The doctrine that God is absolutely simple derives from the metaphysical considerations that God is a being whose existence is self-explanatory, absolutely perfect, and pure actuality. Prior to Thomas, the doctrine has its most influential formulations in Augustine and Anselm.
According to Thomas, God is his essence and his existence. If the existence of a thing differs from its essence, this existence must be caused either by some exterior agent or by its essential properties. The latter seems to be impossible; for nothing, if caused to exist, can be the sufficient and efficient cause of its own existence. Nothing can be self-caused and thus the latter option is insufficient. Therefore, if existence differs from essence then another being must cause existence. This option is also an insufficient explanation for God’s essence and existence because another being cannot cause God because he is the first efficient cause—the uncaused cause.
There are three important claims Thomas commits to concerning the doctrine of divine simplicity.
(1) It is impossible that God have any spatial or temporal parts that could be distinguished from one another as here rather than there or as now rather than then, and so God cannot be a physical entity.
(2) It is impossible that God have any accidental properties.
(3) All of God’s intrinsic properties must be essential to him, it must be acknowledge that whatever can be intrinsically attributed to God must in reality just be the unity that is his essence.
The first claim, (1), removes God from having any spacetime properties. God is completely timeless logically prior and posterior to the moment of creation. From this timelessness it follows that God is absolutely immutable and eternal, which are all entailed from simplicity. The immutability that Thomas is advocating functions with respect to God’s intrinsic esse. If God were to be able to change intrinsically, that would suggest that God’s goodness and omnipotence could change. An extrinsic change may certainly be compatible with Thomas’ notion of immutability. If God were to apply salvation to agent X then God has undergone an extrinsic change in the sense that agent X was once an enemy of God prior to salvation whereas post-salvation agent X is now a friend of God. This is a relational change that has no effect on the intrinsic esse of God. Thomas would argue that all creatures are really sustained, known, and loved by God, but God would be the same whether creatures existed or not. However, it is difficult to reconcile God’s genuine relationship with contingent beings if this modal distinction is permitted. If it is the case that no modal distinction is possible and that modal collapse is a byproduct of simplicity then God really does stand in genuine relations to created beings and creation since it is not the case that what exists could not have not existed. Thus, God does not really undergo an extrinsic change in creating the world. He just exists; creation and creatures come into existence with a real relation to God by being caused by God. This simply makes extrinsic change superfluous to God.
Concerning (2), Thomas states that there can be no accident in God because a subject is related to an accident as potentiality to actuality, for with regard to an accident a subject is in actuality in a certain respect. However, being in potentiality is inapplicable to God. From (2) it follows that (3); God is his own esse or nature. According to Thomas, it is impossible that there be any real distinction between one essential property and another in God or between God and his nature. For all things other than God, there is a difference between what they are and that they are, between their essence and their existence, but concerning simplicity the essence which is God is not different from God’s existence. Therefore, unlike all other entities, God is his own being.
Thomas makes the distinction between what a word predicates (id quod predicat) and that from which the word is taken (id a quo imponitur ad significandum). When we discuss words describing God, they are not predications but identity claims. Aspects of God such as omnipotence, omniscience, aseity, and goodness are all identical. God is goodness and God is omnipotence. This does not violate the law of non-contradiction either. The two subjects (i.e. God and goodness) are of the same ontological essence or substance. We may certainly index these aspects for a distinction of God’s being, but each indexical does not suggest a part of the whole being. To consider the contrary, non-simplistic, each indexical would not be identity claims; rather, they would be predications. Goodness and omnipotence would be predications to the subject of God. In other words, God would possess the property of goodness and omnipotence. These predications are not identical with God; hence the distinction in identity claims and predications within the doctrine of simplicity.
 Augustine, De Trinitate VI, 7-8; Anselm, Monologion XVII.
 For whether or not God exists pertaining to this present discussion of existence and essence Thomas refers to Summa Theologica 1.2.2.
 ST 1.3.4.
 Eleanore Stump, Aquinas (New York: Routledge, 2003), 96-97.
 ST 1.3.1-2.
 ST 1.10.1; 9.1.
 This is not to say that omnipotence does not function modally. It may certainly be the case that God could perform action A in W1 but in W2 God could not do A since the state of affairs, which logically precedes A changes. This is an example of an extrinsic change—God’s relation to contingencies and accidental beings and states of affairs. However, as I will discuss later, simplicity does not really permit any extrinsic change and the notion is superfluous given simplicity.
 William Lane Craig, Time and Eternity (Wheaton, IL: Crossway, 2001), 88. There would seem to be genuine relations if the modal collapse is true.
 ST 1.3.6.
 ST 1.3.3.
 We can use an argument for God’s existence from his essence. If we posit that all contingently exemplified essences must originate from something, then we might get the idea of an essence that does not itself originate from anywhere, an essence that is necessarily exemplified, so that the contingently exemplified essences get their ontology from at least one necessarily exemplified essence or from the exemplifier of such an essence. This goes to show that there must be one necessary essence from a necessarily existent being. Alexander Pruss, “The Leibnizian Cosmological Argument,” in The Blackwell Companion to Natural Theology, eds. William Lane Craig and J.P. Moreland (Oxford: Blackwell, 2009), 43.
 Stump, 97.
 Thomas, De Potentia, 7.4.