Thomas Aquinas, Compendium of Theology

Let us consider how goodness may be wanting in creatures. In two respects goodness is clearly inseparable from the creature: first, goodness pertains to the creature's very essence; secondly, the creature's goodness is something determinate.

As regards the first point of view, the goodness which is the form of simple substances is inseparable from them, since they are essentially forms. As regards the second point of view, such substances cannot lose the good which is existence. For form is not like matter, which is indifferent to existence and nonexistence. On the contrary, form goes with existence, even though it is not existence itself.

Therefore simple substances cannot lose the good of nature wherein they subsist, but are immutably established in such good. But composite substances, which are neither their own forms nor their own existence, possess the good of nature in such a way that they can lose it. This is not true, however, of those things whose matter is not in potency to various forms or to being and non-being; such is the case with heavenly bodies.