The Proof of God's Existence in Summa Theologica, a Book by Thomas Aquinas

In his work Summa Theologica, the 13th-century religious philosopher Saint Thomas Aquinas proposes his proof of the existence of God. He called this theory the Cosmological Argument. (Reichenbach 4.1) This argument consists of five distinct ways in which Aquinas proves the existence of the highest being who has the characteristics most would hold of a God.

One of the most controversial of these ways is the Uncaused Cause Argument, which is his statement of God’s existence as the “first cause” of the universe.

Aquinas holds that if all things must be caused by another thing, then there must be the first creation that everything in the universe can be traced back to; this first creation and subsequent creator is God.

The first premise of the Uncaused Cause Argument states that things that exist are caused by other things; for instance, a chicken is caused by an egg. The next premise is that nothing can cause itself; an individual chicken cannot bring itself into existence.

The third premise is that there cannot be infinitely many things causing other things to exist, which leads to the conclusion that, if all of the first three premises are true, there must be one first uncaused cause. One can conclude that this first cause thatch that begins the series is called God; simply, once God exists, all else in the universe can follow, as God creates it.

Noting that a priori arguments are based on premises that rely entirely on reasoning and that a posteriori arguments are based on at least one premise which is a natural observation, this argument is a posteriori.

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All three of the first premises are observations that can be studied in the world; for instance, one concludes that there cannot be infinitely many things causing others to exist because one finds in nature that a series must have a beginning, as in a first egg from which a chicken hatches, beginning the cycle. Additionally, Aquinas’ Uncaused Cause argument is deductive, as it begins with premises that provide evidence for the conclusion to be true. If Aquinas’s rationalization was inductive, his premises would lead to the conclusion that iGod likely exists however, since Aquinas is guaranteeing that God exists, based on the truth of his premises, his argument is deductive. In Hume’s Dialogues Concerning Natural Religion, Hume presents his views on the existence of God through conversations between characters. One of the characters, named Cleanthes, proposes one version of the Design Argument, in which he proves the existence of God by comparing the universe to human creations. First, Cleanthes states that objects in the natural universe bear resemblance to objects created by man, followed by the statement that it’s likely similar effects likelyhave usess. Cleanthes concludes that thus, the cause of the natural universe must be similar to the cause of a manmade object. Then, Cleanthes asserts that the creator of a manmade object is an intelligent designer; thus, the natural universe is also created by an intelligent designer.

Cleanthes’ Design Argument is a posteriori, as it coconcludensists of premises that are observed in the natural world, rather than those that are purely reasoned. One can observe the premise that natural objects are similar to human artifacts; one can see this in the natural world when they compare the design of a pump to the hrt, the design of a lake to a pool, and so on. In addition, the Design Argument is deductive because, if the premises are accepted as true, it definitively proves that a God must exist to design the universe.

Both the Uncaused Cause Argument and the Design Argument are insufficient proofs of the existence of God. Perhaps one might consider the Uncaused Cause Argument to be superior, at least in that it is not immediately disproved by the philosophy who proposed it. However, Aquinas’ argument has faced a multitude of objections, from several different philosophers over time. Concerning the first premise, which assumes that a series of causes must exist, some such as Bertrand Russell propose that the universe does not need to have come about from a cause, but rather that the universe “just is”. (Reichenbach 4.2) Here, Aquinas assumed that just because things that exist in the universe have a cause, the universe must also have a cause; this assumption is faulty, as one cannot conclude something based on specific details within its composition. (Reichenbach 4.3) Hume even proposes that since one can conceive of an uncaused event, merely by reasoning, and since the conceivable can exist in reality, then one need not assume that the universe demands a cause in the form of a God. (Reichenbach 4.4)

Overall, Aquinas’ argument relies upon weak premises which make great, unprovable assumptions. Much of this weakness seems to lie in their nature as posterior statements.

Because the premises are supposed to be able to be observed in nature, the reasoning fails when one cannot say with complete confidence that an object demands a cause or that there cannot exist an infinite series of causes. Though an immediate rebuttal of these might not come to mind, upon considering them, one can offer instances in which they do not hold, such as Hume’s reference to a conceivable uncaused event and other objections.

Where does this leave the Design Argument? Similar to Aquinas’, Hume’s reasoning can be met with just as many, if not more objections. First, Philo’s parody of the Design Argument must be considered. In Dialogues Concerning Natural Religion, Hume follows Cleanthes’ proposal of the Design Argument with another character, named Philo, objecting to Cleanthes’ reasoning in the form of a parody. To summarize his version of the Design Argument, Philo suggests that, as natural things in the universe are similar to vegetables, and vegetables are caused by natural organic growth, the universe must have been caused by a similar naturally occurring process. (Feinberg) This is absurd; most would object to the idea that the universe was created the same way a carrot is planted and grown. Yet, Philo’s parody relies on the same reasoning as Cleanthes’ argument, hinging on the belief that similar effects demand similar causes. Hume goes on, through the character Philo, to note that human artifacts and the universe differ greatly in their designs. While one can observe consistent arrangements in both manmade things and the universe, there also exist vastly different levels of design in both of these. (Russell 4) For instance, the design of a bound book or a vase differs immensely from the organization of the oceans or the orbital placement of the planets. The argument fails here in that it cannot soundly bridge the gap between human artifacts and the design of the natural universe.

Furthermore, Hume notes the difference between what he calls “the human and the divine mind”. (Russell 4) Why should one assume that the intelligence of a divine creator called God is similar to the intelligence of a human being? Even if one can accept that the universe reflects intelligent design, they must realize that they are making a judgment based on human intelligence, as that is all they can understand as a human. Divine intelligence might materialize entirely differently from that of humans, and divine design might not even be recognizable by a mortal eye. This objection draws upon the commonly held definition of God as a higher, omnipotent being, which supporters of the Design Argument are likely to hold. Thus, if one holds that God exists on an entirely different plane of action than a human, one must confront the idea that God’s intelligence would likely manifest in creation completely differently than a human’s reasoning.

After studying Aquinas’ and Hume’s proofs, I feel that neither argument can successfully prove the existence of God. Based on the critiques of both proofs, and personal consideration, I find the Uncaused Cause Argument to be more persuasive than the Design Argument since the latter is so easily disproved by Hume in his work. However, none can deny the significance of both of these arguments in the history of religious philosophy; even today, there exists no universally accepted counterargument to either theory.

Works Cited

  1. Reichenbach, Bruce, “Cosmological Argument”, The Stanford Encyclopedia of Philosophy (Winter 2016 Edition), Edward N. Zalta (ed.), URL =<>.
  2. Russell, Paul and Kraal, And, er, “Hume on Religion”, The Stanford Encyclopedia of Philosophy (Summer 2017 Edition), Edward N. Zalta (ed.), URL =< 2017/entries/hume-religion/>.
  3. Feinberg, Joel, and Russ Shafer-Landau. “Dialogues Concerning Natural Religion.” Reason and Responsibility: Readings in Some Basic Problems of Philosophy, 15th ed., Wadsworth Publishing, 2013, p. 77.

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The Proof of God's Existence in Summa Theologica, a Book by Thomas Aquinas. (2022, Jun 17). Retrieved from

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