(CG 2/44). That the Variety of Creatures has not arisen from Variety of Me... (Book 2: God The Origin of Creatures) (Summa Contra Gentiles) (Aquinas, Thomas, St.)

SINCE every agent intends to induce its own likeness in the effect, so far as the effect can receive it, an agent will do this more perfectly the more perfect itself is. But God is the most perfect of agents: therefore it will belong to Him to induce His likeness in creation most perfectly, so far as befits created nature. But creatures cannot attain to any perfect likeness of God so long as they are confined to one species of creature; because, since the cause exceeds the effect, what is in the cause simply and as one thing is found in the effect in a composite and manifold way, unless the effect be of the same species as the cause; which is impossible in the case before us, for no creature can be equal to God. Multiplicity therefore and variety was needful in creation, to the end that the perfect likeness of God might be found in creatures according to their measure.

2. As the things that are made of any material are contained in the potentiality of the material, so the things done by any agent must be in the active power of the agent. But the potentiality of the material would not be perfectly reduced to actuality, if out of the material were made only one of those things to which the material is in potentiality. Therefore if any agent whose power extends to various effects were to produce only one of those effects, his power would not be so completely reduced to actuality as by making many. But by the reduction of active power to actuality the effect attains to the likeness of the agent. Therefore the likeness of God would not be perfect in the universe, if there was only one grade of all beings.

3. A creature approaches more perfectly to the likeness of God by being not only good itself, but able to act for the good of others. But no creature could do anything for the good of another creature, unless there were plurality and inequality among creatures, because the agent must be other than the patient and in a position of advantage (honorabilius) over it.

5. The goodness of the species transcends the goodness of the individual. Therefore the multiplication of species is a greater addition to the good of the universe than the multiplication of individuals of one species.

7. To a work contrived by sovereign goodness there ought not to be lacking the height of perfection proper to it. But the good of order in variety is better than the isolated good of any one of the things that enter into the order: therefore the good of order ought not to be wanting to the work of God; which good could not be, if there were no diversity and inequality of creatures. There is then diversity and inequality between creatures, not by chance, not from diversity of elements, not by the intervention of any (inferior) cause, or consideration of merit, but by the special intention of God, wishing to give the creature such perfection as it was capable of having. Hence it is said, God saw all things that he had made, and they were very good (Gen. i, 31); and this after He had said of them singly, that they were good; because while things are good singly in their several natures, all taken together they are very good, because of the order of the universe, which is the final and noblest perfection of creation.