Objection 1. It would seem that the world is not governed by one. For we judge the cause by the effect. Now, we see in the government of the universe that things are not moved and do not operate uniformly, but some contingently and some of necessity in variously different ways. Therefore the world is not governed by one.
Objection 2. Further, things which are governed by one do not act against each other, except by the incapacity or unskillfulness of the ruler; which cannot apply to God. But created things agree not together, and act against each other; as is evident in the case of contraries. Therefore the world is not governed by one.
Objection 3. Further, in nature we always find what is the better. But it "is better that two should be together than one" (Ecclesiastes 4:9). Therefore the world is not governed by one, but by many.
On the contrary, We confess our belief in one God and one Lord, according to the words of the Apostle (1 Corinthians 8:6): "To us there is but one God, the Father... and one Lord": and both of these pertain to government. For to the Lord belongs dominion over subjects; and the name of God is taken from Providence as stated above (Question 13, Article 8). Therefore the world is governed by one.
I answer that, We must of necessity say that the world is governed by one. For since the end of the government of the world is that which is essentially good, which is the greatest good; the government of the world must be the best kind of government. Now the best government is the government by one. The reason of this is that government is nothing but the directing of the things governed to the end; which consists in some good. But unity belongs to the idea of goodness, as Boethius proves (De Consol. iii, 11) from this, that, as all things desire good, so do they desire unity; without which they would cease to exist. For a thing so far exists as it is one. Whence we observe that things resist division, as far as they can; and the dissolution of a thing arises from defect therein. Therefore the intention of a ruler over a multitude is unity, or peace. Now the proper cause of unity is one. For it is clear that several cannot be the cause of unity or concord, except so far as they are united. Furthermore, what is one in itself is a more apt and a better cause of unity than several things united. Therefore a multitude is better governed by one than by several. From this it follows that the government of the world, being the best form of government, must be by one. This is expressed by the Philosopher (Metaph. xii, Did. xi, 10): "Things refuse to be ill governed; and multiplicity of authorities is a bad thing, therefore there should be one ruler."
Reply to Objection 1. Movement is "the act of a thing moved, caused by the mover." Wherefore dissimilarity of movements is caused by diversity of things moved, which diversity is essential to the perfection of the universe (47, 1,2; 48, 2), and not by a diversity of governors.
Reply to Objection 2. Although contraries do not agree with each other in their proximate ends, nevertheless they agree in the ultimate end, so far as they are included in the one order of the universe.
Reply to Objection 3. If we consider individual goods, then two are better than one. But if we consider the essential good, then no addition is possible.
Source: Summa Theologica (New Advent)