Updated: Aug 16, 2021
By Luke Lancaster
St. Thomas Aquinas teaches that there are five “ways” of knowing God. This is covered in part one, question two, of his great Summa Theologica. His arguments are from motion, causality, perfection, contingency, and design. These arguments do not “prove” God’s existence, but rather are “a posteriori” arguments, meaning that they look at effects and reason back to the causality. This is done because effects in the world are better known than “the existence of its proper cause.” For example, the existence of the neighbor's dog is an effect that comes from the dog's parents. So it is “from the effect [that] we proceed to the knowledge of the cause.” God’s existence is seen through what He has effected in the world. The five ways of Aquinas have a long history in Christianity, with nearly every theologian encountering them at some point. Here they are.
His first way is from motion. He states that everything in this created world would have had to be put in motion by another who already was in motion. Motion is moving from “potentiality to actuality.” Look at fire. It is in motion, in act. Wood, on the other hand, is in the potential stage, ready to be heated. It is not in motion until the fire touches it and puts it in motion. So something that is pure act, pure motion, which was unmoved, would have put this world into motion. This motion does not go back infinitely, as if there would be no beginning, for there would need to be a first mover to get the dominoes going. This mover Christians call God.
The second way is from efficient causality. Similar to motion, it has to do with a first cause of reality. There is always a first cause, which moves on to the intermediate cause(s), then to the ultimate cause. Reality follows that process. Now, some claim that there is no first cause to the world, but rather there are an infinity of causes. However, if there was no first cause, then there would be no effect. Everything in this world is an effect based on the way humans experience the world. So a cause would be fitting, which we would call God. According to science, the universe began out of nothing 13.8 billion years ago during the big bang. This lends support for Aquinas's proof for God.
The third way is from possibility and necessity. If something is not necessary to exist, then it is in potentiality to exist. Something’s existence is “possible to be and not to be.” It could or could not exist. If everything was just possible to be, then there could have been nothing in existence. If that happened, then there would be nothing in existence today, for there would not be anything necessary to exist. There needs to be “some being having of itself its own necessity, and not receiving it from another, but rather causing in others their necessity.” This is what we give the name “God” too.
Skipping the fourth one, the fifth way Aquinas used was the argument of design. The world has order and an appearance that someone fashioned it just right. Things have a natural end in sight, like the air giving oxygen to humans. It is as if an archer shot an arrow towards a distinct goal or mark. Something caused strawberries to exist and directed them towards the goal of being full and tasty. That is what we would call God.