St. Thomas Aquinas' fourth way is the Argument from Gradation of Being, from Degree, or from Goodness. It's an argument based on our experience that some things are better than others.
Better or Worse?
Every time, my ophthalmologist flips those lenses back and forth. Better? Five.. or six? (Lenses five and six look exactly the same to me. He knows that; I know that; and we still play this game.)
We naturally experience this. Some things are better than others. This cup of coffee is hotter than that one. This winter was colder than last winter. Her car is faster than his car. I got a better grade in this class than that class.
Some things are better than others. Some are more - hotter, colder, more costly, tastier, prettier, smellier, and so on.
The Best of the Best
How do we know one thing is hotter than other? It's just hotter, right? But why? What makes it hotter?
We know there is absolute lowest temperature: −459.67°F or 0K. "Colder" and "hotter" are relative to that absolute. The temperature 80°F is hotter than 70°F because it is further from that absolute zero.
(Aquinas argues that our measure of degree are based on the "uttermost" case and something is hotter if it more closely resembles the hottest thing. Rather than get into the theorized Planck temperature (1.416785 x 10^32 K), it seems easier to discuss absolute zero, and it doesn't affect the logic. Though its worth noting how, as often happens, we find that Aquinas is ahead of his time and that faith and science get along beautifully.)
I find one pizza tastier than another because I have, in my mind, an ideal pizza. I know what pizza should be (and I'm from New Jersey - don't start with me about pizza). The best pizza is the closest to that ideal.
My wife's shirt is more blue than mine because there is an absolute measure of "blue". There is a pure blue (for instance, in HTML it's #0000FF). Her shirt is closer to that pure blue.
The Summum Bonum
Now, Aquinas takes an interesting philosophical step. He states that "the maximum of any genus (a class or group) is the cause of all in that genus." (emphasis mine) There is a greatest heat which is the source of all heat, for example.
There must be a maximum goodness, a maximum truth, a maximum beauty from which we measure everything. That maximum is God.
If we agree with Aquinas that the greatest is the cause of everything in that class, then we can say, too, that the maximally great being is the cause of all being--and we have the creator God.
We're through four different "ways" to argue for God from contingency. We're going to add a few more bricks to this level of the wall: cosmological argument and ontological argument.
Give a listen to a new playlist of brief excerpts from Dr. Peter Kreeft's talks, all on the subjects of beauty and music.