Is There Only One World?
Summa Theologiae, Pars I, q. 47, a. 3
Article 3: Is There Only One World?
1. It seems there are several, not only one. According to St Augustine (Questions. 83, qu. 46) it is improper to say that God has created things without a reason. For the same reason that He created one He could create many, since His power is infinite, as already aknowledged (Question , Article ), and remains unspent after the creation of one world. Therefore He produced several.
2. Further, if nature works for the best much more does God. Better, it will be admitted, for there to be more worlds than one alone; many good things are better than few. Therefore many worlds were made by God.
3. Again, a thing having a form embodied in matter can be numerically repeated without varying the species, for such multiplication is rooted in matter. But the world has form with matter; when I say "man" I signify form, whereas when I say "this man" I signify the form in matter. Well then, when we speak of "world" the form is signified, and when of "this world" the form with matter. There is nothing then against the existence of many worlds.
On the other hand, the prologue of St John states (Jn 1:10), "The world was made by Him," speaking in the singular, as much as to suggest that there is only one world.
Reply: The reigning order in things established by God's creation manifests the unity of the cosmos. This is because of the single plan ordering some things to others. For all things coming from God have a relation to one another and to Him, as we have already shown (Question , Article ; Question , Article ). Hence all must belong to the same cosmos. In fact those only have been able to propose a plurality of worlds for whom the cause of the world is chance, not governing wisdom; thus Democritus, who taight that that this world and an infinity of others happened from the clash of atoms.
Hence: 1. The objection provides the reason why the world is one, namely that things should be ranked together in one order and trained to one end. Indeed it is from the planned unity of order that Aristotle infers (Metaph. xii, text 52) that God who rules them is one. And Plato (cf. Timaeo) proves the unity of the world from unity of the exemplar after which it is fashioned.
2. No agent intends material plurality as a goal, for numerical multitude has no fixed limit, since its trend is towards indefinite repetition, and to be definite is just what an end is not. The claim that many worlds are better than one is obsessed by the value of mere numbers. Such a best is not in the intention of Got the maker. Indeed if we adopt this mood of looking at things we might argue that had He made two worlds it would have been better had He made three, and so forth indefinitely.
3. The universe stands on the whole of its matter. For there to be an another earth than this one is not possible, for each earth would naturally gravitate towards the same centre, wherever it was. The same applies to other bodies which are parts of the world.
Summa Theologiae, I, q. 47, a. 3, edited by Blackfriars, translated by Thomas Gilby (London-New York: Eyre & Spottiswoode - McGraw-Hill, 1967), vol. 8, pp. 101-105.