In Ia.q3 Aquinas demonstrated that God is absolutely simple. In this question he considers a closely related idea, the unity of God. In approaching the question of God’s unity, he has to consider in great depth and profundity the very notions of unity and multiplicity and he concludes that one and being are convertible when considered as transcendentals (as were good and being in Ia.q5). Scripture tells us that there is only one God; reason supports this teaching and goes on to teach us that His oneness expresses the fullness of unity.
The Thread of the Argument
A1: When Aquinas asks whether one adds anything to being, he is asking whether being and one are convertible as transcendentals. That is, are they just different aspects of the same underlying reality? The essence of his argument is that when we say of something that it is one, what we mean is that it is an undivided being. In other words, that it should be considered in its unity as a being rather than as a collection of beings. A composite being made out of parts is a unified being in the singular; if it is divided into its parts it does not have being as a composite thing.
Replying to the first objection, he considers historical approaches to the relationship between unity (considered as applied to things) and the number one. He sides with Plato and Pythagoras (against Avicenna) in claiming that our notion of unity gives rise to the concept of number.
Against the second objection that argues that we do in fact divide beings into the one and the many (for example, we need to be able to differentiate between the being of this collection of ten horses and the being of this one horse) Aquinas answers that this is to confuse the meaning of the word “being”; what we need to focus on is the unity of things-in-themselves, from which we derive the idea of a multiplicity of things. As with the convertibility of good and being in Ia.q5, Aquinas is willing to say that we can conceptually differentiate between the one and being; but it is a purely conceptual, rather than a real, distinction.
A2: The question posed in the second article may seem curious: is one the opposite of many? What this is about is how we come to our notions of unity and multiplicity. For example, we speak quite freely of “a flock of sheep” treating it as a unity, but also considering it as a multiplicity composed of individual sheep. And which comes first, unity or multiplicity? When we consider our understanding of these things, do we derive one concept from the other, and which way around is the derivation?
Aquinas suggests in the sed contra that unity is associated with indivisibility; something is a unity if it makes no sense to consider it divided up whereas multitudes are inherently divisible. In his answer, Aquinas makes a distinction: the one and the many are opposites but in different ways depending on how they are considered. If one is considered the basis of number then many is a derived concept from one; if one is taken as convertible with being then they differ in the way proposed in the sed contra.
In the reply to the first objection, which pointed out the unity inherent in multiplicities, Aquinas gives an extended discussion of the convertibility of one and being along similar lines to that given for the convertibility of good and being in Ia.q5.a1.ad1. In that discussion Aquinas showed how a privation of the good can be thought of in terms of a privation of being; and conversely that unqualified good corresponds to the fullness of being. Here Aquinas argues in the same way: a lack of unity corresponds in some way to a lack of the fullness of being; the complete actualization of potentiality in something would correspond to it being truly a unified thing-in-itself. However, just as the privation of a good has to be grounded in some actual good, the absence of unity has to be grounded in something that exists. Therefore Aquinas can conclude that just as a bad thing is actually some sort of good (in that it exists), “so the many turn out to be somehow one”.
In the reply to the second objection in which it was argued that a whole is made up of parts, Aquinas distinguishes between homogeneous and heterogeneous wholes. In the former all the parts of the whole have the same form as the whole; in the latter the parts differ from the whole. A body of water can be considered as composed as many sub-bodies of water; but a house is not made up of sub-houses. Even for heterogeneous wholes, we must recognize their existence as unities.
In the reply to the fourth objection Aquinas gives a careful description of how we arrive at the concepts of unity and multiplicity. We tend to come to an understanding of simple things through an analysis of complex things and therefore we might think that the idea of multiplicity is prior to that of unity. But this neglects to take into account that the division of something complex into things that are simple presupposes the unity of the bits into which we are dividing the complex. Our first apprehension is of things that exist; we then discern that this thing is not that thing and thereby arrive at the idea of division, which gives us the idea of multiplicity.
A3: Now Aquinas can turn to the oneness of God, for which he offers three proofs that reinforce the scriptural teaching of Deuteronomy that “The Lord our God is one God”.
The first proof observes that what makes something a singular thing cannot be shared with other things. So Socrates is an individual human being and no other human being is Socrates; there can be other human beings because they can share in the nature of humanity but what individuates Socrates belongs to Socrates alone. God is His own nature, so what individuates God is being-God; to be God is to be this God. There can be no other Gods because this would imply that there would have to be other God-natures.
The second proof observes that if there were more than one God, then they would have to differ from each other in some way. Therefore at least one of them would have to lack something that the other possessed. But God is that which is the fullness of being, lacking any privation. Therefore the hypothesis of a multiplicity of Gods leads to a contradiction.
Finally, when we consider the unity of the world, we find that there is a single ordering of things in the world. Such a unity of ordering must be due to there being a single cause of that order, since a multiplicity of causes would produce a unity of ordering only by accident. Since it is God that provides the ordering of the world, we must conclude that God is one.
A4: Is the unity of God the most unified unity there can be? Having done the hard work in previous articles, proving the affirmative here is straightforward. God is pure actuality and since being and unity are convertible, His unique fullness of being corresponds to a unique fullness of unity. He is also supremely indivisible, as He is completely simple.
Summary & Handy Concepts
- One and being are convertible as transcendentals.
- The idea of the number one is derived from the unity of beings-in-themselves.
- Unity and multiplicity go hand-in-hand with indivisibility and divisibility.
- The one and the many are opposites but in different ways depending on how they are considered.
- Our apprehension of multiplicity is founded upon our apprehension of being and thence of unity.
- The scriptural teaching of the oneness of God is confirmed by reason; what God is by His nature implies that He must be one.
- The fullness of the being of God and His simplicity imply that He is supremely one.
- The convertibility of being and one points to the reality of the being of things in the world. We must not think of the world as an indivisible monad (though we may consider it in its unity); it is not simply our mind that organizes the world into things.
- The meditation on the convertibility of being and one in Ia.q11.a2.ad 1 and the similarities of these transcendentals with the good is profound. Here we get a glimpse of the profound mysticism at the heart of St. Thomas’s theology.