Objection 1. It would seem that the inequality of things is not from God. For it belongs to the best to produce the best. But among things that are best, one is not greater than another. Therefore, it belongs to God, Who is the Best, to make all things equal.
Objection 2. Further, equality is the effect of unity (Metaph. v, text 20). But God is one. Therefore, He has made all things equal.
Objection 3. Further, it is the part of justice to give unequal to unequal things. But God is just in all His works. Since, therefore, no inequality of things is presupposed to the operation whereby He gives being to things, it seems that He has made all things equal.
On the contrary, It is said (Sirach 33:7): "Why does one day excel another, and one light another, and one year another year, one sun another sun? [Vulgate: 'when all come of the sun']. By the knowledge of the Lord they were distinguished."
I answer that, When Origen wished to refute those who said that the distinction of things arose from the contrary principles of good and evil, he said that in the beginning all things were created equal by God. For he asserted that God first created only the rational creatures and all equal; and that inequality arose in them from free-will, some being turned to God more and some less, and others turned more and others less away from God. And so those rational creatures which were turned to God by free-will, were promoted to the order of angels according to the diversity of merits. And those who were turned away from God were bound down to bodies according to the diversity of their sin; and he said this was the cause of the creation and diversity of bodies. But according to this opinion, it would follow that the universality of bodily creatures would not be the effect of the goodness of God as communicated to creatures, but it would be for the sake of the punishment of sin, which is contrary to what is said: "God saw all the things that He had made, and they were very good" (Genesis 1:31). And, as Augustine says (De Civ. Dei ii, 3): "What can be more foolish than to say that the divine Architect provided this one sun for the one world, not to be an ornament to its beauty, nor for the benefit of corporeal things, but that it happened through the sin of one soul; so that, if a hundred souls had sinned, there would be a hundred suns in the world?"
Therefore it must be said that as the wisdom of God is the cause of the distinction of things, so the same wisdom is the cause of their inequality. This may be explained as follows. A twofold distinction is found in things; one is a formal distinction as regards things differing specifically; the other is a material distinction as regards things differing numerically only. And as the matter is on account of the form, material distinction exists for the sake of the formal distinction. Hence we see that in incorruptible things there is only one individual of each species, forasmuch as the species is sufficiently preserved in the one; whereas in things generated and corruptible there are many individuals of one species for the preservation of the species. Whence it appears that formal distinction is of greater consequence than material. Now, formal distinction always requires inequality, because as the Philosopher says (Metaph. viii, 10), the forms of things are like numbers in which species vary by addition or subtraction of unity. Hence in natural things species seem to be arranged in degrees; as the mixed things are more perfect than the elements, and plants than minerals, and animals than plants, and men than other animals; and in each of these one species is more perfect than others. Therefore, as the divine wisdom is the cause of the distinction of things for the sake of the perfection of the universe, so it is the cause of inequality. For the universe would not be perfect if only one grade of goodness were found in things.
Reply to Objection 1. It is part of the best agent to produce an effect which is best in its entirety; but this does not mean that He makes every part of the whole the best absolutely, but in proportion to the whole; in the case of an animal, for instance, its goodness would be taken away if every part of it had the dignity of an eye. Thus, therefore, God also made the universe to be best as a whole, according to the mode of a creature; whereas He did not make each single creature best, but one better than another. And therefore we find it said of each creature, "God saw the light that it was good" (Genesis 1:4); and in like manner of each one of the rest. But of all together it is said, "God saw all the things that He had made, and they were very good" (Genesis 1:31).
Reply to Objection 2. The first effect of unity is equality; and then comes multiplicity; and therefore from the Father, to Whom, according to Augustine (De Doctr. Christ. i, 5), is appropriated unity, the Son proceeds to Whom is appropriated equality, and then from Him the creature proceeds, to which belongs inequality; but nevertheless even creatures share in a certain equality--namely, of proportion.
Reply to Objection 3. This is the argument that persuaded Origen: but it holds only as regards the distribution of rewards, the inequality of which is due to unequal merits. But in the constitution of things there is no inequality of parts through any preceding inequality, either of merits or of the disposition of the matter; but inequality comes from the perfection of the whole. This appears also in works done by art; for the roof of a house differs from the foundation, not because it is made of other material; but in order that the house may be made perfect of different parts, the artificer seeks different material; indeed, he would make such material if he could.
Source: Summa Theologica (New Advent)