The most famous unfinished book in philosophy. Aquinas began it around 1265 and stopped in December 1273 after something happened during Mass that he never explained, saying only that everything he had written seemed like straw. He died three months later. The last part was completed by his secretary out of Aquinas's own earlier lectures.

About 1.5 million words of Latin, longer than the Bible, and — this is the thing about it — written as an introductory textbook, because he thought the existing ones were badly organized.

The form, which is the point

Every one of the roughly 2,700 articles has the same four-part shape, and the shape is the reason the book is still usable.

Objections. It opens with the arguments against the position Aquinas is going to take, stated in their strongest available form, usually three to five of them, frequently quoting authorities who agree with them. He is not setting up straw men; he routinely states an objection better than its proponents did.

Sed contra — on the contrary. One authority on the other side, often brief, often scriptural, establishing that there is a case to answer.

Respondeo — I answer that. The body of the article, where the distinction gets drawn and the position is set out. Usually a page or less.

Replies. Each objection answered individually, in order, by showing what it got right and where it went wrong.

Read one article and you have the shape of a disagreement rather than a conclusion. That is a pedagogical decision and it is unusual: the book is constructed so that a student who reads it encounters every position before being told which one to hold, and is shown that the wrong answers are intelligent.

It is also why the Summa can be read in fragments. Articles are close to self-contained, and nobody except a specialist reads it through.

What reading it is like

Dry, precise, and much easier sentence by sentence than its reputation. The Latin is technical and unornamented; there are no digressions, no images and almost no rhetoric. What makes it hard is not the prose but the density of vocabulary — act and potency, form and matter, essence and accident — all of which is Aristotle's and none of which is explained, because the intended reader had it already.

Where to start: Ia q.2 (the existence of God, including the Five Ways) for the famous passage; Ia-IIae qq.1–5 (happiness as the human end) for the best writing; Ia-IIae q.94 (natural law) for the most consequential. Ia q.75–89, on the human person as one thing rather than a soul with a body attached, is the most underrated.

A practical note: citations look worse than they are. "ST Ia-IIae q.94 a.2" is Part, question, article. Learn that and the whole apparatus opens.

The shape of the whole

The three parts follow a circle rather than a list, which is easy to miss and is the architecture.

Prima Pars — God, then creation, then the human person. Everything proceeding out from its source.

Secunda Pars — by far the longest, and the human being's movement back toward that source: action, the passions, habit, virtue and vice, law, grace. The Prima Secundae handles the structure in general, the Secunda Secundae takes the individual virtues one at a time.

Tertia Pars — Christ and the sacraments, the means by which the return happens. Unfinished.

Out, and back. It is the same shape as the Neoplatonic procession and return, carrying Aristotelian machinery, in a Christian frame — and noticing it makes the organization stop looking arbitrary.

Key passages

Ia q.2 a.3 — the Five Ways. The most quoted page in medieval philosophy, and a page and a half long.

Ia q.3 — divine simplicity; in God, essence and existence are identical.

Ia q.13 — analogical predication; how words can apply to God at all.

Ia q.75–89 — the human person as a body-soul composite rather than a soul using a body.

Ia-IIae q.1–5 — happiness as the last end; the argument that nothing finite can be it.

Ia-IIae q.94 — the precepts of the natural law. The most influential single question.

IIa-IIae q.57–79 — justice, including the treatment of property and of what is owed.

IIIa q.75 — the Eucharist, and the technical deployment of substance that the doctrine of transubstantiation rests on.

What is contested

Whether the Five Ways prove anything. The standard objections — Hume's and Kant's — are aimed at versions Aquinas did not hold, and the more careful readings are correspondingly harder to dismiss. See five ways.

How much depends on Aristotle's physics. The arguments from motion and causation are stated in a vocabulary of act and potency that belongs to a superseded natural philosophy. Whether they can be restated without it is the live question in Thomist scholarship and the answer is not obvious.

Whether the natural law can survive the loss of natural ends. The ethics derives content from what human beings are for, and a reader who does not grant natural ends is not given an alternative route.

What the abandonment means. Aquinas stopped, called the work straw, and gave no explanation. Readings range from a stroke to a mystical experience to exhaustion, and each one implies a different relationship between the book and its author's final view of it.