Summa Contra Gentiles
Aquinas's earlier and more apologetic Summa, in four books, addressing Christian doctrine to readers (Muslims, Jews, pagans) who do not share its scriptural premises.
Who should read it
Readers who want Aquinas arguing rather than instructing, on premises a non-believer could accept for the first three books.
Skip it if
You want the standard Aquinas. The Summa Theologiae is the systematic one and the form is more instructive.
The other Summa, written first, and the one that exists because of a different reader.
The Summa Theologiae is a textbook for Christian students and argues from scripture whenever it likes. This one, composed between 1259 and 1265, is addressed to people who do not accept scripture — the traditional account says it was written for missionaries in Spain, arguing with Muslim and Jewish scholars, and the account is disputed. What is not disputed is the constraint Aquinas works under, because it is stated in the book and visible on every page.
The constraint, and what it produces
If your reader does not grant your authorities, you cannot cite them.
So for three of the four books Aquinas argues from premises a non-Christian could accept: observation, and reasoning about it. Scripture appears, when it appears, as confirmation of something already argued rather than as a premise. Only in Book IV, where the subjects are the Trinity and the Incarnation — things he holds could not be reached by reason at all — does he switch openly to arguing from revelation, and he says that he is doing it.
The effect on the prose is the reason to read it. The quaestio format is gone; this is continuous argument, developed at length, and Aquinas is arguing rather than instructing. Positions get pursued further, objections are handled in the flow rather than in a numbered list, and the writing is more recognizably the work of someone trying to convince.
What reading it is like
Four books, continuous prose, shorter than the Summa Theologiae and considerably less convenient. Without the article structure you cannot open it anywhere and get a self-contained unit; you have to follow a chain.
That makes it the better book for reading and the worse book for consulting, which is the reverse of its sibling and is why it is the less cited of the two.
Where to start: Book III on providence and human happiness, which is the philosophically richest and is where the treatment later compressed into the Summa Theologiae has room to develop. Book II on the unity of the intellect is the most combative.
The four books
Book I: God. Existence, then the divine attributes — simplicity, perfection, goodness, infinity, eternity, unity — and the divine intellect and will. The arguments for existence are the same ones the later Summa compresses into a page and a half; here they are spread across the book with room to breathe.
Book II: Creation. Creation from nothing, the distinction between creator and creature, the angels, the human soul. Its center is a sustained attack on the Averroist doctrine that there is one intellect shared by all human beings — a reading of Aristotle then current in Paris, and one Aquinas regarded as among the most dangerous philosophical errors available, because it makes personal immortality incoherent. He is at his sharpest here.
Book III: Providence. God's relation to creatures, providence and human freedom, happiness as the human end, the goods and evils of a life. The argument that no finite good can be the last end gets its fullest statement in this book.
Book IV: Revealed doctrine. Trinity, Incarnation, sacraments, last things. The method changes and the change is announced.
Key passages
I.13 — the arguments for God's existence, at length.
I.28–43 — the divine attributes; how much can be said about what God is.
II.59–78 — the refutation of the single shared intellect. The most philosophically aggressive stretch in Aquinas.
III.25–63 — happiness; the argument that nothing finite can be a last end, and that the human end is the vision of God.
III.111–113 — providence and human freedom held together.
IV.1 — the announcement that the method is changing, and why.
What is contested
Who it was for. The missionary-handbook account comes from a source written some fifty years later and has been questioned since. Alternatives are that it was a general work of apologetics, or a set of arguments for use in university disputation. The constraint the book operates under is visible whatever the occasion was.
Whether the first three books really avoid Christian premises. The conclusions are conspicuously the Christian ones, and a hostile reader can argue that the destination was fixed and the premises selected to reach it. Aquinas would say the convergence is the point.
Its relation to the later Summa. Whether the two differ in doctrine or only in presentation has been argued for a long time, mostly over the treatment of happiness in Book III against Summa Theologiae Ia-IIae, where the emphasis shifts in a way that may or may not be a change of mind.
Questions
What is the Summa Contra Gentiles?
Aquinas's earlier Summa, in four books, arguing for Christian doctrine on grounds available to readers who do not accept scripture.
How does it differ from the Summa Theologiae?
It argues rather than instructs, proceeding by continuous demonstration rather than the question-and-objection format, and reserves appeals to revelation for the final book.
Who was it written for?
Traditionally described as a manual for missionaries addressing Muslims and Jews, though the evidence is thin and it may simply be aimed at readers who share no scripture with the author.