In the Prima Pars of Summa Theologica, Aquinas says of divine simplicity, “when the existence of a thing has been ascertained there remains the further question of the manner of its existence.” Question three of the Summa is an appropriate place for the Italian theologian to assert this proposition as the previous question deals with Aquinas’s five proofs for the existence of God. Brain Davies notes that these five proofs “are famous and have given rise to a huge amount of literature both expository and critical.” Summarizing the “famous” five arguments, Michael Dodds says they can be categorized as: “(1) motion; (2) efficient causality; (3) contingency and necessity in beings; (4) grades of perfection in beings; and (5) finality in nature.”
The First Mover
The first of Aquinas’s five proofs is pertinent to our discussion here as it pertains to motion. Aquinas argues that “in the world some things are in motion.” This is evident to us, as we perceive our creaturely surroundings and see change and motion everywhere—time, space, matter, etc. If it is true that much is in motion around us, it also must be true that something set these items in motion. Aquinas argues that recounting the causative effect of motion back to its source will run into an abundance of causes until one arrives at the first mover—God himself.God is the unmoved first mover who lacks potentiality, parts, and accidental qualities. Click To Tweet
Aquinas describes the type of motion he has in mind here by utilizing the language of potency and act. He argues that nothing can be in motion except that which poses potentiality, for change is simply the motion from potency to actuality. For explanatory purposes, Aquinas draws to the mind the example of a fire and the logs that make up the fire. “Thus, that which is actually hot, as fire, makes wood, which is potentially hot, to be actually hot, and thereby moves and changes it.” The flames no longer have the potentiality of heat—as it is an actualized property—and it is those flames that actualize the potential property of heat within the logs. This is the exact kind of motion that may not be predicated of God as this form of motion entails potentiality.
Aquinas is building on Aristotle’s argument of a first mover, yet he is forced to nuance Aristotle’s position for theological purposes, for, in Aristotle’s articulation, the first mover inaugurates the chain of motion with a first act of self-motion. However, Aquinas denounces this view on two grounds: first, he writes that it is impossible for an object to both have potentiality and actuality in the same respect. Second, the idea that the first mover must move himself would entail complexity of being. There must be composition such that an actualized part could move a part that possesses potentiality. As Aquinas moves toward question three and addresses divine simplicity, it becomes evident this cannot be said of God. On the contrary, God is the unmoved first mover who lacks potentiality, parts, and accidental qualities.
The concept of an unmoved mover is an important element of Aquinas’ Summa Contra Gentiles as well. In Summa Contra Gentiles, readers might be surprised to find that Aquinas does not treat the doctrine of divine immutability in a stand-alone article or chapter like he does with the ninth question of Prima Pars. However, while not receiving a stand-alone chapter, Thomas’ argument and the development therein depend on divine immutability. This is why, for example, he can say in the fourteenth chapter of Book One, which treats predication by way of remotion, “As a principle of procedure in knowing God by way of remotion, therefore, let us adopt the proposition which, from what we have said, is now manifest, namely, that God is absolutely unmoved.” And again, when Thomas treats divine eternality in Summa Contra Gentiles, he can say without hesitation, “Since, however, we have shown that God is absolutely immutable, he is eternal, lacking all beginning or end.”
While he never addresses immutability explicitly, Aquinas can begin both chapter 14 and 15 with this assumed acceptance of God’s changelessness due his work in demonstrating the necessity of God’s immutability by his virtue of being the unmoved mover in the previous chapter. Chapter 13 is a series of proofs for God’s existence, and again, Michael Dodds is helpful to summarize these proofs: (1) motion, (2) efficient causality, (3) perfection in being and truth, and (4) the government of the world. In his argument of the unmoved mover, Aquinas builds again off of Aristotle to show that (1) “everything that is moved is moved by another” and (2) “that in movers and things moved one cannot proceed to infinity.”
After working through two differing views of how to understand motion, as seen in Aristotle vs. Plato, Aquinas comes to the conclusion, “Therefore, some self-moving being must have a mover that is moved neither through itself nor by accident.” Similar to his conclusion in Summa Theologiae, ultimately, Aquinas resolves the conversation of the unmoved first mover which must not regress into infinity, nor move itself, nor be moved by an accident by stating, “there must, therefore, be an absolutely unmoved separate first mover. This is God.” This theological reasoning on the unmoved mover allows Aquinas to pick up after the proofs of God’s existence on the foundation that God must be immutable.This theological reasoning on the unmoved mover allows Aquinas to pick up after the proofs of God’s existence on the foundation that God must be immutable. Click To Tweet
While we saw Aquinas indirectly treat immutability through the argument of the unmoved mover in Summa Contra Gentiles, He does address the doctrine head on in Summa Theologiae. As Aquinas turns to address divine immutability explicitly in Ia9, he builds his case for immutability “from what precedes,” meaning he constructs his understanding of immutability on the foundation of God’s existence, divine simplicity, perfections, goodness, and infinity—as these are the topics of questions one through eight. Aquinas provides three arguments for divine immutability, all of which are intricately related to another aspect of the divine life. In his first argument he insists that the first mover “must be pure act, without admixture of any potentiality.” He continues, “Everything which is in any way changed, is in some way in potentiality. Hence it is evident that it is impossible for God to be in any way changeable.”
