Can we definitively prove the existence of God? St. Thomas Aquinas thought so, and using his profound intellectual ability, his knowledge of theology, and his masterful command of philosophy, he proceeded to do just that. And he did it not just in one way, but in five ways!
Since I want to concentrate primarily on St. Thomas’s first proof of the existence of God, I won’t say much about the life of Saint Thomas except to give a brief introduction for those who may not be familiar with him.
St. Thomas Aquinas (1225-1274) was a Dominican friar born into a wealthy Italian family in the 13th century. He was sent to study at the Benedictine monastery of Monte Cassino from about the age of five and later continued his studies at the University of Naples. It was there that he encountered the philosophy of Aristotle. Against the wishes of his family, he took the vows of the Dominicans, who were, and still are, committed to obedience, chastity, and poverty. He is known for his devoutness and is famous in the history of Western thought for harmonizing faith and reason, especially through works like his Summa Theologiae, in which he presented his proofs for the existence of God.
St. Thomas’s proofs of the existence of God appear in Question 2, Article 3 of the Prima Pars (First Part) of his lengthy Summa, which includes hundreds Questions, each featuring multiple articles.
In Article 2 of Question 2, before he gives his proofs, he examines the question of whether God’s existence is self-evident to us and discusses how our knowledge of things can proceed from causes to draw conclusions about their effects, or from effects to make determinations about causes. God, being the creator and sustainer of all things, is the first and ultimate cause. We cannot know God as he is in his essence, since man cannot fully conceive of the infinite, but we do have direct experience of the things God has made—His effects—so we can look at those things and come to some conclusions about God. And one of the first questions we might ask is whether He exists.
In Article 3 of the same question, he goes into his proofs of God’s existence, and the first and most famous of these is his argument from motion. He says that it is clear to our senses that some things are “in motion”. By this, he means simply that things change. They change position, they change appearance, they decay, they go in and out of existence, etc. All this he calls motion. And he says it is evident that nothing has ever set itself in motion. It is put in motion by something else. In other words, something causes it to change. And insofar as it is ceasing to be in one state and coming to be in another state, it is always “in potentiality” to that which it is becoming. A thing that is here is always potentially there; a thing that is young is always potentially old, etc.
As a thing transitions from one state to another, it is moving from being potentially in a particular state to actually being in that state, e.g., the young man goes from potentially being old to actually being old, or the kettle on the stove goes from being potentially hot to actually hot. Therefore, motion of any kind is a movement from potentiality to actuality. He then goes on to say that nothing can be made to move from a state of potentiality to actuality, except by something that is already actually in that state. Although St. Thomas uses the case of fire and wood, I’ll take the similar kettle example I gave above. The kettle is potentially hot, and the only way it can be made actually hot is by something that is already actually hot, namely by fire. Likewise, a person cannot be given life and proceed to grow except by already living, growing persons.
St. Thomas goes on to emphasize that a thing cannot be both the mover and the thing moved. In other words, the kettle cannot heat itself. So we observe that absolutely everything that is in motion is put in motion by something else. And so we work our way further and further back from the things we see in motion, and each of them has a prior thing that put it in motion. But this cannot regress infinitely, otherwise there would be no motion now. So there must be a first and original mover. St. Thomas concludes the proof by saying, “Therefore it is necessary to arrive at a first mover, put in motion by no other; and this everyone understands to be God.”
In the Summa, the proof is much more succinct and elegant than I make it out to be here. St. Thomas draws largely on the earlier work of the philosopher Aristotle, and it is these two figures—Aristotle and St. Thomas Aquinas—that have made famous the concept of God as the “unmoved mover.”
Granted, we cannot conclude just from this one proof what the full character of God is; for that, we need divine revelation. But this proof aims to demonstrate philosophically that God, the unmoved mover, does exist. It is now up to us to further inquire into what has been revealed about who this God might be.