Faith & Science Icon Faith & Science

Aquinas’ Third Way: An Analogy to Moonlight


Editor’s note: See also, “Introducing Aquinas’ Five Ways,” by Michael Egnor. For more on Thomas Aquinas, intelligent design, and evolution, see the website Aquinas.Design.

The Cosmological Arguments for God’s existence are proofs from beginnings in nature. Change, causation and existence must have a beginning that is outside nature, because of the impossibility of infinite regress of essentially ordered causal chains. I’ve explained the proofs for Aquinas’ First Way and Second Way in prior posts. Here I’ll explain his Third Way — the proof from Necessary Existence. 

The Third Way can be subtle, because existence is a trickier concept than change or causation. I find that an analogy to moonlight is helpful. I’ll give the analogy, and then the formal proof, for which the analogy will be helpful in gaining an intuitive understanding of the proof. 

An Astronomer and a Moon

Imagine that you are an astronomer on a world with one moon. It is always night on your world, and the moon is the only body in the sky. You note that the moon goes through phases — sometimes it is bright and full, sometimes it is dark and new. 

One night (it’s always night) you are having a conversation with your atheist friend. You raise the question: Where does the moonlight come from? Your atheist friend says “from the moon itself, obviously.” But you point out that the moonlight comes and goes. This implies that there must be something else causing it to come and go. 

Your atheist friend says, “Moonlight doesn’t need a cause. It just exists.” But you point out that your research through your telescope shows that the moon is a rocky body with mountains and shadows, without any light of its own, and all of its light is reflected. 

Your atheist friend replies: “Then there must be other moons — moons we can’t see — that are the sources of the reflected light. Its moons, all the way down!”

All the Way Down

But you point out to your friend that you can’t get light from an infinite regress of mere reflection. If a moon doesn’t make its own light, you can’t provide an ultimate explanation for the light merely by positing the existence of a series of other moons that just reflect light. There must be an ultimate source of the reflected light, something that shines light according to its nature, and doesn’t merely reflect it. There must be a Sun, even though we can’t see it. 

The proof of the Third Way proceeds in the same manner. In nature we see things that exist, and can go into and out of existence. Their existence is contingent (their existence is a reflection). But everything that exists can’t exist by contingency. If anything exists at all, there must be Something that necessarily exists — Something that is a source of existence, not a reflection of existence. Contingent existence — reflected existence — can’t go to infinite regress. There must be Something that exists (shines) by its own nature, and gives existence to all other things. The Source of existence must be something for which existence is not merely contingent but necessary. 

A Contingent Universe

But, you atheist friends may ask, couldn’t the universe itself be the necessary (ultimate) existence? No, any more than all of the moons reflecting light could be the ultimate source of the light. An infinite regress of mirrors can’t be the source of an image in the mirror. 

The universe is contingent — it is a collection of contingent things that can go into and out of existence. Nothing in the universe represents necessary existence — everything in the universe, as well as the entire universe itself, is capable on not existing. That is, we can describe the universe (its essence) without affirmation of its existence. In fact, existence of the universe is finite — it began with the Big Bang and will end with (presumably) the Big Crunch. 

And the argument that the existence of our universe is necessary because we must exist in order to ask the question doesn’t work, because an alien from a different universe could define the essence of our universe, on theoretical grounds, without knowing whether it existed or not. We can do the same with other universes in the multiverse. Essence is absolutely distinct from existence. We could know what our universe is without knowing that our universe is. 

This is one of many remarkable consequences of St. Thomas’ most important metaphysical doctrine — that existence is absolutely distinct from essence. This is, in my view, the cornerstone of modern experimental science. It turned science away from the a priori arguments of the ancients about the natural world to modern a posterori theories of science based on empirical observations of nature. His doctrine leads, in a direct and rather simple way, to his Third Way of proving God’s existence.   

A Formal Statement

The formal statement of the Third Way may be expressed, as in the first two Cosmological Arguments, by observation of nature, by act and potency and by the law of non-contradiction. 

  1. The universe contains things that are capable of beginning to exist and of ceasing to exist.
  2. Things in the universe do in fact begin to exist and cease to exist.
  3. This represents elevation of potency (potential existence) to act (actual existence).
  4. By the law of non-contradiction, potency and act cannot coexist in a thing in the same respect. 
  5. Therefore, the universe cannot cause itself to exist. 
  6. The universe does exist, therefore its cause of existence must be Something Whose existence is necessary and not contingent. 
  7. That is what all men call God. 

Existence of contingent nature cannot go to infinite regress, just as moons reflecting light cannot go to infinite regress. There must be a Sun. The existence we see all around us is merely reflected existence, and it points in a remarkably direct way to the Source of existence. The existence of nature presupposes a Necessary Existence that is outside nature that provides the existence reflected in natural things. This is what all men call God. 

Photo credit: Ganapathy Kumar via Unsplash.