The ascending curve represents positive acceleration...

The ascending curve represents positive acceleration, and the descending curve represents negative acceleration. This picture has a clearer instance in the case of substances which possess two compound forms, in such a way that the underlying form possesses a constant substantial motion, whose level of existence does not become more perfect nor decline, while the higher form possesses rising and declining motion.

For example, the component elements of a plant remain in the same condition in which they began, while the vegetable form gradually becomes more perfect, and then enters the state of withering and decaying, and at last it rots and is destroyed. This is the point at which the descending curve joins the straight line.

Those who rely on some other definitions of motion have inferred the necessity of its becoming perfect, and so, in the case of substantial motion, they have also held that its intensification and becoming more perfect are necessary, even if our senses are not able to perceive this intensification. In the same way, they have considered declining or weakening motions to be accidental. In Lesson Fifty-Seven, this inference was criticized and its weakness was made clear.

There is no reason to repeat it again. The Relation between Substantial Motion and Actuality and Potentiality As was previously explained, the potential and the actual are two abstract concepts abstracted from the relation between two successive existents, and from the persistence of the previous existent or a part of it in the following existent.

Now, regarding the fact that all material existents are constantly in a state of renewal and coming about and passing away, the question will be raised as to how the existence of the previous existent can be imagined, and how the definition of potential and actual can apply to the beginning and end of the motion. Sometimes the answer is given that although the previous existent does not itself remain, the perfection of its existence is preserved in the following existent.

It is concluded that every motion is a becoming perfect and intensification.