Existentialism
A primary goal of all philosophy is the proof of existence: the existence of God, the universe, and the self (see Colish, Macdonald-Cornford, Russell, Nahm, Pieper, Gilson). Although St. Thomas "five proofs" are the most referenced, they are targeted toward a Catholic audience. In Summa Contra Gentiles, Thomas provides a much lengthier and detailed proof that does not rely on divine revelation. God's existence is established based on the proposition that man's reason is capable of apprehending a cause from its effects. After analyzing motion, it becomes apparent that the first mover must be unmoved, without cause, and uncreated. From this, it then follows that the unmoved mover must be without change, and, therefore, must be eternal. The understanding that goodness and being are interchangeable leads to the realization that God is goodness. God is also found to be intelligent because he is not composed of matter nor is the form of any body. Since God is the first cause, His effects can be observed by the diversity found in creation, and the study of this is the study of Divine Providence. After building a foundation of proofs based upon natural reasoning, Aquinas then demonstrates that the Catholic Faith cannot be disproved and goes on to defend various positions against the Incarnation, the sacraments, and the Resurrection. This line of thought is also followed in the Summa Theologica and the Compendium of Theology.
A basic question of Greek Philosophy: Can an organized hierarchy (as is seen in nature) exist in the absence of an intelligence? In the atheistic view, the answer is "yes". If one answers "no", then there is a necessity of an intelligent God. The Greeks called this ultimate intelligence the "gnos" or "nous" (Nahm pp. 1–28). However, the intimate nature of this intelligence cannot be delineated by human contemplation. Many philosophers concluded that the Universe itself, was the God (Gilson). It was basically indifferent the needs of the human. This is very different from the Judeo-Christian concept of an intimate and loving God, aware of all.
Some Greek philosophers such as Zeno of Citium, Chrysippus, Plotinus, Thales of Miletus, Heraclitus, Parmenides, Socrates, Plato and Aristotle came to the conclusion that there had to be a God. Their conclusions were based on various physical observations: there could be no infinity of actions, therefore, there must be an unmoving mover, which moves without moving. This is the prime mover. There has to be a beginning and an end, otherwise, everything would have happened long ago and not now. This is also seen in Jewish philosophy (which may be based on Greek philosophy). There must be an ultimate smallness, of which there can be nothing smaller: the atomus (Leucippus, Democritus). This was in response to the Paradoxes of Zeno: there could be no motion, since every moving body would have to pass through half the distance, then one-quarter the distance, etc. A body could not pass through an infinite number of points. This led to the conclusion that all nature was united: the doctrine of the "One". Therefore, the human mind could reasonably conclude that there was a single God, and that this God existed (For an overview see Gilson p. 29-83; Russell pp. 453–463). This God had to be intelligent.
When Moses wished to know the name of God in order to reveal it to the Jewish people, he asked God directly. Exodus 3, 13-14: "'Lo! I go to the children of Israel, and I shall say to them: the God of your fathers sent me to you. If they ask me His name what shall I say to them?' God replied: 'I Am Who Am.' Then He added, 'Thus will you reply to the children of Israel: He Who Is sends me to you.'" Therefore, the proper name of God is "I Am" or "Who Is". This was a remarkable exposition on several levels. First, the idea that the great, unapproachable God of the Hebrews would provide His name was exceptional. Recall that in these ancient cultures, to know the name of a person was to possess them, or a part of them, in some way. Also, the name is unusual. What did this mean? (Gilson) The only means available to the early Christians for evaluating philosophy was provided by the Greeks. Largely, this was within the Stoic school, which had been re-invigorated by the Persians whom Alexander the Great had brought back to Greece. These in turn, influenced the Roman Stoic Schools which affected both pagan and Christian thought (Colish pp. 9, 12,21, 179-180, 300-301; Russell pp. 252–270). Christianity has been described as a religion without a need for a philosophic basis. The Stoics gave it one (Gilson, p. 84).
St. Augustine was convinced that the God of Exodus was Plato's being. He speculated that Plato must have known of Exodus. "But what makes me almost subscribe to the idea that Plato was not completely ignorant of the Old Testament is that when the Angel conveys the words of God to the holy man Moses, who asks the name of the one who is sending him to proceed to the deliverance of the Hebrew people, the reply is: 'I Am Who Am', and you are to say to the children of Israel: 'it is He Who is Who has sent me to you.' It was as if, in comparison with him who truly is because he is immovable, he who has been made movable did not exist. Now Plato was intensely convinced of this and he took great care to say so." (Augustine De Civ. Dei 8:11, PL 41,236). The Being in Exodus was the same immovable entity of Plato, according to Augustine. he called the immovable being of Plato, "The first and highest being is that which is entirely immovable, and which can say by full right: 'I Am Who Am'; and you will tell them, 'He Who Is has sent me to you.'" (St. Augustine, De doctrina christiana, I, 32,5; PL 34, 32).
St. Augustine had a very deep sense of the difficulty of the problem presented by this tract. He asked the question "I Am" what? From John 8, 24: "If you do not believe that I Am, you will die in your sins." But Augustine asked, "si non credideritis quia ego sum?" (I Am What?) There was nothing added. Augustine found this embarrassing. "We were waiting for Him to say what He was, and He says nothing." Augustine: "If you do not believe that I Am the Christ; if you do not believe that I Am the Son of God; if you do not believe that I Am the Word of the Father; if you do not believe that I Am the author of the world; if you do not believe that I Am the former and the reformer of man, his creator and recreator, He Who made him and remade him; if you do not believe that I Am that, you will die in your sins. This I Am, He says He Is, is embarrassing. Even if Moses could understand this, how could the people to whom he was being sent understand it? In point of fact, he did add: 'I Am the God of Abraham, and the God of Isaac, and the God of Jacob (Exodus 3, 13-15)(Augustine on the Gospel of St. John) (Gilson, p. 85)
This is the source of the doctrine of divine essentialitas, or essential theology of Augustine which would influence Richard of St. Victor, Alexander of Hales and St. Bonaventure. By this method, the essence of God is defined by what God is, and also by describing what God is not (negative theology). St. Thomas took the text of Exodus beyond the explanation of essential theology. He bridged the gap of understanding between the being of essence and the being of existence. In Summa Theologica, the way is prepared with the proofs for the existence of God. All that remained was to recognize the God of Exodus as having the nature of "Him Who is the supreme act of being". God is simple, there is no composition in God. In this regard, Aquinas relied on Boethius who in turn followed the path of Platonism, something Aquinas usually avoided. The conclusion was that the meaning of "I Am Who Am" is not an enigma to be answered, but the statement of the essence of God. This is the discovery of Aquinas: the essence of God is not described by negative analogy, but the "essence of God is to exist". This is the basis of "existential theology" and leads to what Gilson calls the first and only existential philosophy. In Latin, this is called "Haec Sublimis Veritas", "the sublime truth". The revealed essence of God is to exist, or in the words of Aquinas, I am the pure Act of Being. This has been described as the key to understanding Thomism. Thomism has been described (in terms of a philosophic movement), as either the emptiest, or the fullest of philosophies. (For a full discussion, see Gilson, pp. 84–95)
Read more about this topic: Thought Of Thomas Aquinas