Thomas Aquinas’s treatment of the doctrine of God in his Summa Theologiæ clearly has a peculiar and compelling structure that shapes the mind to have a correct understanding of God the Trinity. First, Thomas speaks much on God’s essence, and then he goes on to speak of the distinction of persons. While many may take this format and put essence and person at odds, the content of the first section seems to reject any case to do so. Thomas uses a pedagogical sequence that prepares the reader with redoublement so that he or she may properly understand the divine persons, and much of this understanding hangs on the location and function of simplicity in his treatise on God.
The Divine Essence: The Location and Function of Divine Simplicity
Though the doctrine of divine simplicity (q. 3) appears to be the beginning of Thomas’s actual treatment of the essence of God, it is in a sense not the technical starting place for Aquinas’s teaching on God’s being. Rather, Aquinas begins with the proofs of God (q. 2). Then, he spends a good amount of time in the divine attributes (qq. 4–10), and closes with unity (q. 11). This section will explain the function and effects of the placement of divine simplicity vis-à-vis doctrines such as actus purus, perfection, and unity.
Actus Purus and Divine Simplicity: QQ. 2–3
In showing many proofs of God, Aquinas reasons that God must be the ultimate efficient, exemplary, and final cause of all things. As such, God must be actus purus. God has nothing to achieve to come to completion. He has no lack, no depletion, no possibility of augmentation or growth. Simply put in negative terms: God has no potentiality. Put in positive terms: God is and has always been wholly realized—he is pure actuality. Building upon this positive claim, Aquinas then speaks on simplicity.
For Thomas, divine simplicity indicates that God is not composed. Therefore, simplicity is negative in nature. By placing pure act and simplicity next to each other, one can see that Thomas uses a positive claim to support a negative one. Put differently, simplicity builds off of pure act and clarifies what sort of one this God is. Thomas’s structure reveals that divine simplicity ought to be considered relatively to pure act. Hence, as the one who is pure act and thus the efficient, exemplary, and final cause of all things, divine simplicity negates the notion that God is composed of three causal forces. Indeed, such a notion that God is composed would technically render God as caused and not the ultimate cause. Simplicity, then, does not only build off of pure act, but is a required reality of God. For if God is pure actuality, then he cannot have any potentiality, and therefore, he cannot be divisible of act and potentiality. In other words, God must be simple if he is pure act, for nothing is greater or higher than he who is who he is. However, much more ought to be said on the function of simplicity in Aquinas’s treatment.
Simplicity and Perfection: QQ. 3–4
As noted above, divine simplicity is simply a negative notion that says God is not composed. Put differently, simplicity is a way of speaking about what God is not rather than saying what God is. Thus, simplicity on its own cannot fully describe what Scripture says of God’s being. Something positive must be said about God’s being and his life to ensure that we do not misread Scripture and bring God down to be compared or equated with creation. Aquinas seems to be aware of this, and so it makes sense that the doctrine of God’s perfection immediately follows simplicity.
The doctrine of perfection, unlike simplicity, is a positive statement about God’s life. Perfection could be understood in a sense that is similar to pure act in that it denotes fullness or complete realization or actuality. As Aquinas says, “The first active principle must needs be most actual, and therefore most perfect; for a thing is perfect in proportion to its state of actuality.” Put another way, something is perfect if it “lacks nothing of the mode of its perfection.” Further, God’s perfection, in a restricted sense, includes talk about creation because God is the ultimate cause of all things. To be perfect then, as one who is the ultimate efficient cause, means “the perfections of all things must pre-exist in God in a more eminent way.” Since God is the ultimate cause of all things, God is this one who is perfect. Interestingly, Aquinas links perfection with God’s existence or life, which is important to understand the perfection-simplicity relation. If God is perfect, and perfection really pertains to his life, then he must have life in a more perfect way than creatures. With this in mind, we can see how simplicity and perfection work together to describe God’s life and identity as indicated in Scripture.
Aquinas clearly sees that God is the living God (Matt 16:16). But not only is God living, he himself is life (John 14:6). Hence, in his treatment of simplicity, Aquinas sees two forms of predication: (1) concrete (e.g., living), and (2) abstract (e.g., life). In this regard, he says “we use concrete nouns to signify [God’s] subsistence . . . and we use abstract nouns to signify his simplicity.” Because God is simple and is therefore not other than or severed from his perfections, he is his own perfection. Hence, all that God has, he necessarily is. Put in other terms, what God has is not divisible from who/what he is. Again, “whatever God has God is.” God, therefore, is life or being because he has it. Since it pertains to concrete predication, perfection affirms that God really is being, but says that, because God is perfect, “God is subsistent being itself—that is, God is his own being subsisting in and through himself alone.” One can say this because if God is perfect, then that which exists in creation must be so in God in a more perfect way. Whereas creatures possess life as gift, God’s life is from himself. Further, since God is simple, this life that he has in the most perfect way—from himself—is what God is. And so, God’s “essence is His existence.”
