The Catechism of St. Thomas Aquinas



In the divine law which tells us we must love God and our neighbor, it is commanded that we not only do good but also avoid evil. The greatest evil that can be done to one’s neighbor is to take his life. This is prohibited in the Commandment: “Thou shalt not kill.”[1]

Killing of Animals Is Lawful.–In connection with this Commandment there are three errors. Some have said that it is not permitted to kill even brute animals. But this is false, because it is not a sin to use that which is subordinate to the power of man. It is in the natural order that plants be the nourishment of animals, certain animals nourish others, and all for the nourishment of man: “Even the green herbs have I delivered them all to you.”[2] The Philosopher says that hunting is like a just war.[3] And St. Paul says: “Whatsoever is sold in the shambles eat; asking no questions for conscience’ sake.”[4] Therefore, the sense of the Commandment is: “Thou shalt not kill men.”

The Execution of Criminals.–Some have held that the killing of man is prohibited altogether. They believe that judges in the civil courts are murderers, who condemn men to death according to the laws. Against this St. Augustine says that God by this Commandment does not take away from Himself the right to kill. Thus, we read: “I will kill and I will make to live.”[5] It is, therefore, lawful for a judge to kill according to a mandate from God, since in this God operates, and every law is a command of God: “By Me kings reign, and lawgivers decree just things.”[6] And again: “For if thou dost that which is evil, fear; for he beareth not the sword in vain. Because he is God’s minister.”[7] To Moses also it was said: “Wizards thou shalt not suffer to live.”[8] And thus that which is lawful to God is lawful for His ministers when they act by His mandate. It is evident that God who is the Author of laws, has every right to inflict death on account of sin. For “the wages of sin is death.”[9] Neither does His minister sin in inflicting that punishment. The sense, therefore, of “Thou shalt not kill” is that one shall not kill by one’s own authority.[10]

Suicide is Prohibited.–There are those who held that although this Commandment forbids one to kill another, yet it is lawful to kill oneself. Thus, there are the examples of Samson (Judges, xvi) and Cato and certain virgins who threw themselves into the flames, as St. Augustine relates in “The City of God.”[11] But he also explains this in the words: “He who kills himself, certainly kills a man.”[12] If it is not lawful to kill except by the authority of God, then it is not lawful to kill oneself except either upon the authority of God or instructed by the Holy Ghost, as was the case of Samson. Therefore, “thou shalt not kill.”[13]

Other Meanings of “To Kill.”–It ought to be known that to kill a man may happen in several ways. Firstly, by one’s own hand: “Your hands are full of blood.”[14] This is not only against charity, which tells us to love our neighbor as ourself: “No murderer hath eternal life abiding in himself.”[15] But also it is against nature, for “every beast loveth its like.”[16] And so it is said: “He that striketh a man with a will to kill him, shall be put to death.”[17] He who does this is more cruel than the wolf, of which Aristotle says that one wolf will not eat of the flesh of another wolf.[18]

Secondly, one kills another by word of mouth. This is done by giving counsel to anyone against another by provocation, accusation, or detraction: “The sons of men whose teeth are weapons and arrows, and their tongue a sharp sword.”[19] Thirdly, by lending aid, as it is written: “My son, walk not thou with them . . . for their feet run to evil, and they make haste to shed blood.”[20] Fourthly, by consent: “They are worthy of death, not only they that do them, but they also that consent to them that do them.”[21] Lastly, one kills another by giving a partial consent when the act could be completely prevented: “Deliver them that are led to death;”[22] or, if one can prevent it, yet does not do so through negligence or avarice. Thus, St. Ambrose says: “Give food to him that is dying of hunger; if you do not, you are his murderer.”

We have already considered the killing of the body, but some kill the soul also by drawing it away from the life of grace, namely, by inducing it to commit mortal sin: “He was a murderer from the beginning,”[23] that is, in so far as he drew men into sin. Others, however, slay both body and soul. This is possible in two ways: first, by the murder of one with child, whereby the child is killed both in body and soul; and, secondly, by commiting suicide.


Why We Are Forbidden to Be Angry.–In the Gospel of St. Matthew (chapter V) Christ taught that our justice should be greater than the justice of the Old Law. This means that Christians should observe the Commandments of the law more perfectly than the Jews observed them. The reason is that greater effort deserves a better reward: “He who soweth sparingly, shall also reap sparingly.”[24] The Old Law promised a temporary and earthly reward: “If you be willing and will hearken to Me, you shall eat the good things of the land.”[25] But in the New Law heavenly and eternal things are promised. Therefore, justice, which is the observance of the Commandments, should be more generous because a greater reward is expected.

