Aristotle on the Motion of Projectiles: A Reconsideration Pantelis Golitsis Aristotle is still credited in several studies dealing mostly with medieval sci- ence with the so-called theory of antiperistasis, by which he allegedly explained the motion of the projectiles. 1 Nonetheless, antiperistasis is the first of two theo- ries to which Aristotle refers with regard to the motion of the projectiles, pass- ingly in Physics iv 8 and more amply in Physics viii 10, where it is actually rejected in favour of the second theory. Several commentators fail to notice the strict analogy between the two passages and wrongly take Aristotle as expound- ing and endorsing just one theory in Physics viii 10, which would be (a refined version of) the theory of antiperistasis. As a result, Aristotle’s account of forced motion has been repeatedly misunderstood. 2 As is well known, according to the Stagirite, the natural elements that consti- tute the universe are either heavy or light or weightless; the heavy elements, i.e., earth and water, naturally move towards the centre, the light elements, i.e., air and fire, naturally move away from the centre, and the weightless element, i.e., the ether that occupies the supralunary region of the universe, naturally moves around the centre. 3 The problem with the projectiles, which are heavy by nature (φύσει), was not that they do not immediately move downwards, since this was explained as the result of a forced, unnatural (βίαιος or παρὰ φύσιν) motion pro- vided by the thrower’s hand. The real problem was that the projectile continued to move for some time, while there was no mover to move it. Here is how a histo- rian of science presents the difficulty that Aristotle had to face: Projectile motion posed a…problem for Aristotle. In the case of a thrown object, the force was provided by the hand of the thrower as long as the object was in contact with the hand. But one needed an explanation of why the object continued to move once it had left the thrower’s hand. Aristotle concluded that the medium through which the projectile moved provided the force that kept it moving. This occurred either by antiperis- tasis (replacement), in which the medium rushes around the 1 See, for instance, Gabrovsky 2015, 40: ‘There were many problems with Aristotle’s theory of antiperistasis: in Aristotle’s argument a stone that is thrown moves the air, and the air propels the stone as it fills in the vacuum left behind.’ 2 Various accounts of medieval science treat the theory of antiperistasis as a standard part of a general Aristotelian theory of motion; see, e.g., Grant 1974, 275-280. Nevertheless, medieval thinkers themselves, such as Thomas Aquinas and Jean Buridan, clearly distinguished between the two theo- ries in both passages; cf. Maggiòlo 1965, §1163; Benoît 1996, IV, q. 2, 21-28. 3 For a concise presentation of this theory, see Aristotle On the Heavens i 3. Ancient Philosophy 38 (2018) ©Mathesis Publications 1