The variety and multitude of things is from the intention of God, the First Agent, Who produces them for the communication of His Goodness. This shines forth more eminently through the diversity of creation; for it could not be fully represented by any single creature. Hence goodness, which in God is simple and uniform, is found in creatures multiple and divided, and is more perfectly represented by the universe than by any single creature.
And the Wisdom of God is the Cause of the inequality of things as of their differences. For we find in things a twofold distinction: one formal, inasmuch as they differ in species; the other material, where they differ in individuality only - a material distinction which is for the sake of the formal.
The formal difference always requires inequality, because the forms of things are like numbers, in which species differ by addition or subtraction of unity; therefore in natural things species are graduated, and one is found to be more perfect than another, the inequality of things having been made a law for the perfection of the universe.
And as all those things which God has made are ordered one to another and also to Himself, it follows of necessity that the world is one and belongs to One. Those who deny this unity are driven to ascribe it to chance.
- text taken from Compendium of the Summa Theologica of Saint Thomas Aquinas, by Bishop Berardus Bongiovanni