This pairing of simplicity and perfection thus continues to build off the notion that God is pure act and the ultimate efficient, final, and exemplary cause of creation with whom he shares a measure of his perfection. Perfection therefore makes positive statements of God in Scripture that are clearly evident in creatures “through the via eminentiae,” but before anyone can then think that God is merely like creatures, one is reminded of what perfection builds off of—divine simplicity, which calms our hasty hearts and shows us “through the via remotionis . . . that this perfection exists in God in a manner unlike anything we know.” By placing simplicity and perfection together, Aquinas creates a foundation of how we are to understand God by both speaking positively to help us know this God and by using negations to maintain his transcendence. Such a foundation, or in Wittman’s terms, “a dialectic” between the negative (i.e., simplicity) and the positive (i.e., perfection) enables one to properly think about the divine attributes that follow (qq. 5–10). But again, the placement of simplicity does not only carry weight here, but also shapes how we understand Aquinas’s initial terminus of the divine essence.
Simplicity and Unity: QQ. 3, 11
Aquinas terminates his initial treatment of the divine essence with the attribute of unity (q. 11). By beginning with simplicity and ending with unity—two similar concepts, yet distinct—Thomas has a sort of inclusio or bookend of the divine attributes. That is, simplicity and unity “bracket the intermediate attributes because simplicity belongs to the rationem of divine unity, which is a pure unity corresponding to God’s pure actuality.” Unity and simplicity, then, are not at odds, but they work together. Since simplicity begins the initial treatise of the divine essence and finds its initial termination in unity, simplicity and unity should be understood together.
In article 3 of question 11, Aquinas seeks to answer whether God is one. His answer is certainly scriptural, pointing to the Shema. Further, Aquinas will go on to say that God’s oneness or unity is in fact proven from his simplicity, since “God himself is His own nature.” As such, unity in God is not numerical, but it relates to or is “convertible” with his being. So, what does it mean that something is one?
For a being to be one, it must be “an undivided being.” That is, it must be a whole or discrete unit. Here, as Wittman’s assessment exemplifies, discussion on the distinction between unity and simplicity may further our understanding of what unity entails. Something “one” does not need to be simple, that is, it could be composite. For example, the United States of America is one nation—a discrete whole. But it is not simple, for the U.S.A. is composed of fifty different states. Further, as those strange states like Texas or as the Civil War (1861–1865) indicates, the U.S.A.’s internal conflict shows its potential for dissolution. It has divisibility. Hence, something may be a unit, but yet it can still have the potential to become divided—such is not with simplicity. In Aquinas’s observation, composite things exist as “one” only when its parts “make up and compose it. Hence it is manifest that the being of anything consists in undivision.” However, God is surely simple. So, how might one say that God is one without being composed?
Here, unity must build off the, in Wittman’s terms, negative-positive “dialectic” of simplicity and perfection. In his final article on unity, Aquinas says, “Since one is an undivided being, if anything is supremely one it must be supremely being and supremely undivided . . . God is one in the supreme degree.” One can hear the echoes of both simplicity and perfection here: God has supreme being, and this being is undivided. Because God is the exemplary cause of all things, thus eternally possessing all the perfections in creatures in a simple and therefore different or completely more perfect way than creatures, God is perfectly and simply one. As such, simplicity and perfection inform us that God’s unity is not merely undivided unity, but as ipsum esse subsistens, God’s unity is indivisible—not possessing any potential of dissolution. Hence, the one God of the entire cosmos is an indivisible whole, fully perfect and without potentiality. However, to stop here on the simplicity-unity discussion would seem to stop short of Aquinas’s aims.
Simplicity, Unity, and Trinity
In his last article on unity, Aquinas’s final authority on God’s unity is the Trinity. Quoting Bernard, he says, “Among all things called one, the unity of the Divine Trinity holds the first place.” This naturally prepares Thomas’s readers for what is to come in his section on the distinction of persons (or the so-called de Deo trino). Here, then, it seems that Aquinas is following the order of the revelation God, which first explicates what it means that “the Lord is one,” and then turns to the revelation of the distinction of the divine persons. The so-called de Deo uno thus prepares the reader to consider God as an indivisible unit so that he or she might correctly see how the divine persons relate to the divine essence without falling into the errors of, say, Tritheism or Arius.
