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Aquinas’ Fourth Way: Light in a Mirror


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

Aquinas’ Fourth Way is different from his other Ways, and is probably the most difficult  for us modern folks to understand. That’s a shame, because it’s perhaps his most profound Way. While the first three proofs are from passivity, activity, and contingency, the fourth is from the perfection of things — the capacity for nature to participate in transcendence. It has a distinct Platonic flavor, which is a bit unusual for Aquinas, who was very much an Aristotelian. It is the one proof that focuses more on the conceptual aspects of nature, rather than on nature’s specific physical properties. The Fourth Way is rather abstract. When you do understand it, it opens a door into seeing God in nature in a way that is quite compelling and beautiful. 

Digging in the Backyard

It’s helpful, as with his Third Way, to begin with a metaphor, in order to get an intuitive feel for the proof. Imagine that you are digging in your backyard and you find a mirror. You have led a sheltered life, and you don’t know what a mirror is. So you study the thing. It seems to have two different kinds of properties. 

On the one hand it is a solid inert object. It weighs a pound or so, has a long blue handle, a round frame with some decorations on the rim, a grey metal back, and a shiny front. 

On the other hand, the shiny part has images. The images, unlike the other parts of the mirror, seem to change with time. You see the clouds in the sky, then you see a tree, then you see grass, depending on how you hold the mirror. The images have two other properties that are unlike the “solid” properties of the mirror itself. The images vary quite a bit in intensity, depending on the time of day. At midday, they are very bright and clear. At night, it’s hard to see any image at all. The other difference is that the images aren’t unique to the mirror. You see these images in other things — sometimes you see the clouds shining off a window, or a tree in a smooth pond, or grass in a shiny shard of metal in your hand. There’s something about these images that points beyond the mirror itself. 

The solid properties — the weight, shape, etc. — of the mirror are easy enough to understand. It’s some kind of artifact. But the images are ephemeral. They can shine with a whole range of brightness, from blinding to dark, and the images can be seen in many different things, not just in the mirror. 

Images and Reflections

You surmise that the images are different kinds of properties — they aren’t from the mirror itself. The images are reflections. The mirror seems to possess them — to participate in them in a sense — but to different degrees at different times — sometimes they’re bright, sometimes dark, sometimes clear, sometimes hazy. Furthermore, the images are possessed by many other things besides the mirror — by smooth ponds and shiny glass and clear metal. The images transcend particular things. Particular things participate in them, each in its own way, but the images come from somewhere else. 

Where do the images come from? It makes no sense to say that they come from other images — from images in the pond bouncing off images in your window bouncing off images in a piece of metal and ending up in your mirror. Even if that happened, images bouncing off things can’t go to infinite regress. The images are of real things.  What you see in the mirror are just reflections of things that are completely real. The clouds and trees and grass are perfect in a sense, in that they themselves are not dark or distorted or hazy. The images of them are always hazy and attenuated. 

Images of Light

But then you notice something else, something that takes your breath away. In one sense the images are different reflections of different real things, but they are the same in a fundamental way. The images are all made of light. The images reflected in your mirror are pure Light, instantiated in various forms — shapes of clouds and trees and grass. But the substance of all of the images is one thing — Light Itself. The diversity of the real things that are reflected in your mirror is an illusion, of sorts. What is in your mirror is Pure Light, manifested in various forms. All of the reflected images in mirrors and ponds and windows and pieces of metal are different forms of Light, differentiated and attenuated in the natural world. 

That’s as far as the metaphor will carry us. The Thomistic proof of the Fourth Way proceeds like the metaphor. St. Thomas himself found it subtle and tricky. His longest exposition of it was in Summa Contra Gentiles, one of his early works, and he revised it in Summa Theologica to depend more on actual gradations in nature in its premises and less on pure logic, as his exposition in Summa Contra Gentiles did. 

A Modern Paraphrase

St. Thomas’ explication of the proof, paraphrased best for the modern ear, goes like this (I’ll draw the analogies to the mirror as we go). In nature we see things (mirrors) that are more or less perfect in various ways. Things may be more or less good or true or noble (a Thomistic word for deserving of praise or dignity — the ceiling of the Sistine Chapel is more noble than a paint stain on the floor). These limited perfections (reflected images) are comprehensible only in reference to limitless perfections (real things) of various sorts. And even the images themselves don’t explain the real things (the actual clouds, etc.) they are images of. These individual limitless perfections (clouds, trees, grass) are themselves just different manifestations of Perfection (Light) Itself.  

To go deeper into the proof, we note that the perfections in the Fourth Way have two distinguishing characteristics. They are possessed by different things, and they are possessed in different degrees. All sorts of things in nature can be good or true, and each may be good or true across a spectrum of perfection. The goodness or truth isn’t inherent to the thing itself — it’s something the thing seems to participate in. Goodness or truth isn’t a category or genus in the same way in the same way that being an animal or a plant or a rock is. And there is a very real sense of potentiality in (for example) the goodness of a thing. No matter what the degree of goodness is in something, there is always the possibility for it to be more or less good. A man can always be more or less good than he is, but he can never be less an animal or a man than he is. Perfections like goodness and truth and nobility have a different ontology from genus or species. 

Potency and Act

By the principle of potency and act, and the law of non-contradiction, the potency inherent in degrees of the perfections necessarily implies a fully actual perfection in which they (incompletely) participate. That fully actual perfection — pure Goodness, pure Truth, pure Nobility — is what all men call God. 

The Fourth Way proves what we intuit: that the goodness and truth and nobility that we see in nature are reflections of the perfections of nature’s Creator. 

There is another important aspect to this proof, implied in the metaphor about the mirror. The reflected images are really reflections of the same substance — Pure Light — instantiated in different images. On the deepest level, the images in the mirror are Light itself. In Thomism, this is called the Interconvertibility of Transcendentals. Grist for another post. 

Image credit: StockSnap via Pixabay.