Does God Exist? Thomas Aquinas’s Five Ways, Part One

Does God Exist? Thomas Aquinas’s Five Ways, Part One February 23, 2022

As both a saint and a doctor of the Church, Thomas Aquinas is arguably the most significant figure in Catholic theology. No less significant are Aquinas’s contributions to philosophy and science.

Aquinas’s opus is the Summa Theologica (Summary of Theology). It is a compendium of Catholic theology and philosophy. While the Summa contains several arguments for the existence of God, the most enduring of these are the quinque viae or five ways. The five ways are logical arguments, and, as such, are as much philosophical as theological. 

In order to possess a complete understanding of Aquinas’s philosophy and theology, it is necessary to have a firm understanding of Aristotelian philosophy. Despite this, in the following series, I will provide an introduction to Aquinas’s five ways. 

The First Way

Aquinas’s first way is called an argument from motion. Unfortunately, what Aquinas means by motion has been frequently misinterpreted. For Aquinas, as for Aristotle, motion included how a thing may change, not just its movement from one place to another.

For now, I will provide Aquinas’s formulation of the argument. 

“It is evident by observing the universe that some things are in motion in the world. Now whatever is in motion is put in motion by another, for nothing can be in motion except as it is in potentiality to that towards that which it is in motion, whereas a thing moves since it is in act. 

Motion is nothing else than reducing something from potentiality to actuality. But nothing can be reduced from potentiality to actuality, except by something in a state of actuality. Thus, which is actually hot, as fire, makes wood, which is potentially hot, to be actually hot, moving and changing it. 

It is not possible that the same thing should be in actuality and in potentiality in the same respect and time, but only in different respects. What is actually hot cannot simultaneously be potentially hot, but it is simultaneously potentially cold. Therefore, it is impossible that a thing should be both mover and moved in the same respect and in the same way. That is to say that nothing moves itself. 

Consequently, whatever is in motion must be put in motion by another.

If the agent that puts another into motion, is itself in motion, it too must be put in motion by another. Yet there cannot be an infinite regress because then there would be no first mover and, consequently, no other mover, seeing that subsequent movers move only after they are put in motion by the first mover. Therefore it is necessary to arrive at a first mover, set in motion by no other, which everyone understands to be God.”

Analyzing the First Way

As one can see, the argument is, at least partially, a posteriori in nature. That is, it is based on experience and observation. Still, Aquinas’s first way is somewhat complex and requires some unpacking. In particular, we need to understand three terms often used in metaphysics. 

The first word is motion. As indicated above, for Aristotle and Aquinas, the word motion includes change within its definition. The second and third words are often used in conjunction; potency and act

Potency refers to the possibility or capacity for a thing to complete or fulfill its nature. Act refers to the completion or fulfillment of a thing’s nature. For example, a seed exists potentially as a tree. The tree exists in actuality in relation to the seed.

Moreover, the tree exists only because of the seed. The seed exists only because of another tree. Yet even the tree exists in potency. For example, a tree is potentially a table or a chair if one uses the wood from the tree for that purpose.

Understood this way, we can formulate Aquinas’s first way: Whenever something changes, it is caused to change by something else (a seed causes a tree). Nothing can cause its own change since something cannot have a quality of both potentially and actually at the same time (the seed can’t both be a seed and a tree simultaneously and in the same way). Therefore, whenever something changes, this change must have been brought about by an extrinsic cause. Moreover, that cause must already be in act (the seed can only form from an existing tree). 

All of this leads to the question of whether there can be an infinite regress of causes. To address the question of an infinite regress, we need to understand what an efficient cause is. For a definition, we turn to Aristotle, “An efficient cause is the primary source of the change or rest.” (Aristotle. Aristotle’s Physics. Oxford University Press, 1983). Put another way, an efficient cause is an agent that causes a change in another. For example, an artist is the efficient cause of a painting.

Aquinas builds upon Aristotle’s definition by categorizing efficient causes into essential and accidental. An example of an accidental efficient cause would be a father begetting a son. While the accidental efficient cause brings an effect into existence, it does not sustain the effect. A child does not cease to exist if his parents die. Therefore, Aquinas thought it theoretically possible that an accidental efficient causal chain could be infinite since each accidental efficient cause possess causal power (the child can later become a parent). 

Because Aquinas concludes that an infinite regress of accidental efficient causes is possible, the first way does not rule out the possibility of an eternal universe. Ultimately, however, Aquinas accepts the biblical cosmology that describes a contingent, and, therefore, temporal (not eternal) universe. 

We will see in Aquinas’s second way that he argues that an infinite regress of essential efficient causes is impossible.

What Caused God?

A point about potency and act as it relates to God. One could (as some have) argue that if all things are subject to change, then God too, must be in potency or subject to change.

However, such a claim is to misunderstand an important aspect of Aquinas’s argument. Aquinas does not argue that everything moves from potency to act, but only those things that do not exist necessarily (i.e., created things). Because of this, God is “defined” (one cannot properly define that which is infinite) as actus purus (pure act). What is pure act does not change since its nature has been perfected. Put another way, God exists necessarily, and therefore, must exist. Here Aquinas is drawing on Aristotle’s “unmoved mover”, that which is the ground or cause of change, but which itself does not change. This is not just a philosophical claim, but a theological one as well, for we read in scripture that “Jesus Christ [i.e., God] is the same yesterday and today and forever.” (Hebrews 13:8).


In this paper, I have provided an introduction to Aquinas’s five ways or arguments for the existence of God. I have furnished Aquinas’s formulation of the first way and explained the metaphysics that provide the foundation for Aquinas’s arguments.

In the following exposition, I will discuss the second way.

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