Trinity: QQ. 27–43
By having the distinction of persons follow the talk of God’s simple and singular being, Aquinas already clarifies what the divine persons are not. That is, by beginning his treatise of God with simplicity, one can know that the life of the triune persons is not constituted by different parts: paternity, filiation, and spiration. Further, while God’s one, simple life rejects composition, it does not reject distinction in God. It seems to be that the simplicity of the one God enables the saints to distinguish the persons without falling into or confessing Tritheism or Arianism. As such, Thomas can affirm the equality of the persons. Further, by first showing that God is the ipsum esse subsistens, Aquinas can give his readers an understanding what the divine persons are. So, this section will look at what it means, for Aquinas, to be a divine person, which will necessarily involve how they are distinguished, and how the persons relate to the essence.
Divine Persons: QQ. 27–29
Before discussing a definition of the divine persons, Aquinas discusses the divine processions of the Son and the Spirit (q. 27). According to Thomas, the names of the divine persons in Scripture signify processions. After this, Aquinas then speaks of the divine relations (q. 28). Here, he says that there are real relations in God. Thus, he corrects the mind that is apt to fall into Sabellianism. According to Aquinas, these relations in God are the only grounds to distinguish the persons. Further, the persons are “denominated” by their relative or personal properties indicated by their relations. By affirming distinct relations in God, Aquinas can then say that the relations are not other than the divine essence, which prepares the mind to understand the divine persons.
The term “person” for Aquinas, drawing from Boethius’s tested definition, highlights a distinct, subsisting individual. Since the persons share the divine essence and are distinguished only by relation, “person” applied to God must refer to relation. Thus, when we are speaking about the persons of the Trinity, we are speaking about the relations. As Aquinas previously said, relations in God are real, not accidental. Therefore, fitting with divine simplicity, a relation in God “is the divine essence itself; and so it is subsistent, for the divine essence subsists.” Therefore, given that persons can only be distinguished by relation, “person” refers to a relation and since these relations are not other than the divine essences and are thus subsistent, “a divine person signifies a relation as subsisting. And this is to signify relation by way of substance, and such a relation is a hypostasis subsisting in the divine nature, although in truth that which subsists in the divine nature is the divine nature itself.” In short, “the persons are the subsisting relations.” This necessarily sets up the relation of the persons to the divine essence.
Persons and Essence: Q. 39
Building off of what he said in questions 27–29, Aquinas can now further dedicate space to the relation of persons and essence. Because relations in God are real, the relations are the divine essence. And since person refers to the relations and since God is simple, “it follows that in God essence is not really distinct from persons [even though] the persons are really distinguished from each other.” Here, one enters into the great difficulty of the Trinity. Even Aquinas feels this by saying, “difficulty seems to arise from the fact that while the divine persons are multiplied, the essence retains its unity.” So how might the plurality of persons and singular essence cohere? Aquinas goes on to say, “relation as referred to the essence does not differ therefrom really, but only in our way of thinking; while as referred to an opposite relation, it has real distinction by virtue of that opposition. Thus there are one essence and three persons.” In other words, “in God relations are subsistent, and so by reason of the opposition between them they distinguish the supposita; and yet the essence is not distinguished, because the relations themselves are not distinguished from each other so far as they are identified with the essence.”
Redoublement, De Deo Uno, and De Deo Trino
Two things must be taken from this in order to understand the order of Aquinas’s doctrine of God and a proper understanding of God the Trinity. First, there is redoublement. As Gilles Emery says, “Trinitarian theology effects a sort of ‘reduplication.’ To express the Triune mystery, one must use two words, two formulas, in a reflection that joins the aspect of the unity of the divine substance to that of the distinctions of persons.” Emery once again says, “it is imperative to consider the double perspective of the common nature and the Trinitarian relations.” Hence, to perceive the trinitarian mystery, one must have a conceptual distinction of what is common and proper, and one must know that these two really cannot be severed from each other. This is why Aquinas will say things like “in our way of thinking.” So, to speak of the Trinity, there must be two modes of thinking: on the common and on the relative property—“This is precisely what Thomas does in the structure of his treatise on God.” Thus, unity and indivisibility does not deny distinction, and distinction does not distort simplicity.
Second, if the persons are “subsisting relations” and if the relations are identical to the essence, then the persons are nothing other than the divine essence subsisting in a particular manner. Thus, in a sense, when Thomas speaks of the divine persons, he does not stop speaking of the essence or esse of God.
