Aristotle in his Physics and St. Thomas in his First Way both claim that anything in motion is moved by another, though they give differing but complementary accounts of why it is true.
For St. Thomas, the premise follows from the fact that “what is in motion”, when taken strictly, is potential. The thing that moves from A to Z can’t be by nature A or Z, but has to have both as possibilities. The “other” that moves this thing is primarily the “Z” that serves as the terminus of the motion, and secondarily whatever it is that has the power to push the thing to Z. The argumentation here is extraordinarily tight: all it requires is the minimal claim that motion means to move from one thing to another, and that what undergoes this is neither necessarily the one thing or the other.
Aristotle appeals to a different grounding premise. For him, “what is in motion” is necessarily a whole extended body, and each part of the extension is other than the body. All of this rests on the interesting claim that it is impossible for a non-extended body (a Euclidean point) to change in any way, even to change place. One can imagine say, this dot
moving across the screen, but Aristotle would claim that it cannot move. What exactly it can do is unclear, and there are several logical possibilities: either we are not imagining a point doing anything, or whatever the point was doing could not be considered motion, or any possible motion could not be a physical process (a mathematical one, perhaps?) etc. But at any rate the point can’t move: since in order for anything to move it has to move to the next place, but a point could only be here and then be there, it couldn’t move from one thing to another.
Here again, we encounter the minimal claim that motion means moving from one thing to another, but it is taken as meaning that what moves is an extended whole that requires “others” to move, namely, its parts.
Notice that on the Thomistic account, the mobile is considered as incomplete; on Aristotle’s account the mobile is considered as whole and complete but dependent on its parts. The Thomistic account gives us a mobile that is an open system receiving energy and intentional direction from the outside, Aristotle’s account is of a closed system that has an activity in virtue of its parts. I use closed and open in a broader sense than they are used nowadays – for example, a falling stone, considered as just the stone, is a closed system in the sense we are using here, though it includes the complex moving closed systems of modern physics too.
And so the differing accounts of “everything in motion is moved by another” give us a metaphysical foundation for our ability to consider nature as both an open and closed system. The differing accounts lead us to different accounts of the first source of motion as well. For Aristotle, the first source is partless and non-physical; for St. Thomas it is purely actual and in conscious possession (as opposed to physical possession) of the terms defining the motion.