Dr. Michael Sudduth
St. Thomas Aquinas: The Doctrine of Divine Simplicity
Doctrine of Divine Simplicity: God lacks all metaphysical composition (compositio). God is omnino simplex.
I. Context/Background of Doctrine
St. Thomas's exposition of divine simplicity is integrally related to his account of whether God exists (prima pars, question 2), according to which Thomas reaches the positive conclusion that we can know that God exists (an est).
A. Terminological Preliminaries:
 Substance: a thing as that which can exist on its own (e.g., house, car, person, etc.)
 Accident: modification(s) of a thing which exists on its own, where the modifications can only exist in the thing (i.e. substance), and not on their own. (e.g., red or velocity: no redness or velocity without a thing possessing it). (also called a property)
 Act: things as they are (things are actual by virtue of their form).
 Potency: capacity a thing has for change (or to be acted upon) whether substantially (i.e., a thing of one kind becoming a thing of another kind) or accidentally (i.e., a thing of one kind altering without becoming a thing of another kind).
 accidental change: a change which a thing can undergo without turning into a thing of a different kind (i.e. without a change in essence, substance, or nature). E.g.: A cow in a field undergoes accidental change if it moves into the barn. A cow in the field is actually in the field, though potentially in the barn. If the cow moves into the barn, it is now actually in the barn and has undergone accidental change. Moving into the barn doesn't change the nature of the cow. It was a cow in the field and it is still a cow in the barn.
 substantial change: a change which a thing can undergo by turning into a thing of a different kind (i.e. a change in essence or nature). If the cow in the field dies, it undergoes substantial change (it looses its substantial form). The beef at the butcher shop is not a cow, but is something which once was a cow.
 matter: the capacity (or potentiality) a thing has for substantial change, that by which a thing which has a nature might cease to have it and turn into something else. Matter is the opposite of actuality.
 form: that by which a thing actually is what it is, whether substantially or accidentally. Substantial form is what brings about substantial existence, whereas accidental form is what brings about accidental existence. Cow, substantial form; brown, accidental form. Form gives things their positive intelligibility (nature and qualities). We have form whenever we have a definite thing, property, or attribute predicable of things.
"Matter, for Aquinas, is opposed to form. Form is that by which something actually is (e.g., cow), while matter is that by which what is might not be (that by which a cow can become a corpse)." (Davies, The Thought of Thomas Aquinas, p. 48).
B. Five Ways: The Positive Grounds
According to Thomas, human reason can demonstrate (demonstratio quia, from effects) that there exists a first existent--a uniquely necessary immutable sustaining cause (efficient and final) of the world, which is the ultimate explanatory being.
 Observational starting points: features of the world known to us through sensory experience. (e.g., it is evident to the senses that some things are in motion)
 Observational features of the world entail the existence of some other thing(s) which accounts for what we observe. (e.g., omne quod movetur ab alio movetur).
 There can no infinite regression of things that account for any particular datum of experience.
 To explain the existence of the features presented to us in empirical observation, we are led to a First Existent that lies beyond the mundane realm.
The features of the world we come to know through sensory experience cannot be accounted for in ordinary mundane terms, in terms of those very features of the world.
 This First Existent is called God (Deus).The First Existent (if it is to explain the data of empirical experience) must be wholly actual, that is lacking all passive potency, sine ulla potentialitate. Accordingly, the first cause is wholly immutable, not subject to either substantial or accidental change. It is Pure Act--there is nothing it could be which it isn't.
The first existent (FE) must be the sort of thing for which causal questions do not arise, for otherwise it could not constitute the ultimate explanation of things that do change in the world. The immutability of the first existent follows clearly. If FE were not immutable, then it would undergo change. If it undergoes change, then FE is changed wither by itself or by another. If by another, then we would be forced to concede another changer prior to FE, which is contrary to FE qua FE. If FE is changed by itself, then either (a) FE is a changer and changed in the same sense or (b) FE is a changer in one sense and changed in another. Since whatever is changed is in potency and whatever changes is in act, (a) cannot be the case since it would entail that FE is in potency and in act in the same sense. If, however (b) is the case, then one part of FE is causing change and another part of FE is being changed. But then there would be no first changer, except in the sense that a part of FE causes change. FE itself couldn't be a real first cause, since it itself would be a composite of changer and changed, and the changed part of FE would require a prior changer. So the first cause must be wholly immutable.
C. Via Negationis: the Eliminative Method
Having established that there is a First Existent (an est), Thomas turns to the question of the way in which the First Existent exists so that an understanding may be gained of what the First Existent is (quid est). Here Thomas follows the common Aristotelian method of scientific treatment, first an est then quid est. But Aquinas states that de deo scire non possumus quid sit, sed quid non sit. Consequently, the task of the theologian will be to consider the ways in which God does not exist. Thus Thomas embarks on the via negationis, denying of God things which are true of the creature, the core of which is the doctrine of divine simplicity.
