Potentiality and Contraries
There is difficulty in the question how the matter of each thing is related to its contrary states. It is the matter of one in virtue of its positive state and its form, and of the other in virtue of the privation of its positive state and the corruption of it contrary to its nature. The corpse comes from the animal, and vinegar from wine, as night from day. And all the things which change thus into one another must go back to their matter; e.g. if from a corpse is produced an animal, the corpse first goes back to its matter, and only then becomes an animal; and vinegar first goes back to water, and only then becomes wine.
One primary kind of potency is an originative source of change in another thing or in the thing itself qua other; Actuality in the strict sense is thought to be identical with movement. And so people do not assign movement to non-existent things. Eternal movement, if there be such, does not exist potentially; if there is an eternal mobile, it is not in motion in virtue of a potentiality, except in respect of ‘whence’ and ‘whither’. For it, movement is not connected with the potentiality for opposites, so that the continuity of the movement should be laborious, as it is for perishable things, for it is matter and potency, not actuality, that causes this.
The non-rational potencies are all productive of one effect each, but the rational produce contrary effects, so that if they produced their effects necessarily they would produce contrary effects at the same time; but this is impossible. There must, then, be something else that decides; I mean by this, desire or will.
The delimiting mark of that which as a result of thought comes to exist in complete reality from having existed potentially is that, if the agent has willed, it comes to pass if nothing external hinders, while the condition on the other side-viz. in that which is acted on-is that if nothing prevents it from becoming, and if there is nothing which must be added or taken away or changed.
The capacity for contraries is present at the same time; but contraries cannot be present at the same time, and the actualities also cannot be present at the same time. Therefore, while the good must be one of them, the capacity is both alike, or neither; the actuality, then, is better. In the case of bad things the end or actuality must be worse than the potency; The bad does not exist apart from bad things; for the bad is in its nature posterior to the potency. And therefore we may also say that in the things which are from the beginning, i.e. in eternal things, there is nothing bad, nothing defective, nothing perverted.
The Problems with Actuality
Aristotle’s explanation of change revolves around his notion of potentiality and actuality. A boy is potentially what a man is in actuality. The change from boy to man is irreversible, since a man cannot change back into a boy. Once a thing has been actualized, it no longer changes, for otherwise, actuality doesn’t exist; Pythagoreans and Plato, OTOH, believe all changes are reversible. Through reincarnation, a man can change into a boy.
There are two problem with “actuality”:
1. How can actuality be identified and known? A boy is born, grows, ages, dies and returns to dust. Wherein lies the “actuality”? All things seem to be changing continually (i.e., “coming to be”) in the phenomenal world, but nothing actually “come to be”, therefore actuality doesn’t exist.
2. One question remains unanswered: What causes potentiality to become actuality?