Arthur Harris (University of Cambridge): Multiple motions and mechanical explanation in the Aristotelian corpus

A body cannot be in two places at once, but can a body undergo two changes of place simultaneously? Take a simple example. Does a sailor walking across the deck of a moving ship undergo one motion or two? From one point of view, there are two changes, the ship’s motion and the sailor’s walking, and both should feature in an explanation of what is occurring. From another point of view, the sailor has only one motion, since he traverses only one path through space. The relation between two component motions and the resultant motion is given by the parallelogram rule. One reason for thinking that all three cannot truly exist is that, if this were so, the moving thing would traverse twice the distance owing to the composition of three motions. Whether one regards only the components or only the resultant as real will depend in part on one’s account of change.

It is well known that puzzles concerning composition and superposition in scientific explanations have appeared in various guises from the sixteenth century to the present day. Ancient perspectives on these issues have so far remained relatively unexplored, although the earliest extant instantiations of the ‘parallelogram rule’ are found in the Aristotelian Corpus. It is often assumed that Aristotle believed a body can undergo only one motion at a time. I aim to overturn this assumption, arguing that Aristotle believed a body can undergo multiple motions simultaneously and took a realist view of component motions.

First, I argue that Aristotle’s account of change in Physics 3 points towards a realist account of component motions. I show that this interpretation can clarify Aristotle’s remarks on ‘mixed’ motion in Physics 8 and De Caelo 1, as well as some further passages in the De Caelo and Meteorology. Next, I consider three challenges for Aristotle’s account of the composition of motions. Can the account be reconciled with (i) teleology; (ii) passages where Aristotle says that one motion is overpowered and destroyed by another; (iii) Aristotle’s claim that it is impossible for a thing to undergo opposite motions simultaneously?

Finally, I show how this understanding of Aristotle’s account of the composition of motions sheds light on the explanatory project of the Mechanica, attributed to Aristotle but more likely the work of an early follower. In this text, various natural and unnatural phenomena are explained in terms of the lever and ultimately of the abstract model of the rotating radius which traces out a circular path. Mech. problem 1 argues that this rotation results from two component motions, one radial and one tangential. Previous scholarship has treated these component motions as theoretical fictions. I argue that a realist reading makes better sense of Mech. problem 1’s arguments and hence of the place of mechanics in early Peripatetic scientific investigations.

In my closing remarks I briefly discuss some implications of my argument for future work in the history and philosophy of science.


This lecture will be given on Fri, 2 July 2021, 14:40 (UK time) as part of the workshop Change and Changemakers in Ancient Philosophy. The workshop is a collaborative initiative of the Change and Changemakers Network (Siegen) together with the Mereology of Potentiality Project (Oxford).