A First Glance at St. Thomas Aquinas by Ralph McInerny

A First Glance at St. Thomas Aquinas by Ralph McInerny

Author:Ralph McInerny [McInerny, Ralph]
Language: eng
Format: epub
Publisher: University of Notre Dame Press
Published: 1989-12-29T16:00:00+00:00


A TEXT OF THE MASTER

The Definition of Motion

[3] It has been pointed out that every genus is divided by act and potency. Since they are among the primary differences of being, act and potency are naturally prior to motion and the Philosopher uses them in order to define motion.

It should be considered that something is in act alone or something is in potency alone or it is midway between potency and act. What is in potency alone does not yet move; what is in complete act is not moved but has been moved. Therefore that which moves is midway between pure potency and act, and is indeed partly potential and partly actual, as is clear from alteration. For when water is only potentially hot, it is not yet moved; when it has been heated, its being heated is done, but when it has something of heat, though only imperfectly, then it is moved toward heat; for what is being heated little by little has more and more heat. The imperfect act of heat existing in the heatable is motion, not indeed insofar as it is act alone, but insofar as, according to the act it already has it is ordered to further act. If this order to further act were taken away, however imperfect its actuality, it would be the term of motion, and not motion, as happens when something is lukewarm. The order to further act belongs to that which is in potency to it.

Similarly, if the imperfect act were considered only as ordered to further act, and it compares to that further act as potency, it would not have the mark of motion but of the beginning of motion. Heating can begin from the lukewarm as well as from the cold.

Thus it is that imperfect act has the mark of motion, both insofar as it compares to further act as potency and insofar as it compares to something less perfect as act.

Therefore it is neither a potency of something existing in potency nor the act of something existing in act, but the act of something existing in potency. By ‘act’ the order to the previous potency is designated, and by ‘of something existing in potency’ the order to further act.

The Philosopher thus defines motion most properly when he says that motion is the entelechy, that is, the act of something existing in potency insofar as it exists in potency.

Commentary on Physics, Book Three, lesson 2



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