De Deo uno and de Deo trino, then, do not seem to be pitted against one another. To think they are somehow opposed would appear to mistakenly ignore God’s simplicity or indivisibility. Why then would Thomas begin with the common essence? The simple answer seems to be that Aquinas wanted to prepare the reader’s mind for understanding the person, for “one cannot conceive of the person without the substance or without the nature belonging to the very ratio of the divine persons.”
The notion that Thomas’s treatise on God is severed into two sections—essence and distinction of persons—seems to be mistaken. For it consists of three: the essence, distinction of persons, the procession of creatures from God. The first two cover God in se, and last covers God’s works ad extra where God is still God. The first two sections treat God “considered under the aspect of the essence and under the aspect of the distinction.” Thus the first two are not opposed or in tension, but are treating God from two different perspectives being faithful to Scripture.
Conclusion: John Webster’s Application
To conclude, we might look to how John Webster applies one of Thomas’s insights. When we construct a theology of God’s life, we ought not think that his ontological relations are something we merely attach. Rather, they are God’s life. In forming a theology of aseity, Webster will say, “aseity is life: God’s life from and therefore in himself.” Here, one would be wrong to think this is an underlying perfection anterior to the persons. To ensure one does not go down that route, Webster then says such life “is the relations of Father, Son and Spirit.” Then, after expounding the intratrinitarian relations, Webster writes, “these acts and relations are God’s self-existence. Aseity is not only the quality of being . . . underived; it is the eternal lively plenitude of the Father who begets, the Son who is begotten, and the Spirit who proceeds from both.” Thus in affirming simplicity of the one God, we must know that relations in God are not accidental, but are real. Therefore, the intratrinitarian relations are not something optional in the doctrine of God, but are necessary, for the relations are God’s being.
 Much of the following content in this section is indebted to the original thought of Tyler R. Wittman, “‘Not a God of Confusion, but of Peace’: Aquinas and the Meaning of Divine Simplicity,” in MT 32:2 (2016): 151–169.
 Tyler R. Wittman, “‘Not a God of Confusion, but of Peace’: Aquinas and the Meaning of Divine Simplicity,” in MT 32:2 (2016): 154.
 Commonly known as the “Five Ways.”
 Ibid., 154.
 Ibid., 154.
 Ibid., 154.
 For if God was composed, there would need to be something prior or “above” him to cause and compose him.
 The implication of simplicity in the repetition, here, is not to be missed. Thus, Tyler Wittman will say, “God is pure act and therefore not a composite of whole and parts, supposit and nature, genus and species, matter and form, substance and accident, nor essence and existence” (Wittman, “‘Not a God of Confusion, but of Peace,’” 155). One could summarize this as such: By placing pure act just before simplicity, Aquinas lays the metaphysical foundation to support the claims of simplicity. Further, the location of simplicity, here, helps one see that simplicity clarifies pure actuality and it is logically essential if indeed God is pure act.
 Wittman, “‘Not a God of Confusion, but of Peace,’” 156.
 St. Thomas Aquinas, Summa Theologiæ, Prima Pars I, 1–49, Latin/English Edition of the Works of St. Thomas Aquinas 13, trans. Fr. Laurence Shapcote, eds. John Motensen and Enrique Alarcón (Lander, WY: The Aquinas Institute, 2012), Ia.3.prol.
 ST Ia.3.prol.
 ST Ia.4.1; cf. Ia.5.1. In relation to first cause, which must be the one who is pure act, Aquinas says, “Now God is the first principle, not material, but in the order of efficient cause, which must be perfect” (ST Ia.4.1).
 ST Ia.4.1.
 This is not to say that God’s perfection is his creation, nor does it mean that God becomes perfect by creating. No, God is perfect in and of himself and he would remain so if creation did not exist. But, because God has so acted, there is in a mysterious sense that God’s perfection “includes” creating—but this inclusion here is unnecessary (again, this is mystery). Put another way, this inclusion is “fitting,” that is, it is not constitutive or essential, but it is fitting for who God is as the one who is ipsum esse subsistens. And since God so acts, we have to deal with this mystery in some measure that fits with Scripture, and simply stay silent where revelation is silent. Often this place is where the mind falls numb, as I have learned.
 ST Ia.4.2.
 ST Ia.2.3.
 ST Ia.4.
 See ST Ia.4.1, ad. 3; Ia.4.2; Ia.5.1.
