According to the Apostle Paul, “For what can be known about God is plain to them, because God has shown it to them. For his invisible attributes, namely, his eternal power and divine nature, have been clearly perceived, ever since the creation of the world, in the things that have been made” (Romans 1:19-20). This argument by the Apostle Paul is defended by Aquinas in many of his writings. His best known arguments for the existence of God are his Five Ways that Aquinas offers “near the beginning of his Summa theologiae to establish the existence of God.” The Five Ways “amounts to only a few paragraphs” in the Summa theologiae. Many readers might think the Five Ways exhaust his arguments for the existence of God. This is definitely not true; in fact, arguments for the existence of God are located in many of his works in a more extensive format. (Summa Contra Gentiles, Existence and Existent) Edward Feser, speaking on the topic of the Five Ways asserts, “But it is crucial to understand that they are summaries. Aquinas never intended for them to stand alone, and would probably have reacted with horror if told that future generations of students would be studying them in isolation, removed from their immediate context in the Summa Theologiae and the larger context of his work as a whole.” It is important to remember that the Five Ways takes up space of a few pages in a work that is more than one thousand pages. Second, it is in a book written for beginners in theology.
2 The First Way
The first and more manifest way is the argument from motion. It is certain, and evident to our senses,
that in the world some things are in motion. Now whatever is in motion is put in motion
by another, for nothing can be in motion except it is in potentiality to that towards which
it is in motion; whereas a thing moves inasmuch as it is in act. For motion is nothing else than the
reduction of something from potentiality to actuality, except by something in a state
of actuality. Thus that which is actually hot, as fire, makes wood, which is potentially
hot, to be actually hot, and thereby moves and changes it. Now it is not possible that
the same thing should be at once in actuality and potentiality in the same respect, but
only in different respects. For what is actually hot cannot simultaneously be potentially
hot; but it is simultaneously potentially cold. It is therefore impossible that in the same
respect and in the same way a thing should be both mover and moved, i.e., that it should
move itself. Therefore, whatever is in motion must be put in motion by another. If that
by which it is put in motion be itself put in motion, then this also must be put in motion
by another, and that by another again. But this cannot go on to infinity, because then
there would be no first mover, and, consequently, no other mover; seeing that subsequent
movers move only inasmuch as they are put in motion by the first mover; as the staff
moves only because it is put in motion by the hand. Therefore it is necessary to arrive at
a first mover, put in motion by no other; and this everyone understands to be God.
Aquinas says that the First Way is the most evident way. The way starts from things that are “evident to our senses.” It starts “from the fact that some things in the world undergo change (motus), meaning that they vary in place, quantity, and quality.” In other words, people move from one place to another, a tree grows from an acorn to a tree, people can slim down, individuals can go from not knowing to knowing. These are all examples of change that occur in the world. It is clear that change occurs in the world. As Aquinas says, “For motion is nothing else than the reduction of something from potentiality to actuality.” For example, people have the potential to learn how to read, write, add, and think. However, these things are not realized in them when they are born.
How can these changes be explained? Aquinas says, “For motion is nothing else than the reduction of something from potentiality, except by something in the state of actuality.” Aquinas is saying if that something or someone changes in the state of potentiality, the change must occur from something in actuality. Feser asserts, “it is impossible for anything to be at the same time and in the same respect both that which is moved or changed and that which does the moving or changing.” It seems something in the process of change cannot be the cause of that change. In the next part of the argument, Aquinas argues that there must be an unmoved mover. Feser explains why: “By the same token, if that which puts something else in motion is itself moving, there must be yet something further moving it and so on. But if such a series went on to infinity, then there would be no first mover; and if there were no first mover, there would be no other movers.” This seems to make sense. If everything is in motion and dependent on something else, it seems there has to be a immovable mover not in motion to move another. Aquinas concludes that this first mover is God. It is important that in these Five Ways of proving the existence of God Aquinas is not arguing for a “developed doctrine of God.” Aquinas’ intention is more limited than this. He is not trying to describe the essence of God or what God is in Himself. Instead, the Five Ways argue from effect to cause to show that God exists, not the existence of God.