Objection 1. It would seem that the multitude and distinction of things does not come from God. For one naturally always makes one. But God is supremely one, as appears from what precedes (11, 4). Therefore He produces but one effect.
Objection 2. Further, the representation is assimilated to its exemplar. But God is the exemplar cause of His effect, as was said above (Question 44, Article 3). Therefore, as God is one, His effect is one only, and not diverse.
Objection 3. Further, the means are proportional to the end. But the end of the creation is one--viz. the divine goodness, as was shown above (44, 4). Therefore the effect of God is but one.
On the contrary, It is said (Genesis 1:4-7) that God "divided the light from the darkness," and "divided waters from waters." Therefore the distinction and multitude of things is 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 (Question 44, Article 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 (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 (Question 45, Article 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."
Reply to Objection 1. The natural agent acts by the form which makes it what it is, and which is only one in one thing; and therefore its effect is one only. But the voluntary agent, such as God is, as was shown above (Question 19, Article 4), acts by an intellectual form. Since, therefore, it is not against God's unity and simplicity to understand many things, as was shown above (Question 15, Article 2), it follows that, although He is one, He can make many things.
Reply to Objection 2. This reason would apply to the representation which reflects the exemplar perfectly, and which is multiplied by reason of matter only; hence the uncreated image, which is perfect, is only one. But no creature represents the first exemplar perfectly, which is the divine essence; and, therefore, it can be represented by many things. Still, according as ideas are called exemplars, the plurality of ideas corresponds in the divine mind to the plurality of things.
Reply to Objection 3. In speculative things the medium of demonstration, which demonstrates the conclusion perfectly, is one only; whereas probable means of proof are many. Likewise when operation is concerned, if the means be equal, so to speak, to the end, one only is sufficient. But the creature is not such a means to its end, which is God; and hence the multiplication of creatures is necessary.
Source: Summa Theologica (New Advent)