Summa Theologiae · II-II, q. 64
Question 64. Of Murder
Thomas Aquinas, Summa Theologiae, Second Part of the Second Part. 8 articles.
Text: Summa Theologiae, Project Gutenberg #18755, Second Part of the Second Part (Secunda Secundae), translated by the Fathers of the English Dominican Province. The Second Part of the Second Part of the Summa in the English Dominican translation (first printed 1911 to 1925; second and revised edition from 1920). A sample check on 2026-11-09 against pre-1931 printings at the Internet Archive (first printings of 1915 and 1916 and the second and revised edition of the 1920s) found that the article text matches, with no 1947 introduction, synopsis, or index. Question and article numbers were added by the transcriber. Rights: Public domain (US)
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In due sequence we must consider the vices opposed to commutative justice. We must consider (1) those sins that are committed in relation to involuntary commutations; (2) those that are committed with regard to voluntary commutations. Sins are committed in relation to involuntary commutations by doing an injury to one's neighbor against his will: and this can be done in two ways, namely by deed or by word. By deed when one's neighbor is injured either in his own person, or in a person connected with him, or in his possessions.
We must therefore consider these points in due order, and in the first place we shall consider murder whereby a man inflicts the greatest injury on his neighbor. Under this head there are eight points of inquiry:
(1) Whether it is a sin to kill dumb animals or even plants?
(2) Whether it is lawful to kill a sinner?
(3) Whether this is lawful to a private individual, or to a public person only?
(4) Whether this is lawful to a cleric?
(5) Whether it is lawful to kill oneself?
(6) Whether it is lawful to kill a just man?
(7) Whether it is lawful to kill a man in self-defense?
(8) Whether accidental homicide is a mortal sin?
Source: Thomas Aquinas. Summa Theologica, Second Part of the Second Part. Project Gutenberg #18755. Project Gutenberg record. Public domain in the USA. Outside the U.S., check local copyright rules. Project Gutenberg notices retained. The full Project Gutenberg license is kept in the hosted text.