Aquinas sets up this argument in his discussion of whether or not God exists. His five proofs set up the framework for much of his later writings in the Summa Theologica. As with the five proofs in their entirety, most of Aquinas' reasoning stems from the third proof concerning the existence of God. The first two proofs lead to the third's conclusion that God is "esse a se", or to be of itself. From this conclusion of God as an infinite being, Aquinas moves to the third question, concerning the simplicity of God. In article four of question three, Aquinas determines that God is ultimately simple in that his essence does not differ from his being. He writes, "Therefore, since in God there is no potentiality, it follows that in Him essence does not differ from being. Therefore, His essence is His being." God is an unchanging, infinite being. There is no conceivable way in which he could have parts, such as a separate being and a separate essence. From these proofs and others, Aquinas determines that God is an all knowing, perfectly good, perfectly powerful being. Moving back to the third proof of the existence of God, Aquinas determines that God is the ultimate being and that his existence precludes the existence of contingent beings. The notion entails the idea that without infinity, finite beings would not exist.
Aquinas also addresses the issue of the simplicity of God. From a series of logical steps, he concludes that God is altogether simple. He says, God is "neither a composition of quantitative parts, since He is not a body; nor composition of form and matter; nor does his nature differ from his suppositum." It only makes logical sense that God, not existing in any physical sense, could not have physical parts or even have any parts at all. I cannot physically separate my thoughts inside my head because they do not exist in space. With these conclusions in mind, Aquinas determines that God is completely simple.
From the conclusion that God is ultimately simple, Aquinas goes on in Question 14 to discuss the knowledge of God. In article three, Aquinas discusses whether or not God comprehends himself and he arrives at the idea that God does. Since God is altogether simple, then his intellect as well as his being are one and the same. Therefore, God must know himself perfectly. The intellect must perfectly comprehend all of the other elements of God. Through this concept, God must be all knowing because it is the nature of his being to do so. God's knowledge extends to contingent beings and everything else insofar as he is the first cause of all of them. The argument follows that if anything is perfectly known, then its power must necessarily be known as well. The conclusions are as follows: God must understand himself and understand all other things besides himself and that this understanding must not differ from his being.
Free will is a hotly debated concept. There are many plausible rejections to the notion that God gave human beings free will. For example, if God knows everything and everything that is to be, then are human beings really truly free to make their own decisions? Many other contradictions exists. The idea of a necessary being giving rise to contingent beings, the existence of a perfectly good being and evil in the world that being created, and the effects that human beings have on God...