Aquinas inherited a problem that looks technical and was, at the time, a threat to the whole enterprise.

Aristotle had been largely unavailable in the Latin West for centuries. Across the twelfth and thirteenth he arrived in bulk, in translation, with the commentaries of Avicenna and Averroes attached — and he was obviously the most powerful systematic thinker anyone had encountered, and his conclusions looked wrong. The world eternal rather than created. The soul the form of the body, which appears to rule out its surviving the body. One shared intellect for all human beings, on the Averroist reading, which rules out personal immortality outright. Paris banned the natural philosophy in 1210 and was still arguing about it decades later.

The available responses were to reject Aristotle, to accept him and let the theology take the damage, or to do what Aquinas did.

What the synthesis actually claims

Not a compromise. A claim about how the two kinds of knowing relate.

Grace does not destroy nature but perfects it. Reason, working properly on the world, reaches real truths — and it reaches a limit. Revelation does not contradict what reason establishes; it tells you things reason could not have reached, and it presupposes the nature reason was describing.

Which means there is no double truth, no two-track arrangement where something is true in philosophy and false in theology. There is one truth, and two ways of getting at parts of it. When a philosophical argument and a doctrine conflict, one of them is being read wrong, and the work is finding out which.

That confidence is the thing to notice. It is a bet that the world is intelligible all the way down and that careful reasoning cannot arrive anywhere that faith has to fear, and it is why his books look the way they do: the objections are stated first, in their strongest form, because if the position is right the objections can survive being stated.

Essence and existence

His most consequential technical move, and the one that does the most work.

In any created thing, what it is and that it is come apart. You can know perfectly well what a phoenix is and not know whether there are any. The essence does not include existence; existence is something received.

In God alone they are identical — God's essence is to exist, which is what Aquinas takes the name given at the burning bush to mean.

This does two things at once. It makes creation a continuous act rather than an event at the beginning: a thing whose essence does not include existence is being held in existence right now, not merely started. And it makes God categorically different from everything else rather than the largest item in the inventory — which is the move that protects the whole project from the objection that it is describing a very big object.

The argument from it to God's existence is on the five ways page, and the two books have their own.

Analogical predication

The problem: if God is categorically different, how can any word we use mean anything when applied to God?

Say God is good, and either you mean good in the sense you understand, in which case you have made God an item in a category, or you mean something else entirely, in which case you have said nothing.

Aquinas's answer is that the terms are analogical — neither univocal nor equivocal. When you call a person healthy and a climate healthy, the word is not being used identically and is not being used at random; there is an ordered relation between the senses. Talk about God works like that: real reference, no univocal grasp.

It is the most contested thing he says. Duns Scotus argued within a generation that at least one concept has to be strictly univocal or the whole of natural theology is invalid, and that dispute has never closed.

Natural law

The ethics, stated compactly: the natural law is the rational creature's participation in the eternal law. Its first precept is that good is to be done and pursued and evil avoided, and everything else is worked out from the ends a human being actually has — to live, to reproduce and raise children, to know the truth, to live in society.

Two things follow that matter outside theology. The content is derived from human nature rather than from divine command, so in principle it is available to anyone reasoning carefully, believer or not. And because it is derived from ends, it is a teleological ethics and stands or falls with the claim that human beings have natural ends — which is exactly what the modern objection denies.

It is also the line through which the concept of natural rights reaches the Enlightenment, which is why a thirteenth-century Dominican is in the ancestry of documents that would have appalled him.

The objection

Three years after he died, Étienne Tempier, bishop of Paris, condemned 219 propositions. Some of them were Aquinas's.

The Condemnation of 1277 was aimed at the radical Aristotelians and it caught him in the sweep, and the reason is worth understanding because it is the structural objection to the whole project. If Aristotle is that reliable, then the philosophy starts constraining what God can have done — and a bishop looking at a system where necessary truths about nature bind the creator is looking at a theology that has given something away.

Aquinas was canonized in 1323 and the condemnation lifted as it applied to him in 1325, which settled the politics and not the question. The question is whether the synthesis holds or defers the tension, and the honest answer is that it holds while Aristotle's physics and biology hold. Those did not. What survives — essence and existence, analogy, the account of action and virtue — survives because it was never as dependent on the science as the system implied it was.