Thomas Aquinas, Compendium of Theology

A being is noble and perfect in the measure that it approaches likeness to God, who is pure act without any admixture of potency. Therefore beings that are supreme among entities must be more in act and must have less of potency, whereas inferior beings must be more in potency. How this is to be understood, we must now examine.

Since God is eternal and immutable in His being, those things are lowest in the scale of being, as possessing less likeness to God, which are subject to generation and corruption. Such beings exist for a time, and then cease to be. And, since existence follows the form of a thing, beings of this kind exist while they have their form, but cease to exist when deprived of their form. Hence there must be something in them that can retain a form for a time, and can then be deprived of the form. This is what we call matter. Therefore such beings, which are lowest in degree among things, must be composed of matter and form. But beings that are supreme among created entities approach most closely to likeness with God. They have no potency with regard to existence and non-existence; they have received everlasting existence from God through creation. Since matter, by the very fact that it is what it is, is a potency for that existence which is imparted through form, those beings which have no potency for existence and nonexistence, are not composed of matter and form, but are forms only, subsisting in their being which they have received from God. Such incorporeal substances must be incorruptible. For all corruptible beings have a potency for non-existence; but incorporeal beings have no such potency, as we said. Hence they are incorruptible.

Furthermore, nothing is corrupted unless its form is separated from it, for existence always follows form. Since the substances in question are subsisting forms, they cannot be separated from their forms, and so cannot lose existence. Therefore they are incorruptible.

Between the extremes mentioned, there are certain intermediate beings which have no potency for existence and nonexistence, but which have a potency for ubi, or presence in place. Such are the heavenly bodies, which are not subject to generation and corruption, since contrarieties are not found in them. However, they are changeable according to local presence. Thus in some beings there is found matter as well as motion. For motion is the act of a being in potency. Accordingly such bodies have matter that is not subject to generation and corruption, but is subject only to change of place.