Although Thomas argues that God's existence is not self-evident to us, he argues that his existence can indeed be proved through reason alone.
St. Paul tells us of God that “ever since the creation of the world God's invisible nature, namely, his eternal power and deity, has been clearly perceived in the things that have been made” (Rom. 1:20). The Catholic Church agrees. The First Vatican Council would declare ( as dogma ) that certain knowledge of God can be attained “through the light of reason,” and we read in our current Catechism of the Catholic Church that “by natural reason man can know God with certainty, on the basis of his works” (50). Thomas declares, “The existence of God can be proved in five ways.” (I like to think they are “as easy as 1-2-3”—not that they are so easy, but because we can find them in ST, I, Q. 2, a. 3!) Here Thomas tersely presents, in just a couple of pages, five proofs that he explicates in more detail in other writings, most notably his Summa Contra Gentiles.
Thomas begins with a couple of objections. One states that God's existence cannot be proved by reason because God is supposedly infinitely good, yet there is evil in the world (the age-old problem of evil). Another holds that “it is superfluous to suppose that what can be accounted for by a few principles has been produced by many.” (That's a concept prescient of Occam's razor, a principle elucidated by William of Occam, a Franciscan born about thirteen years after Thomas's death.) In brief, all of our reason relies on natural phenomena—what we can sense—but we can explain natural phenomena by nature itself, and therefore, we don't need to bring in God.
Thomas argues to the contrary that we can indeed prove God's existence through reason, and then he briefly lists five ways. Before I describe them in our next question, let's note two important characteristics about all of them.
First, they all employ a posteriori reasoning. That is, they start with the undeniable evidence of our human senses and then rigorously reason backward from these phenomena to their ultimate cause. In other words, they do not begin upon some abstract philosophical premise with which we may or may not agree. Second, many pagan philosophers believed that the universe itself was not created, but existed always, whether or not with God. Thomas believes that reason alone could not address whether the universe was created, though we know definitively through revelation that it was. Nonetheless, all five of his arguments are designed to prove God's existence even if the universe existed eternally with God. Even if the universe were not created in time, the universe could not be sustained in existence without the causal power of God! Further, there is no possibility that God created the world and then left it alone as he went about other business. All movement or change, causation, perfection, order, and purpose require a Prime Mover, first efficient cause, necessary being, ultimate formal cause, and final cause for their existence—not merely sometime in the past, but at this very moment.
“We live, and move, and have our being” (Acts. 17:28) right now through the grace, love, and power of an eternal God. Now let's look at the five proofs.
3 What are Thomas's five ways to prove the existence of God?
Please be aware that I present them here summarized in the most concise of nutshells. To examine them more deeply, please see ST, I, Q. 2, a. 3 and consult some of the writings of the greatest modern philosophical minds who have explained them in depth (Fr. Reginald Garrigou-Lagrange, Etienne Gilson, and Edward Feser being among my favorites).
Note as well that to fully grasp them requires a deep understanding of fundamental Aristotelean-Thomistic principles, including the distinction between potency and act (potentiality and actuality) and the nature of the four causes (material, formal, efficient, and final)—a comprehension I have never encountered in the writings of modern atheists who try to discredit the proofs. Now let's get ready to find God in five ways through the powers of reason he so graciously gave us.
A. The argument from motion or change: Our senses tell us with certainty that some things are in motion. They change, such as in their location or in qualities such as size and temperature. They move from some state of potentiality (what they could change into) to an actuality (what they actually are). Anything that moves from a potentiality to an actuality must be made actual by something else. A thing cannot give what it does not already have. These outside sources of change cannot go on indefinitely, though, so there must be a first agent of change, a first mover to put the series of changes in motion, as a stick moves something else only because it is put in motion by a hand. There must be a first, unmoved mover, already completely actualized and put in motion by no other. This mover we call God.
B. The argument from efficient cause: Our senses reveal an order of efficient causes in the natural world by examining the effects produced. A thing cannot cause itself, because then it would exist prior to itself, which is impossible. The chain of causation cannot go into infinity because without a first cause, no intermediate causes would exist, and to take away the cause is to take away the effect. But there clearly are effects. Therefore, there must be a first efficient, uncaused cause, and this we call God.
C. The argument from necessary being: We find in nature things that are possible to be or not to be—things that come to be, but pass away. (Who among us gave himself his own existence or will live on earth eternally?) If everything in the universe is possible not to be (that is, is contingent, being dependent on something else), at one time there could have been no existing thing. If that were true, there would be nothing now, because something that does not exist cannot give itself its own existence. There must therefore be some being that not merely possibly, but necessarily exists, having received its existence not from another thing, but which causes other things to exist. We call this necessary being God.
D. The argument from degrees of being: Everything that exists has some measure of goodness by the fact that it exists. (It is truly better to be than not to be!) Still, we clearly see that some things in the world are better than others. They are more good, noble, true, or complete. (Consider, for example, the increasing powers and abilities of living organisms as we move from plants to animals to human beings.) But there is no standard to appreciate degrees of perfection unless there is an unchanging maximum. There must be some utmost being against whom we measure the various goodness and perfections in every other being. This standard of perfection we call God.
E. The argument from the governance of the world (also known as the argument from final cause): There are an order and seemingly purposeful behavior even in inanimate natural bodies that follow the regular laws of nature. Although they lack awareness, they act in the same way, over and over again, in ways that achieve effective ends or goals. Unintelligent beings cannot reach specific goals unless directed by a being with intelligence, “as the arrow is shot to its mark by the archer. Therefore, some intelligent being exists by whom all natural things are directed to their end; and this being we call God.”
In essence, Thomas's five ways prove not that God well may exist, but that he absolutely must exist. Otherwise, the world brought to us by our senses, our own selves and senses included, simply could not exist.
For those who recall that Thomas started his article with objections against God's existence based on the existence of evil and the idea that nature needs no explanation, Thomas responds to the first with the argument from St. Augustine that since God is indeed the highest good, he would only permit evil to exist in his works if his omnipotence and goodness were such that he could bring goodness even out of evil. Thomas notes, “This is part of the infinite goodness of God, that he should allow evil to exist, and out of it produce good.”
As for nature explaining its own existence, Thomas responds that nature works for particular goals or ends only as determined by a higher agent (a being with the capacity to act), so whatever nature does can be traced back to God, the first cause. Indeed, even the voluntary acts of human beings can be traced back to a higher cause than human reason and will “since these can change and fail; for all things that are changeable and capable of defect must be traced back to an immovable and self-necessary first principle, as was shown in the body of the article.”
* Please note that Thomas used a biblical translation based on the Latin Vulgate, and the fathers of the English Dominican Province translated the Summa Theologica into English in 1911. Some of the wording of texts, names of Bible books (e.g., Ecclesiasticus or Ecclus. for Sirach and Apocalypse or Apoc. for Revelation), and numbering of verses differ from some modern English translations. For those who would like to track down the verses cited in the Summa, the English translation most similar to the Bible Thomas used is the Douay-Rheims edition, which translates the Latin Vulgate.
Kevin Vost, What Is God? : Answering the World's Most Important Question (with help from Thomas Aquinas) (pp. 36-37). Catholic Answers Press. Kindle Edition.