His second argument is concerned again with divine simplicity. Aquinas writes that whatever changes “remains as it was in part, and passes away in part.” This would necessitate composition, which cannot be properly predicated to God for he has been shown—in Ia3—to be “altogether simple.” His third and final argument deals with divine infinitude; Aquinas writes that “everything which is moved acquires something by its movement, and attains what it had not attained previously.” Of course, this means that God must not be moved as “God is infinite, comprehending in himself all the plenitude of perfection of all being, he cannot acquire anything new, nor extend himself to anything whereto he was not extended previously.”
In question nine, then, Aquinas concludes that God is altogether unmovable. His lack of potentiality, together with his simplicity and his infinitude, means he may not be said to move into any greater actuality, move in any part, nor move into acquiring anything new.God’s operation is unlike that of creatures since it is his very substance. Click To Tweet
Thomas’s writings concerning immutability are not confined to Summa Theologica and Summa Contra Gentiles. On the contrary, Aquinas explicitly refers to or alludes to the doctrine of divine immutability in nearly all his published work. In his commentary on Lombard’s Sentences, Aquinas echoes a similar argument to his first of three in Summa Theologica, “One ought to say that Every motion or mutation, howsoever it be said follows upon some possibility, since motion is an act of an existent in potency. Hence, since God is pure act, having nothing of potency admixed, there can be no mutation in him.” Relating this conception of God’s pure actuality to that of temporal creation and divine action, Dodds helpfully summarizes one of Aquinas’s responses to objectors of divine inalterability:
Thomas shows how God’s “transient action” in the production of creatures is directed by his immanent activity of knowing and willing. God’s operation is unlike that of creatures since it is his very substance. Things whose operation is different from substance must be moved to action since in acting they acquire a new actuality they cannot give themselves. Since God’s operation is his substance, he does not need to be moved by another to operate. As his substance is eternal, so is his operation. The effect of his operation, however, need not be eternal. It proceeds from him not eternally, but according to the order of wisdom in God’s immanent activity of knowing and loving.
It is not without reason that most articulations of divine immutability make reference to the great medieval Italian theologian. Aquinas aids readers in articulating God’s unchanging essence as it relates to philosophical inquiry and other divine corollaries of being.
 Aquinas, ST, Ia3.2.
 Brian Davies, Thomas Aquinas’s Summa Theologiae: A Guide and Commentary (Oxford: Oxford University Press, 2014), 34.
 Dodds, The Unchanging God of Love, 94.
 Aquinas, ST, Ia2.3.
 Aquinas, ST, Ia2.2.
 Aristotle, Metaphysics, in Metaphysics, Books X-XIV: The Oeconomica, Magna Moralia, Trans. Hugh Tredennick and G. Cyril Armstrong, (Cambridge: Harvard University Press, 1935), 150.
 Aquinas, ST, Ia2–3.
 Aquinas, Summa Contra Gentiles, Book I (Indiana: University of Notre Dame Press, 1975), 97.
 Aquinas, SCG I, c15.1
 Dodds, 86.
 Aquinas, SCG I, c13.4. Aquinas defends both of these points with three subpoints each. For the first proposition, see SCG I, c13.5-10 and for the second proposition, see SCG I, c13.12-16.
 Aquinas, SCG I, c13.24. cf. also, SCG I., c13.10.
 Aquinas, SCG I, c13.28.
 Aquinas, ST, Ia9.1.
 Aquinas, ST, Ia9.1, cf. Ia3.
 Aquinas, ST, Ia9.1, cf. Ia7.
 Dodds, The Unchanging God of Love, 46. Dodds says this nearly verbatim, “Thomas refers to the immutability of God in almost all his major works.” Dodds then proceeds to work through most of Aquinas’s major publications detailing the role of immutability throughout his corpus (pp. 46–105).
 Aquinas, Thomas Aquinas’s Earliest Treatment of the Divine Essence: Scriptum super libros Sententiarum, Book 1 Distinction 8, trans. E. M. Macierowski (New York: University of New York Press, 1997), 75.
 Dodds, The Unchanging God of Love, 49.