Two Arguments for Monotheism
To my mind, the standard contemporary argument against polytheism involves an appeal to parsimony: because the simplest explanation is often the most preferable, and because a multiplicity of “Gods” seems to be as good an example of multiplying entities beyond necessity as any, we may well ask: why invoke more than one God when one God will suffice? In other words, what is it about a multiplicity of Creators that offers a better explanation of the existence of the world than the resources of a single Creator, all-powerful, all-knowing, and so on?
Here are two arguments for monotheism which do not rely on the principle of parsimony. Like the argument from parsimony, they can be used to complement the conclusion of any theistic proof. Unlike the argument from parsimony, they carry greater probative force.
The first argument takes as its starting point St. Anselm’s conception of God as “that than which no greater can be thought”. Note carefully that this is taken purely as a cognitive claim. There is no further resemblance to Anselm’s ontological argument, since we are not here concerned with whether God exists, but only whether there can in principle be more than one God.
- God exists. [Assumption]
- God is “that than which no greater can be thought”. If you can think of something greater than God, then that is what is meant by God.
- If more than one God existed, there would have to be some way of individuating one from another.
- The only way of individuating one from another is by reference to something one has that the other lacks.
- But to admit that one or more of them lack something is to admit that there is a greater that can be thought.
- So, there can only be one God. [Per premise 2]
Premises 3 and 4 can be defended as follows: whereas to be man is something communicable to many, to be this particular man is only communicable to one. Certainly there are many men; but there is only one Socrates. Why? Because there is something communicable to him — being informed by a certain parcel of matter; being born in a particular time and place; being a philosopher; having been put to death at the hands of the Athenian democracy; and so on — that just isn’t communicable to any other man . There are hundreds upon thousands of people who share my name, yet I am distinguishable from them by having certain attributes that they do not share, and being subject to outside conditions that do not apply to them.
You can extend this type of individuation to things that share the same essential properties, such as triangles, circles, and so on. Take two triangles, for example. Though essentially identical — they are both made up of 3 sides, and three angles equalling 360 degrees — they will nevertheless differ with respect to spatial location, size, color, etc.
The second argument rests on a number of principles defended by Aquinas in the context of the Fourth Way, a henological argument for God’s existence. See Michael Augros’ “Twelve Questions about the “Fourth Way””
- To be God is to have a certain nature or essence, such that if there were many “Gods”, this essence would be shared equally.
- But to speak of a multiplicity of “Gods” is to admit difference between them, and difference is always a matter of inequality.
- Moreover, where any perfection X belongs more to A than to B, X cannot belong to both of them essentially. For if X belonged to A and B essentially, it could not be said to exist less perfectly in one than in the other. Essence does not admit of degree.
- So, if X belongs more to A than to B, then X does not belong to B of its own essence, but through some outside cause.
- So, any “God” who has any perfection to a lesser degree than another would be caused or acted upon in some sense, and hence not be “God” in the first place.
- But this process of elimination will always exist so long as there is more than one God. [Since difference implies inequality, per premise 2]
- So, there can only be one God.
This argument begins by asking us to consider what it would mean to say that there are many “Gods” with a capital “G”. Presumably one prerequisite would be to say that they are essentially the same. If to be God is to be Absolute Being — unchanging, completely actualized being — then we must say as much of every God that exists.
Premise 2 raises the obvious question: ‘just what is it that distinguishes this God from that God?’ Surely there must be some way of telling them apart — perhaps God1 is more powerful than God2; or perhaps God1 is temporal, whereas God2 is atemporal; or perhaps God1 is more knowledgeable than God2.
In defense of premises 3 and 4, we may say the following: if you add (or subtract) anything to a number, you will get a new number. If you add (or subtract) anything to the substantial being of a thing, you will get a new species. In the same way, if you add (or subtract) anything to an essence, you will get a new essence. It makes no sense to say, of two triangles, that one is more three-sided than the other, since to be three-sided just is what it means to be a triangle. It either has three sides, and is thus a triangle, or it does not have three sides, in which case it is not a triangle. Essence does not admit of degree.
Now, say there is some perfection X that belongs both to God1 and God2, but it belongs less to God2 than to God1. This would mean that X belongs to God2, not of itself — again, since essence does not admit of degree — but through some outside cause.
 Summa Theologica, I, q. 11, a. 3.