Augustine and Aquinas attempted to prove the existence of God through different approaches, but their arguments were faith-based and, by modern standards of rationality, are more a means to faith than proof.

Aurelius Augustine’s proof of God’s existence

Augustine’s view, summarized by the phrase “believe in order to understand,” aimed to make theology the ultimate goal of all scholarship. A native of North Africa, he traveled through numerous pagan religions and religious controversies on his way to becoming a Christian bishop. In doing so, he was able to combine Neoplatonism with faith and establish the primacy of faith over reason without separating the two.
Augustine’s proofs for the existence of God are twofold, with the latter playing a supporting role to the former, and the former being the centerpiece of his argument. This stems from his aforementioned prioritization of faith over reason.
Augustine’s first proof of God’s existence is from eternal truth and reason. It centers on the premise that God’s existence can be found within human beings, not outside of them, and centers on the premise that there are certain necessary and immutable truths and that the human mind grasps these necessary and immutable truths.
Augustine builds his proof of God’s existence on these two premises. First, he argues that “inevitable and immutable” truths must, by their very nature, be unaffected by anything. The second premise is that the human mind has the capacity to “grasp” these truths, and therefore the human mind must accept them as they are. Since truth is immutable and the human mind has the capacity to grasp it, the conclusion that follows is that the human mind accepts truth as it is. Therefore, truth is a higher concept that cannot be changed by the human mind, and the human mind cannot modify it. Rather, the human mind can only be fixed by truth.
How, then, can the human mind realize truth? Augustine argued for ‘illumination’. Illumination means that God illuminates the truth so that humans can see it. Augustine believed that God reveals truth to humans, and humans can then use reason as a tool to recognize that truth more clearly.
Thus, Augustine’s proof of God’s existence does not prove God as an object of the senses. Rather, it might be more appropriate to say that it “cannot” be proven by the senses. In any case, Augustine sees God as a being beyond time and space and as the source of truth, and as this necessary and immutable source of truth, God is a concept that precedes the human mind, thought, and reason.
Now let’s look at Augustine’s second proof of God’s existence. This is called the proof from creatures and common consent. In his De Deologia, Augustine argues that the world and its “creatures,” represented by the movement, order, change, and arrangement of all things in it, were created by God. His argument is that the natural order of the world is a great work that cannot be produced by humans, and that only God can produce such results. Therefore, Augustine argues that humans can feel the breath of God here. This is a “cosmological” and “natural theological” argument.
However, the reference to “creatures” in his second proof of God’s existence can be understood less as a logical proof and more as a way of reasserting the inevitability of God’s existence to Christians who already believe in God.
In his second proof of God’s existence, the proof of God’s existence by general consent, he argues that humans, as rational creatures, cannot help but feel the existence of God if they use their reason. In this respect, he argues, humanity already recognizes the existence of God: some may worship many different gods, but the God Augustine speaks of is nonetheless the God of Gods.
However, this proof is not a proof of the existence of God itself, as it is merely an assertion that God’s existence is a given, assuming that the world is the result of God’s activity. As such, Augustine’s proof of God’s existence is not an academic or rational proof of God’s existence, but rather an aid to remind us of God’s existence. His proof of God’s existence is fundamentally religious.

