intended to show that in empty space—which Aristotle rejects—and contrary to Aristotle's opinion, a moving body would continue to move. It should be pointed out that Philoponus in his book uses two different expressions for impetus: kinetic capacity (dynamis) and kinetic force (energeia). Both expressions designate in his theory a concept, which is close to the today's concept of energy, but they are far away from the Aristotelian conceptions of potentiality and actuality.