Thomas Aquinas, Compendium of Theology

All things are directed to the divine goodness as to their end, as we have shown. Among things ordained to this end, some are closer to the end than others, and so participate in the divine goodness more abundantly. Therefore lesser creatures, which have a smaller share in the divine goodness, are in some way subordinated to higher beings as to their ends. In any hierarchy of ends, beings that are closer to the ultimate end are also ends with respect to beings that are more remote. For instance, a dose of medicine is administered to procure a purge, the purge is designed to promote slimness, and slimness is desirable for health; and thus slimness is, in a sense, the purpose of the purging, as the purging is the purpose of the medicine. And such subordination is reasonable. As in the order of efficient causes, the power of the first agent reaches the ultimate effects through intermediate causes, so in the order of ends, whatever is farther removed from the end attains to the ultimate end through the intermediacy of beings that are closer to the end. Thus, in our example, the medicine has no relation to health except through purging. Similarly, in the order of the universe, lower beings realize their last end chiefly by their subordination to higher beings.

The same conclusion is manifest if we turn our attention to the order of things in itself. Things that come into being by a natural process, act as they are equipped by nature to act. As we observe, however, imperfect beings serve the needs of more noble beings; plants draw their nutriment from the earth, animals feed on plants, and these in turn serve man's use. We conclude, then, that lifeless beings exist for living beings, plants for animals, and the latter for men. And since, as we have seen, intellectual nature is superior to material nature, the whole of material nature is subordinate to intellectual nature. But among intellectual natures, that which has the closest ties with the body is the rational soul, which is the form of man. In a certain sense, therefore, we may say that the whole of corporeal nature exists for man, inasmuch as he is a rational animal. And so the consummation of the whole of corporeal nature depends, to some extent, on man's consummation.