Be sure and review and understand what our saint meant by “the good”.
1 Now, if a celestial body is moved by intellectual substance, as we have shown, and if the motion of a celestial body is ordered to generation in the realm of things here below, it must be that the processes of generation and the motions of these lower things start from the intention of an intelligent substance. For the intention of the principal agent and that of the instrument are directed toward the same thing.
Now, the heavens is the cause of the movements of inferior bodies, by virtue of its own motion in which it is moved by an intellectual substance. It follows, then, that the heavenly body is like an instrument for intellectual substance. Therefore, the forms and movements of lower bodies are caused by intellectual substance which intends them as a principal agent, while the celestial body is like an instrument.
2 It must be, then, that the species of things caused and intended by the intellectual agent exist beforehand in his intellect, as the forms of artifacts pre-exist in the intellect of the artist and are projected from there into their products. So, all the forms that are in these lower substances, and all their motions, are derived from the intellectual forms which are in the intellect of some substance, or substances. Consequently, Boethius says in his book, The Trinity, that “forms which are in matter have come from forms which are without matter.” And on this point, Plato’s statement is verified, that forms separated from matter are the principles of forms that are in it. Although Plato claimed that they subsist in themselves and immediately cause the forms of sensible things, we assert that they exist in an intellect and cause lower forms through the motion of the heavens.
Notes One wishes our good saint had known of things like string theory, and could make these arguments from below, arguments which must be of the same character—and same result.
3 Since everything that is moved directly and not merely accidentally by another being is directed by that being to the end of its motion, and since the celestial body is moved by an intellectual substance, and, moreover, the celestial body causes, through its own motion, all the motions in these lower things, the celestial body must be directed to the end of its motion by an intellectual substance, and so must all lower bodies be directed to their own ends.
4 So, then, it is not difficult to see how natural bodies, devoid of knowledge, are moved and perform actions for an end. They tend to the end as things directed to that end by an intellectual substance, in the way that an arrow tends toward the target when it has been aimed by the archer. Just as the arrow attains its inclination to a definite end from the archer’s act of shooting it, so do natural bodies attain their inclination to natural ends, from natural movers; from which movers they also receive their forms, powers, and motions.
5 Consequently, it is also evident that every working of nature is the work of an intelligent substance, because an effect is more fundamentally attributed to the prime mover, which aims at the end, than to the instruments which have been directed by it. And because of this we find that the workings of nature proceed toward their end in an orderly way, as do the actions of a wise man.
6 Hence, it becomes obvious that even things which lack knowledge can be made to work for an end, and to seek the good by a natural appetite, and to seek the divine likeness and their own perfection. And there is no difference between saying one of these things or the other. For, by the fact that they tend to their own perfection they tend to the good, since a thing is good to the extent that it is perfect.
Moreover, by virtue of tending to be good it tends to the divine likeness, for a thing is made like unto God in so far as it is good. And this or that particular good thing becomes an object of desire according as it is a likeness of prime goodness. So, too, for this reason it tends to its own good, because it tends to the divine likeness, and not conversely. Hence, it is clear that all things desire the divine likeness as an ultimate end.
7 Now, the good that is proper to a thing may be received in many ways. One way depends on what is appropriate to the essential character of the individual. It is thus that an animal seeks his good, when he desires the food whereby he may be kept in existence. A second way depends on what is appropriate to the species. It is in this way that an animal desires his proper good, inasmuch as he desires the procreation of offspring and the nourishment of the same, or the performance of any other work that is for the preservation or protection of individuals belonging to his species.
A third way depends on the essential character of his genus. It is in this way that an equivocal agent seeks its proper good by an act of causation, as in the case of the heavens. And a fourth way depends on the analogical likeness of things produced, in relation to their source. And it is in this way that God, Who is beyond genus, gives existing being to all, because of His own goodness.
Notes That second way is appropriate for us, too. Hence the natural law.
8 It is evident, next, that the more perfect something is in its power, and the higher it is in the scale of goodness, the more does it have an appetite for a broader common good, and the more does it seek and become involved in the doing of good for beings far removed from itself.
Indeed, imperfect beings tend only to the good proper to the individual, while perfect beings tend to the good of their species.
But more perfect beings tend to the good of the genus, while God, Who is most perfect in goodness, tends toward the good of being as a whole. Hence it is said by some people, and not inappropriately, that “the good, as such, is diffusive,” because the better a thing is, the more does it diffuse its goodness to remote beings. And since, “in every genus, that which is most perfect is the archetype and measure of all things belonging in the genus,” God, Who is most perfect in goodness and Who diffuses His goodness in the broadest way, must be in His diffusion the archetype for all diffusers of goodness. Now, inasmuch as a thing diffuses goodness to other beings, it comes to be their cause. As a result, it is also clear that a thing which tends to become the cause of others tends toward the divine likeness, and nonetheless it tends toward its own good.
Notes It cannot be too often repeated that “imperfect beings tend only to the good proper to the individual, while perfect beings tend to the good of their species.”
9 Therefore, it is not unfitting to say that the motions of the heavenly bodies and the actions of their movers are in some sense for the sake of these generable and corruptible bodies which are less worthy than they. They are not for the sake of these bodies, in the sense of an ultimate end; rather, by intending the generation of these bodies they intend their own good and the divine likeness as an ultimate end.