Thomas Aquinas’s proof of God’s existence

Unlike Augustine, Aquinas wanted to first explore the truths that the human mind can grasp and whether God’s existence is self-evident through them. This is in contrast to Augustine’s position in the Eternal Truths and Proofs from Reason, where he believed that God’s existence is naturally grasped by the human mind. If God’s existence were self-evident, it would not need to be proven. However, according to Aquinas, since humans cannot intuit the existence of God, his existence must be proven.
Aquinas proposed two ideas to prove God’s existence. The first is the “cause” method, and the second is the “effect” method. However, since Aquinas was a theology professor, his belief in God’s existence was already firmly established, so for him, God was already a concept of absolute immutability. This makes the first method of proof through cause unusable, because the “absolute immutability” of God cannot have a “cause”. For example, if the moon is the cause of the tilt of the earth’s axis of rotation, then the earth’s axis of rotation cannot be ‘absolutely immutable’. If the Earth’s axis of rotation is tilted by the Moon, it has already changed. If the Earth’s axis of rotation were an ‘absolute constant’, it would remain the same even if the Sun were next to the Earth. This is the relationship between ’cause’ and ‘absolute immutability’. Therefore, it becomes impossible to prove the existence of God by ’cause’ because of the nature of God, which is already presupposed to be ‘absolutely immutable’.
In the end, Aquinas uses the method of proof through effects to prove the existence of God. It is important to note that Aquinas was heavily influenced by Aristotle. Borrowing from Aristotle’s ideas of a posteriori and empiricism, Aquinas argued that God’s existence is not a priori, known by God himself, but a posteriori, known by humans. Thus, Aquinas sought to prove God’s existence through the arrangement, order, etc. of the sensory world.
Aquinas argued that the proof of God’s existence through “consequences” can be demonstrated in five ways. Whereas in Augustine’s proof of God’s existence, the latter was developed as a complement to the former, in Aquinas’ proof of God’s existence, each method proves God’s existence independently.
The first method starts with the sensory facts of motion in the universe. We can see all things in the world moving. And we can see that they cannot move by themselves, and that each movement has a cause: as I am moved by you, so you are moved by that person, who is moved by another person, who is moved by another person, who is moved by another person, who is moved by another person, and so on. Here, we see that the first cause of motion, reasoning with reason, was itself moving. This first cause is the first mover, which is God. Therefore, God exists.
The second way is to deduce the first cause by cause and effect. This is essentially the same as the first method. Aquinas argues that every effect in the universe has an “agent,” i.e., everything in the world has a cause in its birth, which must have a supreme “agent,” otherwise we would have to search for the cause indefinitely higher up. Therefore, the supreme “agent” must be God. Therefore, God exists.
The third way is to deduce necessary existence from contingent existence. All beings in nature are born and die. It is an endless cycle, and they are all contingent. Aquinas says that these contingent beings are brought into being by a necessary being. If there were no necessary being, there would be no natural beings that come and go by chance. Therefore, if the process of natural beings coming and going by chance continues, it follows that the necessary being continues to exist. Aquinas sees this necessary being as God.
The fourth way concerns the concept of the value and perfection of things. This is reminiscent of Plato’s Idea of the world. Each thing exists in degrees, even if it is the same thing. For example, a woman may be the most beautiful woman in a local district in the eyes of the locals, but there may be a more beautiful woman on a national level. In this case, the nationally recognized beauty is more beautiful than the locally recognized beauty. Similarly, all things in the universe exist with different grades of perfection. So we can imagine something with the highest degree of perfection. Aquinas saw God as the highest kind of unrestricted, all-encompassing perfection.
The fifth way has to do with the purpose of everything in the universe. This has to do with the order of the world. All beings in the universe move toward their proper purpose. Even unintelligent things exist within this purpose. Aquinas asks how this can be explained if it is not planned, so a higher being with intelligence is naturally deduced, and this supreme intelligence can be seen as God.
Aquinas was a theology professor, so he basically thought that the things in the empirical world are dependent on God, so his proof of God’s existence starts with the things in the world that we experience and then moves on to the transcendent. This makes him an empiricist, but not a modern empiricist.

