Whether the multitude and distinction of things come from God?

I answer that, The distinction of things has been ascribed to many causes. For some attributed the distinction to matter, either by itself or with the agent. Democritus, for instance, and all the ancient natural philosophers, who admitted no cause but matter, attributed it to matter alone; and in their opinion the distinction of things comes from chance according to the movement of matter. Anaxagoras, however, attributed the distinction and multitude of things to matter and to the agent together; and he said that the intellect distinguishes things by extracting what is mixed up in matter.

But this cannot stand, for two reasons.

First, because, as was shown above (I:44:2), even matter itself was created by God. Hence we must reduce whatever distinction comes from matter to a higher cause.

Secondly, because matter is for the sake of the form, and not the form for the matter, and the distinction of things comes from their proper forms. Therefore the distinction of things is not on account of the matter; but rather, on the contrary, created matter is formless, in order that it may be accommodated to different forms.

Others have attributed the distinction of things to secondary agents, as did Avicenna, who said that God by understanding Himself, produced the first intelligence; in which, forasmuch as it was not its own being, there is necessarily composition of potentiality and act, as will appear later (I:50:3. And so the first intelligence, inasmuch as it understood the first cause, produced the second intelligence; and in so far as it understood itself as in potentiality it produced the heavenly body, which causes movement, and inasmuch as it understood itself as having actuality it produced the soul of the heavens.

But this opinion cannot stand, for two reasons. First, because it was shown above (I:45:5) that to create belongs to God alone, and hence what can be caused only by creation is produced by God alone—viz. all those things which are not subject to generation and corruption. Secondly, because, according to this opinion, the universality of things would not proceed from the intention of the first agent, but from the concurrence of many active causes; and such an effect we can describe only as being produced by chance. Therefore, the perfection of the universe, which consists of the diversity of things, would thus be a thing of chance, which is impossible.

Hence we must say that the distinction and multitude of things come from the intention of the first agent, who is God. For He brought things into being in order that His goodness might be communicated to creatures, and be represented by them; and because His goodness could not be adequately represented by one creature alone, He produced many and diverse creatures, that what was wanting to one in the representation of the divine goodness might be supplied by another. For goodness, which in God is simple and uniform, in creatures is manifold and divided and hence the whole universe together participates the divine goodness more perfectly, and represents it better than any single creature whatever.

And because the divine wisdom is the cause of the distinction of things, therefore Moses said that things are made distinct by the word of God, which is the concept of His wisdom; and this is what we read in Genesis 1:3-4: "God said: Be light made . . . And He divided the light from the darkness."

Thomas Aquinas, Summa Theologica (Part 1, Question 47, Article 1)