Thomas Aquinas, Compendium of Theology

We can see from this that lower creatures are ruled by God through the agency of higher creatures. Some creatures are said to be higher, because they are more perfect in goodness. Creatures receive their order of good from God, inasmuch as they are under His rule. Consequently higher creatures have a greater share in the order of divine government than lower creatures. But what has a greater share in any perfection is related to what has a smaller share in that perfection, as act is related to potency, and agent to patient. Therefore higher creatures are related to lower creatures in the order of divine providence as agent is related to patient. Accordingly lower creatures are governed by higher creatures.

Divine goodness has this characteristic, that it communicates a likeness of itself to creatures. This is the sense in which God is said to have made all things for the sake of His goodness, as is clear from a previous chapter. The perfection of divine goodness entails the double truth that God is good in Himself, and that He leads other beings to goodness. He communicates goodness to creatures under both aspects: they are good in themselves, and some lead others to goodness. In this way God brings some creatures to goodness through other creatures. The latter must be higher creatures; for what receives a likeness of both form and action from some agent, is more perfect than what receives a likeness of form but not of action. Thus the moon, which not only glows with light but also illuminates other bodies, receives light from the sun more perfectly than do opaque bodies, which are merely illuminated but do not illuminate. Accordingly God governs lower creatures by higher creatures.

Likewise, the good of many is better than the good of an individual, and so is more representative of the divine goodness, which is the good of the whole universe. If a higher creature, which receives more abundant goodness from God, did not cooperate in procuring the good of lower creatures, that abundance of goodness would be confined to one individual. But it becomes common to many by the fact that the more richly endowed creature cooperates in procuring the good of many. Hence the divine goodness requires that God should rule lower creatures by higher creatures.