# What are the best arguments for the defense of the "principle of motion"?

How would someone (I would love to hear how Aquinas and/or Aristotle would) defend the principle of motion (that is, whatever is changed is changed by another or only actual being can actualize potency)?

Could it be reduced to the principle of non-contradiction? Did anyone defend that position, and what would be the argument?

If the principle is regarded as self-evident, what would it mean that any principle is self-evident?

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## 1 Answer

St. Thomas defends "quidquid movetur ab alio movetur" ("whatever is moved is moved by another") in his commentary on the beginning of Aristotle Physics book 7:

1. Then at (677 [=242a44]) he [i.e., Aristotle] proves directly that whatever is being moved is being moved by some other. This is his argument: Nothing that is being moved by itself rests from its motion on account of some other mobile’s resting. (He takes this as per se evident). From this he further concludes that if a mobile rests on account of the rest of another, then the mobile is moved by another. On this ground he concludes that. necessarily whatever is being moved is being moved by some other. And that this follows from the premises, he now proves.

That mobile which we have supposed as being moved by itself, i.e., ABI must be divisible, for whatever is being moved is divisible, as was proved above. Hence, because it is divisible, nothing prevents it from being divided. Therefore, let it be divided at the point C so that one part of it is PC and the other part AC. Now, if PC is part, of AB, then when the part BC rests, the entire AB must rest. But if upon the part resting, the whole does not rest, let us grant that the whole is being moved and one part is at rest. But because we have assumed that one part is resting, the whole could not be granted as being moved except by reason of the other part. Therefore, when BC (which is one part) is at rest, the other part AC is being moved. But no whole of which one part only is being moved is being moved primarily and per se. Therefore AB is not being moved primarily and per se, as we originally assumed. Therefore while BC is at rest, the entire AB must be at rest. Thus, what is being moved ceases to be moved upon the occasion of something else resting. But above we held that if something rests and ceases to be moved on the occasion of another's resting, it is being moved by that other. Therefore, AB is being moved by some other.

The same argument applies to any other mobile, for whatever is being moved is divisible and, for the same reason, if the part rests the whole rests. Therefore, it is clear that whatever is moved is moved by some other.

Also, St. Thomas calls the principle of non-contradiction the "primum principium indemonstrabile" ("the first indemonstrable principle"), so it would follow that it could not be proven by the "quidquid movetur ab alio movetur" principle.

See also (from here):