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Thomas Aquinas and Natural Theology

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Natural theology has been used throughout history to prove the existence of God, mainly the Christian God. Natural theology appeals to reason and logic in order to prove God’s existence while staying within the theological boundaries of Scripture. It takes a more evidential approach to apologetics rather than a presuppositional approach. Also, it does not necessarily have to appeal to the Bible for arguments or authority. There have been many medieval philosophers that have developed and used natural theology in their apologetic of the Christian faith. Saint Thomas Aquinas is one of the more prominent of the medieval philosophers. Born in 1225 and growing up in Italy, Aquinas was heavily influenced by the Greek philosopher Aristotle and borrows many ideas from him to harmonize with Christianity. He borrows Aristotelian concepts such as potentially and actuality, and the doctrine of causes, and uses them to defend the concept of God (Saint). This paper will focus on and analyze Aquinas’ five proofs for God’s existence which are all taken from Summa Theologica. The five proofs can be summed up in the following categories: first mover argument, impossibility of an actual infinite, argument from contingency, God‘s ontology, and the teleological argument.

The first proof Aquinas presents is the first mover argument. Aquinas took this argument directly from Aristotle. He first starts off by presupposing that motion actually exists and is not an illusion as Parmenides and Zeno thought. His first premise to this argument is “whatever is in motion is put in motion by another, for nothing can be in motion except it is in potentiality to that towards which it is in motion; whereas a thing moves inasmuch as it is in act.” He explains this by saying that motion is basically just potentiality becoming actuality. Motion actualizes things which were potential. His second premise is, “it is therefore impossible that in the same respect and in the same way a thing should be both mover and moved, i.e. that it should move itself.” He says it is impossible for something to move itself because in order for something to be moved an outside movement must move it and this would be his third premise. His fourth premise explains that “this cannot go on to infinity, because then there would be no first mover, and, consequently, no other mover; seeing that subsequent movers move only inasmuch as they are put in motion by the first mover; as the staff moves only because it is put in motion by the hand.” This premise is an entire argument altogether which he will explain in further detail in his second proof for God’s existence. This premise is basically a debate between potential and actual infinites within time and how causes work. Because an infinite amount of motions cannot occur in a finite amount of time one must concede that there must be a finite amount of motions. Aquinas concludes this argument that God must be this unmoved mover of the universe.

This argument really hinges on the fourth premise. When Aquinas was alive, people believed that the universe was eternal and that it had no cause. They never conceded a beginning to the universe in any way. This makes this premise extremely difficult for Aquinas to really prove. In the second proof, he will show that this premise is true based on how efficient causes work and the properties of causes.

The second proof is an argument devoted to proving that fourth premise of Aquinas’ first mover argument. He talks about efficient causes and how they work exactly within time. Efficient causes cannot be infinite according to Aquinas. The first cause must produce the intermediate cause which will then produce the ultimate cause. If one takes away the cause then one would have to take away the effect as well. Aquinas continues by saying that if one takes the first of the efficient causes away, the intermediate and ultimate causes would not exist. He then says one has to concede that God is the first efficient cause of the universe. This argument though somewhat rests on the assumption that a first cause is really needed rather than an infinite regress of causes. But he shows the infinite regress to be absurd because no starting cause means no effects exist. Aquinas does know that the effect does exist though. The universe exists rather than nothingness. This question “why is there something rather than nothing?” is answered in the next proof Aquinas postulates for God’s existence.

In his third proof, Aquinas presents the argument from contingency. For his first premise he says that in nature humans find things which are contingent — things which could possibly be and possibly not be. If everything is possible not to be, then at one time nothing would have existed. But Aquinas knows this is utterly impossible not only because nothing cannot exist, but rather because out of nothing, nothing comes. He knows it is impossible to get something out of nothing without some type of cause. He says that there must be something that exists which exists necessarily, that is to say, not everything can exist contingently. Since he already proved that an infinite amount of efficient causes is impossible, the necessary cause must have caused the universe sometime in the finite past. This necessary cause is the explanation for the rest of existence and this cause must be God. Aquinas goes into further detail of the definition of this cause in his next proof.

