Thomas Aquinas believed that there was an appropriated assimilation or likeness to God found in creatures and creation. Some likeness must be found between an effect and its cause. It is in the nature of any agent to do something like itself. Thus, God also gives to creatures and creation all their perfections; and thereby he has with all creatures a likeness.
Additionally, the cause of variety and the multitude of things in creation find their cause in God. Thomas contrasts himself with early Greek philosophers such as Democritus and the other atomists who argued that the distinction of things come from chance according to the movement of matter. Thomas follows Anaxagoras in attributing the multitude to matter and to the agent involved. Thomas identifies this agency as God since he is the creator of matter and thus the efficient cause behind the existence of the matter. Additionally, the universality of things and the perfection of the universe must precede forth from the intention of the first agent—God. Thomas states that the distinction and variety reflects the divine goodness.
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.
Thomas’ contention is that creation participates in God’s goodness. It reflects the nature of God. The reason why there is a variety and multitude of existents is because one existent thing could not properly and sufficiently represent God’s goodness.
This is where modal realism strengthens Thomas’ doctrine of variety. Thomas states that one existent could not be sufficient in depicting God’s goodness. What about two existents? What about a billion existents? The point is that not even universes could properly reflect the divine goodness. What is needed to sufficiently reflect the divine goodness is either an actual infinite or a potential infinite of existents. Even if there were an actual infinite set of existents it could only reflect the divine goodness because it is still the effect of God’s cause. If this doctrine of variety is to be taken seriously then no finite amount of existents could ever sufficiently reflect the divine goodness.
Recall Thomas’ commitments in his doctrine of divine simplicity:
(1) It is impossible that God have any spatial or temporal parts that could be distinguished from one another as here rather than there or as now rather than then, and so God cannot be a physical entity.
(2) It is impossible that God have any accidental properties.
(3) All of God’s intrinsic properties must be essential to him, it must be acknowledge that whatever can be intrinsically attributed to God must in reality just be the unity that is his essence.
Let us consider Thomas’ commitments in more detail. (1) merely states that God is a non-spatiotemporal seamless whole. (2) states that it is impossible for God to have any accidental properties. A logically equivalent rendition is that it is necessary for God to have only necessary properties. (3) is an entailment from (2)—namely that these necessary properties are intrinsic and essential to him. We have the tendency to project a posteriori knowledge back onto God so let us approach this task with an Anselmian notion of God and remove all a posteriori knowledge of the world from the present task. If Thomas articulates simplicity in such a manner making commitments to (1), (2), and (3) and that God is identical to his essence, which is identical to his existence—the very pure act of being then what would we expect creation to look like? Again, given simplicity, an answer to this question may very well be an infinitely various and multitudinous creation.
Gottfried Wilhelm Leibniz developed a similar idea, which is known as the principle of plenitude. He argues that there must be diversity in that which changes. This change and diversity is what produces the specification and variety of simple substances. This diversity must involve a multitude in the unity or in the simple. For, since all natural change is produced by degrees, something changes and something remains. As a result, there must be a plurality of properties and relations. The principle of plenitude entails absolutely every way that a world could be is a way that some world is and absolutely every way that a part of a world could be is a way that some part of some world is.
 Summa Contra Gentiles 1.29.
 Summa Theologica 1.47.1, 2.
 Leibniz argued that every monad is constantly changing, which composes creation.
 Leibniz, The Principles of Philosophy or, the Monadology 1.12-13; Discourse on Metaphysics 2-5.
 David Lewis, On the Plurality of Worlds (Oxford: Blackwell Publishers, 1986), 86.