In part 1, question 3 of Summa Theologica, Thomas Aquinas delves into God's simplicity. We must, he asserts, examine God's mode of being from the perspective of what it is not, for our human minds cannot grasp what it is.
In article 1, Aquinas asks, “Is God a body?” He answers this question to the negative: God is not a body. God is the “unmoved mover,” and bodies must be moved by something else; therefore, God is not a body. There is nothing in potentiality in God, yet bodies are always in potentiality; therefore, God is not a body. A body cannot be “the most noble of beings” because it is living by virtue of a soul, yet God is “the most noble of beings”; therefore, God is not a body. Aquinas responds to objectives to his assertion by explaining that Scripture uses “likenesses drawn from corporeal things” to teach “spiritual and divine things,” but this is only analogical. It does not mean that God is actually a body.
In article 2, Aquinas asks, “Is there a composition of form and matter in God?” His answer is that there is no matter in God. Matter has potentially, but God does not. Further, matter has goodness only through its form, but God is good in Himself and not by participation. Finally, God is the first agent and first efficient cause. He is pure form and not matter. Again, Scripture speaks of God in analogical terms when it suggests that God has matter. He does not.
In article 3, Aquinas asks, “Is God the same as His essence or nature?” He responds that God is indeed “the same as His essence or nature” because God does not have matter, and matter is what distinguishes between individuals and essence. God is simply “His own divinity” and “His own life.” Our minds have difficulty grasping this, so we speak in terms we can understand. Also, God's effects exhibit composition, but God does not.
In article 4, Aquinas asks, “Is God's essence the same as His esse?” He replies to the affirmative. God is the “first efficient cause” and is, therefore, the esse that causes all things. Further, there is no potentiality or participation in God, so His essence and esse must be the same. Sometimes terms like esse can be taken in two different ways, so we must be precise in how we apply it to God.
In article 5, Aquinas asks, “Is God in a genus?” He responds that God is not in a genus because being in a genus suggests something prior, and there is nothing prior to God. God is being, and being cannot be in a genus. Further, things in a genus differ in esse, and God's essence is the same as His esse.
In article 6, Aquinas asks, “Are there any accidents in God?” He replies that there are not, for accidents are only in subjects, and God is not a subject that is related to accidents by potentiality. There is no potentiality in God. He is His own esse. He simply is.
In article 7, Aquinas asks, “Is God altogether simple?” He answers that God is indeed “altogether simple.” There is no composition of form and matter in God. He is not a body. He is the first being and the first cause. He has no potentiality as composite things do. As such, He is perfectly simple, and this is a higher state than composition.
In article 8, Aquinas asks, “Does God enter into composition with other things?” He responds that God does not do so. God is the cause of all things; He can not also be an effect. He is also “first among beings” and acts primarily in Himself.