Aristotle's Theory of Motion Explained

“The solution of the difficulty is plain: motion is in the movable. It is the fulfilment of this potentiality by the action of that which has the power of causing motion; and the actuality of that which has the power of causing motion is not other than the actuality of the movable; for it must be the fulfilment of both. A thing is capable of causing motion because it can do this, it is a mover because it actually does it. But it is on the movable that it is capable of acting. Hence there is a single actuality of both alike, just as one to two and two to one are the same interval, and the steep ascent and the steep descent are one – for these are one and the same, although their definitions are not one. So it is with the mover and the moved.”
Aristotle Physics 3.3, 202a13-21
Introduction
The seven words that bring in this passage are typically Aristotelian. They declare that all is about to become clear, whilst introducing a chapter that could hardly be more opaque. But to understand Aristotle’s concept of agency, the ability to cause motion, it is essential to grasp this passage which purports to make plain the idea of motion. This essay will focus on unpacking the passage in two sections. The first will work towards explaining his initial definition of motion, pausing where necessary to explain background concepts, which I will indent to indicate that they merely supplement the examination of motion and are not part of the definition itself. The second will then look closely at the idea that the passage in question centres on, “that there is a single actuality of both [the mover and the movable] alike”. What this means linguistically will be clear by section two, but what it entails ontologically will be seen to be contested. I will examine three competing interpretations of this idea, from Waterlow, Marmodoro, and Coope, and conclude that future readers of Aristotle’s Physics should side with the final reading.
Section I
1.1 Kinesis
The word we translate

Bibliography: 1 (Aristotle), Aristotle Complete Works: The Revised Oxford Translation, edited by Jonathan Barnes, Princeton University Press, Princeton NJ, 1984; two volumes (Cited in text with name of Aristotelian text and Bekker number) 2 Coope, U., 2007, Aristotle on Action, Aristotelian Society Supplementary Volume 81.1 3 Gill, M. L., 1980, Aristotle 's Theory of Causal Action in "Physics" III 3, Phronesis, Vol. 25, No. 2 4 Hussey, E., 1983, Aristotle: Physics Books III and IV, Oxford University Press 5 Lear, J., 1988, Nature, from Lear, Jonathan, Aristotle: the desire to understand pp.15-54, Cambridge: Cambridge University Press 6 Marmodoro, A, 2007, The Union of Cause and Effect in Aristotle: Physics 3.3, Oxford Studies in Ancient Philosophy 32 7 Smith, B., 2012, Aristotle, Online Resource: http://www.mycrandall.ca/courses/grphil/Aristotle.htm 8 Waterlow, S., 1982, Nature, Change, and Agency in Aristotle’s Physics, Clarendon Press, Oxford