The fourth proof is based on God‘s ontology. This fourth proof is simple and easy to understand. Aquinas says that God, by definition, must be the most maximally greatest being. This maximally great being must be completely perfect and is the explanation of everything in existence. God is also the source of all good, love, and any perfection. This argument is different from Saint Anselm’s because Aquinas never talks about conceiving of God existing in reality but rather he takes a more basic route and proves God by definition. This fourth proof is more of a support to the third proof and gives more of a definition to the necessary cause that he postulated.

The last proof Aquinas presents in his Summa Theologica is the teleological argument. He says in his first premise that humans see things that have a lack of intelligence act intelligently and get the best result. These things which lack inherent intelligence could be things of biological nature such as plants and animals. Such things do not necessarily have a will nor a mind yet they still work with a supernatural intelligence that is built into them. Aquinas talks about the final causes of such things — their purpose and end in life. He says that these non-intelligent things in the world have purposes which seem to come about intelligently. He makes an extremely interesting point when he says, “now whatever lacks intelligence cannot move towards an end, unless it be directed by some being endowed with knowledge and intelligence; as the arrow is shot to its mark by the archer.” This is really interesting because on a naturalistic worldview, where did intelligence even come from? He concludes by saying that this supreme intelligence directs the telos of everything in existence. It gives purpose and meaning to life. And he concludes that this intelligence must be God.

Aquinas’ five arguments are good and well constructed. Since all of these arguments are deductive rather than inductive arguments, it makes the arguments much harder to avoid if all the premises are true. His arguments are sound in their reasoning and logic. Thomas commits no fallacies in his reasoning. His arguments are good for showing the atheist or agnostic that a god does exist. Again, these five proofs were not as convincing at the time he was alive because of the worldview people held before the big bang theory emerged. People have always thought the universe to be eternal and uncaused and would thus make it harder for these arguments to really convince people regardless of the good rhetoric and true premises found in the proofs. Aquinas had to really appeal to an eternal universe rather than a finite one. Today, much better arguments have been formulated and built upon the groundwork that the Christian medieval philosophers created. For example, philosopher William Lane Craig has recently breathed new life into the kalam cosmological argument for God’s existence. This argument works much better under current modern astronomy which implies a beginning to the universe rather than a beginning less one. Also, Alvin Plantinga has formulated a much better version of the ontological argument than the one Aquinas presents as a proof. There’s a huge glaring problem in how Aquinas has argued for God’s existence. He never explained which God or gods should be accepted. This leaves a lot of his conclusions extremely open for more questions from the skeptic reading these arguments. It leaves the reader with a lot of questions that are not answered by Thomas. The aim of Christianity is not to prove the existence of some undefined being of the universe but rather the aim is to prove that the true God is the triune God found in scripture. Aquinas never shows arguments for the Christian God or the historicity of Jesus Christ. If Christianity is true, then people need to accept Christ as their Savior and not this deistic god Aquinas seems to imply at the conclusion of his arguments. Although this is a huge problem, Thomas Aquinas did help Christian philosophy, theology, and did prove the existence of some supernatural god. Both Catholics and Protestants have benefited from the proofs Thomas presented in the thirteenth century.

Works Cited

“Cosmological Argument.” Stanford Encyclopedia of Philosophy. Web. 02 Dec. 2009. <;.

Craig, William Lane. The Kalam Cosmological Argument. New York: Wipf & Stock, 2000. Print.

“Ontological Arguments.” Stanford Encyclopedia of Philosophy. Web. 02 Dec. 2009. <;.

“Saint Thomas Aquinas.” Stanford Encyclopedia of Philosophy. Web. 02 Dec. 2009.     <;.

“Summa Theologica.” Welcome to the Christian Classics Ethereal Library! | Christian     Classics Ethereal Library. Web. 12 Feb. 2010.     <;.


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