The Summa Theologiae of Thomas Aquinas: a guide
The Summa Theologiae is St Thomas Aquinas's great summary of Christian theology, written from about 1265 until he stopped writing in December 1273, and left unfinished. It is organised in three parts, divided into questions and then articles, each a short disputation. Aquinas wrote it, he says, "for the instruction of beginners."
Quick facts
| Author | St Thomas Aquinas, Dominican friar (born 1225 or 1227, died 7 March 1274) |
| Written | From about 1265. He laid aside his pen on 6 December 1273 |
| Parts | First Part; First Part of the Second Part; Second Part of the Second Part; Third Part (unfinished); Supplement |
| Size | The Catholic Encyclopedia counts 612 questions, 3,120 articles and "about 10,000 objections" in the whole work, Supplement included |
| Free English text | The 1920 translation by the Fathers of the English Dominican Province, online at newadvent.org/summa |
| Title | Summa Theologiae and Summa Theologica are both used for the same work |
Why Aquinas wrote it
The prologue explains the purpose. Because "the doctor of Catholic truth ought not only to teach the proficient, but also to instruct beginners," Aquinas proposes "to treat of whatever belongs to the Christian religion, in such a way as may tend to the instruction of beginners." He names three problems with the textbooks of his day: "the multiplication of useless questions, articles, and arguments," material not taught "according to the order of the subject matter," and "frequent repetition" that brought "weariness and confusion." The Summa is his answer: everything in order, as "briefly and clearly as the matter itself may allow."
How the Summa is organised
| Part | Reference | Questions | What it covers (headings from the Dominican translation) |
|---|---|---|---|
| First Part (Prima Pars) | I | 119 | Sacred doctrine; the one God; the Trinity; creation; the angels; the six days; man; the government of creatures |
| First Part of the Second Part (Prima Secundae) | I-II | 114 | Man's last end; human acts; passions; habits; vice and sin; law; grace |
| Second Part of the Second Part (Secunda Secundae) | II-II | 189 | Faith, hope, charity, prudence, justice, fortitude, temperance; acts that pertain to certain people |
| Third Part (Tertia Pars) | III | 90 | The Incarnation; the life of Christ; the sacraments, breaking off in the treatise on Penance |
| Supplement | Suppl. | 99, plus appendices | Penance (continued); extreme unction; holy orders; matrimony; the resurrection and last things |
A reference such as I-II, q. 94, a. 2 means First Part of the Second Part, question 94, article 2.
How one article works
Every article follows the same five steps:
- The question, always beginning "Whether": Whether God exists?
- Objections: the strongest arguments for the answer Aquinas will reject.
- On the contrary (sed contra): an authority on the other side.
- I answer that (respondeo): Aquinas's own argument, usually turning on a distinction.
- Replies to the objections, one by one.
Read the objections first and take them seriously. Our full guide, How to read the Summa Theologiae, walks through one article step by step and lists the common mistakes, such as quoting an objection as if it were Aquinas's view.
The Summa explained, article by article
Each explainer reads one article in its own order, objections first. They are paraphrases and reading guides, not translations, checked against the Dominican translation on newadvent.org.
| Explainer | Reference | The article's question |
|---|---|---|
| The Five Ways | I, q. 2, a. 3 | Whether God exists? |
| What is the soul? | I, q. 75, a. 2 | Whether the human soul is something subsistent? |
| Natural law | I-II, q. 94, a. 2 | Whether the natural law contains several precepts, or only one? |
| Just war | II-II, q. 40, a. 1 | Whether it is always sinful to wage war? |
| Is every lie a sin? | II-II, q. 110, a. 3 | Whether every lie is a sin? |
New to it? Start with Is the Summa Theologica worth reading?, then the Five Ways.
Where to begin
- If you want the big questions: First Part, question 2 (the existence of God), then question 3 (God's simplicity).
- If you want ethics: First Part of the Second Part, questions 1 to 5, on the last end and happiness. Everything in the moral part builds on them.
- If you want law: First Part of the Second Part, questions 90 to 97: the essence of law, eternal law, natural law and human law.
- If you want the virtues: Second Part of the Second Part, questions 57 and 58, on right and justice.
Pick one article, not one part. Ten careful minutes on a single article teaches more than an hour of skimming.
Why is the Summa unfinished?
Aquinas never completed the Third Part. The Catholic Encyclopedia records that on 6 December 1273 "he laid aside his pen and would write no more," after an unusually long ecstasy during Mass, and that when his companion Reginald urged him to continue he replied: "I can do no more. Such secrets have been revealed to me that all I have written now appears to be of little value." The Third Part stops at question 90, in the treatise on Penance. Aquinas died on 7 March 1274.
The Supplement completes the plan. The New Advent editor's note says it "was compiled probably by his companion and friend Fra Rainaldo da Piperno," and "was gathered from St. Thomas's commentary on the Fourth Book of the Sentences of Peter Lombard." So the Supplement is Aquinas's thought, but not text he wrote for the Summa.
Theologiae or Theologica?
Both. Summa Theologiae ("summary of theology") is the form most scholars use. Summa Theologica is the older form, used by the 1920 Dominican translation and by many libraries. Search for either and you will find the same book.
Key ideas to know before you read
The Summa uses a technical vocabulary consistently: act and potency, form and matter, essence and existence, analogy. Two short pages help:
- What is Thomism?, on the tradition that follows Aquinas.
- What does Thomistic mean?, on how the word is used today and the ideas that make an argument Thomistic.
Studying the Summa with Aquinian
Aquinian Studio is a study companion for the Summa that keeps the order of the article: objections, sed contra, respondeo, replies. Ask about a question or article in the language you think in, and Aquinian distinguishes the terms, orders the authorities, and can put the reading into the form of a Disputation or a Lecture. Investigate takes a second pass that is allowed to wound the first.
Aquinian is modelled on the method of St Thomas. It is not St Thomas, and it is not a priest or an ecclesial authority. Named passages and editions should be checked against primary sources before formal use.
Open the Summa with Aquinian: 150 free credits, no card.
FAQ
What is the Summa Theologiae? St Thomas Aquinas's systematic summary of Christian theology, written from about 1265 to 1273 and left unfinished. It is divided into parts, questions and articles, and Aquinas wrote it for the instruction of beginners.
What are the parts of the Summa Theologiae? The First Part (God and creation), the First Part of the Second Part (the last end, human acts, virtue in general, law and grace), the Second Part of the Second Part (the particular virtues), and the unfinished Third Part (Christ and the sacraments), completed after his death by the Supplement.
Can I read the Summa Theologiae online for free? Yes. The 1920 English translation by the Fathers of the English Dominican Province is free on newadvent.org.
Why did Aquinas not finish the Summa? On 6 December 1273 he stopped writing after an experience during Mass, saying that all he had written seemed to him of little value. He died on 7 March 1274. The Supplement was compiled afterwards, probably by his companion Reginald of Piperno, from his earlier commentary on the Sentences.
Sources
Sources (all checked on newadvent.org, 21 Oct 2026 AEDT) - Summa Theologiae, prologue and contents: https://www.newadvent.org/summa/ - Part contents and question counts: https://www.newadvent.org/summa/1.htm (I), /2.htm (I-II), /3.htm (II-II), /4.htm (III, with editor's note), /5.htm (Supplement) - Articles: /summa/1002.htm (I q2), /summa/1075.htm (I q75), /summa/2094.htm (I-II q94), /summa/3040.htm (II-II q40), /summa/3110.htm (II-II q110) - Catholic Encyclopedia (1912), "St. Thomas Aquinas": https://www.newadvent.org/cathen/14663b.htm