On Physics

...capacity so that it brings forth acts not suddenly; and of such a nature is motion. This, which is thus in potentiality The state of being able to become something else, like a seed being a potential tree., has a double potentiality: one regarding the motion itself, and another regarding that which is acquired through the motion. Thus it is clear how motion is the "first perfection" relative to what follows. For motion is an act The reality or fulfillment of a thing. which, while it exists, something of it still remains to be achieved. However, the Form The completed nature or essence of the object. which is acquired through motion is the perfection which, once it exists, nothing remains of it to be acquired.

And note that motion is not properly measured except by time, as if by its own proper attribute original: "passione". Since time is continuous, motion is also continuous. A continuum is that whose parts are joined at one common boundary; therefore, motion has a part, another part, and a middle. Because of this, whatever is moved was properly being moved in the past, and will be moved in the future. For it is partly at the starting point, partly at the endpoint, and partly in the middle. Such motion belongs only to bodies which have parts and a middle.

And note the difference between continuous, contiguous, and consecutive things:

• Continuous things are those joined at a single common boundary, like the parts of a line at a point, the parts of a surface at a line, and the parts of a body at a surface.
• Contiguous things, however, are those whose boundaries are "together"—that is, they touch but do not merge—such as a pen and parchment.
• Consecutive things are those which have nothing of their own kind original: "sui generis" between them. However, nothing prevents something else from being in the middle. "Kind" is used broadly here; for example, two donkeys are of the same kind, as are two houses and similar things.

Also note that for a motion to be "one" (numerically), as Aristotle says in the fifth book of the Physics, three things are required:
1. The object being moved must be one.
2. The time measuring the motion must be one and continuous.
3. No rest must intervene.

Hence, as Aristotle says in the same place: it is many motions and not one if a period of rest sits in the middle. Therefore, if a single motion is occupied by a stop, it is neither one nor continuous. This logic is useful for knowing when there is one sin and when there are many. For some say that two sins can be committed by a single act: for instance, if someone kills a virgin while raping her, or if by fornicating in front of another he provides them an occasion for moral ruin The concept of "scandal" or leading another into sin.. It must be said that this is not "one motion" (one act), because it is not directed toward a single form or end; for in this way he both fornicates and causes scandal.

Also note that something is moved by its own nature original: "per se" and something is moved incidentally original: "per accidens". A subject moves by its own nature. Something moves incidentally in two ways: first, like a "form" in a subject—as when "whiteness" moves because the object it belongs to moves. This follows the saying: "When we move, all things within us move." In the second way, a thing moves like a part in a whole, such as a nail moving because the ship moves.

On the divisions of motion. Chapter 9.