Comparing Augustine’s and Aquinas’ proofs of God’s existence

Although Augustine and Aquinas lived about 1,000 years apart, they shared the same worldview: Christianity. In Christianity, God was the answer to the world’s beginning, the cause of its course, and the hint of its end. Under Christianity, Augustine and Aquinas were working toward the same goal: the connection between God and humanity. As such, they were no different in their attempts to prove the existence of God, which is what any Christian would do.
However, the time span of about 1,000 years is not a short one, and Augustine and Aquinas differed in their activities, ideological backgrounds, and the relationship between faith and reason. Augustine was heavily influenced by Platonic philosophy, while Aquinas drew heavily on the ideas of Aristotle; Augustine came to Christianity through a series of journeys, while Aquinas spent his life as a theology professor learning and practicing Christianity. These differences led to many differences in their ideas about God and, by extension, their views on the proof of God’s existence, even though they were both great Christians.
The most notable difference between their proofs of God’s existence is the difference in approach. Augustine’s proof of God’s existence was a priori and ontological, using human reason with an emphasis on faith. Aquinas’s proof of God’s existence is a posteriori and cosmological, emphasizing human experience. This shift in the way God is proved over the course of about 1,000 years means that the center of the proof has shifted from faith to reason. It also indicates that the starting point of proof for God has shifted from reason to experience.

Conclusion

Proof presupposes rationality, but faith goes beyond rationality, so from this perspective, Augustine’s and Aquinas’ proofs of God’s existence are not strictly speaking proofs, but rather a means to faith. Therefore, we must adopt a medieval perspective when discussing their proofs of God’s existence.
Both Augustine and Aquinas genuinely believed in God; they must have genuinely wanted God to exist before they could prove it. Nevertheless, the reason I find their proofs of God’s existence meaningful is that there is a slight modern shift between them.
Augustine established the existence of God in terms of the inevitable and immutable truths that override human reason, and in terms of the harmony of the world that is beyond human understanding. For him, God was a matter of course. Denial of God’s existence would have led him to ask, “Then where does all this truth, this harmony, come from?”
His proof of God’s existence, however, is questionable. His “illumination theory” is the source of that doubt. In my opinion, the illumination theory is an irrational hypothesis. The idea that God only reveals truths to humans that are barely perceptible through reason is illogical. This makes it difficult for me to accept Augustine’s proof of God’s existence.
Aquinas, on the other hand, seems to go a step further than Augustine, because he presents a much more logical proof than Augustine. He grounded his proofs in the practical experience of human beings, presented God as the first cause of almost every movement, and argued for God as the first agent. His conclusions and proofs are heavily influenced by Augustine, but they are also characterized by an effort to find clues in human experience. But it seems that he was not a modern man.
It is 2019, and the time gap between Augustine and Aquinas is as wide as the time gap between Aquinas and the present. Here’s an interesting idea for how to prove the existence of God from a modern perspective. Elon Musk, the CEO of the American company SpaceX, and some scientists argue for the possibility that this world is a virtual world. In particular, Elon Musk believes that it’s millions of times more likely that this world is a virtual world than that it’s real.
The idea comes from the fact that the game graphics we see on our computers resemble the real world. In a realistic game world, everything outside the player’s field of view freezes to reduce the load on the computer. However, when you turn around, everything there appears to move as if it had been moving before. This is very similar to the phenomenon of quantum entanglement in our world. It’s from this commonality that the idea that this world might be virtual was born.
Considering this, it’s possible that the “God” that medieval theologians claimed was a programmer or engineer from beyond who created this world. This may seem far-fetched, but how is this idea inherently different from their version of God? It’s all the same: it’s unverifiable. In fact, it’s possible that the virtual world hypothesis will be verified sooner.
Being non-religious in the first place, and having an antipathy towards religion due to my personal upbringing, I find it difficult to accept ‘Western medieval philosophy’ because of the medieval worldview based on ‘God’. So the most important thing for me was their answers to the question of whether the God they were talking about really existed. I needed to know if the God they were talking about really existed, and how they presupposed it. So I read the proofs of the existence of God by Augustine and Thomas Aquinas, who are revered as saints among Christians.
As a modern person, I found Augustine’s and Aquinas’ proofs of the existence of God very disappointing. It felt blind to me. Someone once said, “We can’t know that there is a God, but we can’t know that there isn’t a God.” Then we shouldn’t reject it as if it doesn’t exist, and we shouldn’t believe it as if it doesn’t exist. But we either believe or reject it. I don’t think “God” is a concept that should be treated that way. I think it should be left as it is and not given much meaning, and that my life should be my own.