Thomas does not mean to say that we can make no true assertions about the First Existent, but only that we cannot have any defining or comprehensive knowledge of him on the scientific model of genus and species--no "quidditative" knowledge. First, all our knowledge is derived from and limited to sensory experience, but God is not an object of sensory experience. Secondly, as the immutable and uniquely necessary sustaining cause of the world, it will not be appropriate (or intelligible) to place the First Existent in a category, as the First Existent transcends all human conceptual schemes.
II. Four Essential Claims
The propositions involved in the doctrine of divine simplicity, though being essentially negative in nature, rest on the positive assertions reached under an Deus sit. Principally, Deus est purus actus non habens aliquid de potentialitate, from which follows the absolute immutability of the First Existent.
A. God has no extended parts (Spatial simplicity)
God can have no extended parts, for the extended is potentially divisible, for in God there is no passive potentiality. So, God cannot be a body.
B. God is not a composition of form and matter
Since matter is the potentiality a thing has for changing substantially, God cannot be a composition of form and matter; for God is without all passive potentialities. "The very existence of matter is a being potential; whilst God, as we have seen, contains no potentiality, but is sheer actuality" (Aquinas). Since matter is, for Aquinas, the principle of individuation within a species, it follows that God is not, at least in any ordinary sense, an individual. God is pure form.
C. God is not a composition of nature and suppositum
God's not being a person or individual is also exemplified by God's lacking any composition of nature and suppositum (a distinct subject or individual). Two human individuals, say Paul McCartney and Elvis Presley, though they are not the same individuals or particular things (supposita), they do share a common nature. Elvis is human and Paul is human. But the nature and individuality of each one is distinct in each. The one's being Elvis does not follow from Elvis's being human. In short, although Elvis and Paul both have a nature, they are not identical with their natures; neither one is human nature. WHO they are can be distinguished from WHAT they are. The First Existent, however, is identical with its nature. Who the First Existent is and What the First Existent are indistinguishable. God is What He is.
Things composed of form and matter are distinct from their natures (form relating to the nature and matter individuation). Since nature only defines a species of a thing, an individual man will have something above and beyond what his human nature has. This makes him distinct from others who share his nature, belong to the species of human beings. Since God is not composed of form and matter, it follows that he is identical with his nature.
D. God is not a composition of nature and esse
There is also in God no composition of nature and esse (existence). The potentiality that is involved with individual things in the world is such that they are potentiality non-existent. Individuals are not only a composition of form and matter and nature and suppositum, but a mixture of nature and esse. Neither who Elvis is nor what he is guarantees that he is. As individuals have "natures" they also have "existence." Again, what a creature is does not ensure that it is. Even the angels (which, being pure form, are not contingent), which are identical with their natures, are not identical with their existence, having received it from God. They are also potentially non-existent (even if they have no in-built tendency to perish).
But God is not the type of being who might not have been or who might be. There is no potentiality in him. He must be his own existence, or as Aquinas says, Ipsum esse subsistens. If a things existence were different from its nature, then its existence would have to be externally caused. But the First Existent is the first cause. God's existence is not derived from another. God is not dependent upon anything or anyone else. Not only is his existence necessary, but it is necessary through itself (not a derived necessity like that of the angels). In short, God is not created.
III. Criticisms and Reflections
Alvin Plantinga (in Does God have a Nature?) understands divine simplicity (especially the claim that God is identical with his nature) to entail that different properties are not different when applied to God or that God himself is nothing but a property.
But the doctrine involves nothing like this. When we speak about God using sentences which differ in meaning, the reality to which are talk of God refers is not to be thought of as something distinct from its nature (cf. Davies, Aquinas, p. 53). "God" and "the wisdom of God" are two ways of referring to one and the same reality, much like "morning star" and "evening star" (clearly different in sense) are two different ways of referring to one and the same reality--venus. They differ in sense, but not in reference. Hence, we are to understand that the attributes referred by expressions such as "the wisdom of----" and "the power of----" are indeed different, but the wisdom of God and power of God are identical. In other terms, the reality signified by "the wisdom of God" is the same thing as signified by "the power of God." (See Davies, The Thought of Thomas Aquinas, p. 53, 54, 68-69).
"The square of----" and "the double of----" signify two quite different functions, but for the argument 2 these two functions both take 4 as their value. Similarly, "the wisdom of----" and "the power of----" signify different forms, but the individualizations of these forms in God's case are not distinct from one another; nor is either distinct from God, just as the number 1 is in no way distinct from its own square. (Geach, Three Philosophers, p. 122).
What is denied by divine simplicity is:
(1) What is signified by "the wisdom of God" is possessed by God as a property distinct from that of being powerful.
© Michael Sudduth 1997