 Wittman, “‘Not a God of Confusion, but of Peace,’” 157; cf. Aquinas, ST Ia.3.3, ad. 1. This notion, as Wittman continues, does not only apply to “life” but to others as well: “God loves and is love . . . God enlightens and is light . . . God is wise and is wisdom” (Wittman, “‘Not a God of Confusion, but of Peace,’” 157).
 Aquinas, ST Ia.3.3.
 This notion is not foreign to classical or orthodox theology proper. See St. Augustine, The City of God, trans. Marcus Dods (Peabody, Mass.: Hendrickson, 2017), XI.10.
 Wittman, “‘Not a God of Confusion, but of Peace,’” 157.
 Ibid., 157.
 Again, ST Ia.4.2.
 ST Ia.3.4.
 Wittman, “‘Not a God of Confusion, but of Peace,’” 157.
 Ibid., 157.
 Ibid., 154, 158.
 Ibid., 158.
 Since, of course, God is simple.
 “Hear O Israel: the Lord our God, the Lord is one” (Deut 6:4). Aquinas, ST Ia.11.3, sed contra.
 ST Ia.11.3.
 ST Ia.11.1, ad.1; cf. Wittman, “‘Not a God of Confusion, but of Peace,’” 158 (also see 158n33).
 Aquinas, ST Ia.11.4.
 Wittman, “‘Not a God of Confusion, but of Peace,’” 159.
 Ibid., 159. The following example is indebted to Wittman, “‘Not a God of Confusion, but of Peace,’” 159, where Wittman talks about the U.K. being an undivided whole with potential of divisibility.
 Despite the Pledge of Allegiance to the Flag: “one nation, indivisible. . . .”
 Aquinas, ST Ia.11.1.
 Wittman, “‘Not a God of Confusion, but of Peace,’” 154, 158.
 Aquinas, ST Ia.11.4; cf. Wittman, “‘Not a God of Confusion, but of Peace,’” 159.
 ST Ia.11.4, sed contra; quote of Bernard, De Consid. v.
 As Aquinas will quote Hilary (De Synod.): “The oneness of nature does not result from division, or from union or from community of possession, but from one nature being proper to both Father and Son” (ST Ia.39.2, ad. 6).
 Wittman, “‘Not a God of Confusion, but of Peace,’” 160.
 Aquinas, ST Ia.42.
 ST Ia.27.1. Procession, according to Thomas, is indeed a scriptural word providing a basis for a theology of the divine processions (ST Ia.27.1, sed contra; cf. John 8:42).
 ST Ia.28.1, sed contra
 ST Ia. 28.2–3, 29.4, 40.2; cf. Gilles Emery, “The Dignity of Being a Substance: Person, Subsistence, and Nature” in Nova et Vetera 9 no. 4 (2011): 997.
 ST Ia.28.1, sed contra; cf. Scott R. Swain, “Divine Trinity,” in Christian Dogmatics: Reformed Theology for the Church Catholic, ed. Michael Allen and Scott R. Swain (Grand Rapids: Baker Academic, 2016), 98.
 ST Ia.28.2, sed contra.
 Swain, “Divine Trinity,” 98.
 Aquinas, ST Ia.29.4.
 ST Ia.29.4.
 ST Ia.40.2, ad. 1.
 ST Ia.28–29.
 ST Ia.29.4.
 ST Ia.39.1.
 ST Ia.39.1.
 ST Ia.39.1.
 ST Ia.39.1.
 ST Ia.39.1, ad. 1.
 Gilles Emery, The Trinitarian Theology of St Thomas Aquinas trans. Francesca Aran Murphy (New York: Oxford University Press, 2010), 46.
 Gilles Emery, “Essentialism or Personalism in the Treatise on God in Saint Thomas Aquinas?,” in The Thomist 64 (2000): 528. This notion is not new to Thomas, but finds its roots even back to Basil of Caesarea (see Basil, Against Eunomius, 2.28).
 Aquinas, ST Ia.39.1.
 Emery, “Essentialism or Personalism in the Treatise on God in Saint Thomas Aquinas?,” 534.
 Ibid., 535. Emery will go on to say: “In treating of the divine essence, Thomas thus treats of what is fundamentally required in order to account for the person and for the esse of the relation in God, and therefore in order to elaborate what is the pinnacle of the divine persons: the subsisting relations” (ibid., 535–536).
 Ibid., 532.
 Ibid., 532.
 John Webster, God Without Measure: Working Paper in Christian Theology, vol. I God and the Works of God (London: Bloomsbury T&T Clark, 2016), 19.
 Ibid., 19.
 Ibid., 20.