Thomas Aquinas, Compendium of Theology

The goodness of a creature may also be regarded otherwise than as the creature's subsistence in its nature; for the perfection of its goodness is realized in its destiny to its end. And since creatures are to attain their end by their activity, we have still to inquire how creatures may be lacking in goodness from the point of view of their actions, whereby they are destined to attain their end.

In this connection we should first note that a judgment concerning natural operations is equivalent to a judgment concerning the nature which is the principle of these operations. Therefore in beings whose nature cannot suffer defect, no defect in natural operations can develop; but in beings whose nature can admit of defect, a defect in activity can occur.

Hence in incorruptible substances, whether incorporeal or corporeal, no defect in natural activity can take place. Thus angels forever retain their natural power of exercising their proper activity. Likewise the movements of heavenly bodies are found never to leave their appointed orbits. But in lower bodies many defects in natural activity result from the corruptions and defects incidental to their natures. Thus from a defect in some natural principle come the sterility of plants, monstrosities in the generation of animals, and other such disorders.