Say that God is good. What does the word mean?

Two answers are available immediately, and both are disasters.

Say it means what it means of a person. Then God is good in the way a generous neighbor is good — better, presumably, and by a great deal, but along the same scale. God has become a being among beings, differing in degree rather than in kind, and the difference between God and a very impressive creature is quantitative. Classical theism cannot accept this, because the God it is talking about is not an item in the universe.

Say it means something unrelated. Then the word is a homonym, like bank, and the sentence carries no information whatever. Everything said in theology becomes noise with a reverent tone, and every argument from the world to its source fails at once, because you could never know what the conclusion meant.

Univocal predication makes God finite. Equivocal predication makes theology empty. Aquinas's answer is that there is a third option, and it is not a compromise between the two.

Healthy

His example is the best short route in.

Call a man healthy. Call a diet healthy. Call a urine sample healthy.

The word is not being used in three unrelated senses — the uses are obviously connected, and anyone who knows the language sees how. Nor is it being used in one sense: a diet is not healthy in the way a man is, because a diet does not have health. What is happening is that one use is primary — the man, who possesses the health — and the others are named from their relation to it. The diet causes the health; the sample indicates it.

That is analogy: related senses, ordered to a primary one.

The theological application inverts what you might expect. The primary case is God, who is goodness rather than possessing it; the creature is called good derivatively, by relation to that source. So analogical language about God is not a weakened version of ordinary language. Ordinary language is the derivative case.

What it is for

This is not a piece of decorative scholasticism. It is the load-bearing member under the whole project of natural theology.

Every argument from the world to God — the five ways, the cosmological argument, the argument from design — moves from features of creatures to a conclusion about their source. If the key terms shift sense between premise and conclusion, every one of those arguments equivocates and none of them works. If the terms keep exactly the same sense, the conclusion is about a very large creature.

Analogy is the claim that there is a way for the terms to be neither, and that inference can survive it.

It also does work on the other side. Divine simplicity says God has no parts and no distinct attributes — God does not have goodness and wisdom as separable properties, but is what those words name. That doctrine makes the ordinary sense of every predicate inapplicable by definition, so some account like this one is not optional for anyone who holds it.

The objection that would not go away

Duns Scotus thought the middle way collapses.

His argument is about inference. In any syllogism, the middle term must mean the same thing in both premises, or the conclusion does not follow — that is not a convention but the whole basis of deduction. So if being means something merely analogous when said of God and of creatures, then no argument from creatures reaches God, and analogy has not saved natural theology but quietly ended it.

Scotus concluded that at least one concept — being itself — must be strictly univocal, applying in the very same sense to God and creature, with the difference lying in what is added to it: infinite in one case, finite in the other.

The dispute has never been settled, and it is not a small one. It is an argument about whether reasoning can cross the gap between the world and its source at all, and each side is pointing at a genuine cost in the other.