The Lord mentioned this Commandment in particular among the others when He said: “You have heard that it was said to them of old: Thou shalt not kill. . . . But I say to you that anyone who is angry with his brother, shall be in danger of the judgment.”[26] By this is meant the penalty which the law prescribes: “If any man kill his neighbor on set purpose, and by lying in wait for him; thou shalt take him away from My altar, that he may die.”[27]

Ways of Avoiding Anger.–Now, there are five ways to avoid being angry. The first is that one be not quickly provoked to anger: “Let every man be swift to hear, but slow to speak and slow to anger.”[28] The reason is that anger is a sin, and is punished by God. But is all anger contrary to virtue? There are two opinions about this. The Stoics said that the wise man is free from all passions; even more, they maintained that true virtue consisted in perfect quiet of soul. The Peripatetics, on the other hand, held that the wise man is subject to anger, but in a moderate degree. This is the more accurate opinion. It is proved firstly by authority, in that the Gospel shows us that these passions were attributed to Christ, in whom was the full fountainhead of wisdom. Then, secondly, it is proved from reason. If all the passions were opposed to virtue, then there would be some powers of the soul which would be without good purpose; indeed, they would be positively harmful to man, since they would have no acts in keeping with them. Thus, the irascible and concupiscible powers would be given to man to no purpose. It must, therefore, be concluded that sometimes anger is virtuous, and sometimes it is not.

Three Considerations of Anger.–We see this if we consider anger in three different ways. First, as it exists solely in the judgment of reason, without any perturbation of soul; and this is more properly not anger but judgment. Thus, the Lord punishing the wicked is said to be angry: “I will bear the wrath of the Lord because I have sinned against Him.”[29]

Secondly, anger is considered as a passion. This is in the sensitive appetite, and is twofold. Sometimes it is ordered by reason or it is restrained within proper limits by reason, as when one is angry because it is justly fitting to be angry and within proper limits. This is an act of virtue and is called righteous anger. Thus, the Philosopher says that meekness is in no way opposed to anger. This kind of anger then is not a sin.

There is a third kind of anger which overthrows the judgment of reason and is always sinful, sometimes mortally and sometimes venially. And whether it is one or the other will depend on that object to which the anger incites, which is sometimes mortal, sometimes venial. This may be mortal in two ways: either in its genus or by reason of the circumstances. For example, murder would seem to be a mortal sin in its genus, because it is directly opposite to a divine Commandment. Thus, consent to murder is a mortal sin in its genus, because if the act is a mortal sin, then the consent to the act will be also a mortal sin. Sometimes, however, the act itself is mortal in its genus, but, nevertheless, the impulse is not mortal, because it is without consent. This is the same as if one is moved by the impulse of concupiscence to fornication, and yet does not consent; one does not commit a sin. The same holds true of anger. For anger is really the impulse to avenge an injury which one has suffered. Now, if this impulse of the passion is so great that reason is weakened, then it is a mortal sin; if, however, reason is not so perverted by the passion as to give its full consent, then it will be a venial sin. On the other hand, if up to the moment of consent, the reason is not perverted by the passion, and consent is given without this perversion of reason, then there is no mortal sin. “Whosoever is angry with his brother, shall be in danger of the judgment,” must be understood of that impulse of passion tending to do injury to the extent that reason is perverted–and this impulse, inasmuch as it is consented to, is a mortal sin.

Why We Should Not Get Angry Easily.–The second reason why we should not be easily provoked to anger is because every man loves liberty and hates restraint. But he who is filled with anger is not master of himself: “Who can bear the violence of one provoked?”[30] And again: “A stone is heavy, and sand weighty, but the anger of a fool is heavier than both.”[31]

One should also take care that one does not remain angry over long: “Be ye angry, and sin not.”[32] And: “Let not the sun go down upon your anger.”[33] The reason for this is given in the Gospel by Our Lord: “Be at agreement with thy adversary betimes whilst thou art in the way with him; lest perhaps the adversary deliver thee to the judge, and the judge deliver thee to the officer, and thou be cast into prison. Amen, I say to thee, thou shalt not go out from hence till thou repay the last farthing.”[34]

We should beware lest our anger grow in intensity, having its beginning in the heart, and finally leading on to hatred. For there is this difference between anger and hatred, that anger is sudden, but hatred is long-lived and, thus, is a mortal sin: “Whosoever hateth his brother is a murderer.”[35] And the reason

is because he kills both himself (by destroying charity) and another. Thus, St. Augustine in his “Rule” says: “Let there be no quarrels among you; or if they do arise, then let them end quickly, lest anger should grow into hatred, the mote becomes a beam, and the soul becomes a murderer.”[36] Again: “A passionate man stirreth up strifes.”[37] “Cursed be their fury, because it was stubborn, and their wrath, because it was cruel.”[38]

We must take care lest our wrath explode in angry words: “A fool immediately showeth his anger.”[39] Now, angry words are twofold in effect; either they injure another, or they express one’s own pride in oneself. Our Lord has reference to the first when He said: “And whosoever shall say to his brother: ‘Thou fool,’ shall be in danger of hell fire.”[40] And He has reference to the latter in the words: “And he that shall say: ‘Raca,’ shall be in danger of the council.”[41] Moreover: “A mild answer breaketh wrath, but a harsh word stirreth up fury.”[42]

Finally, we must beware lest anger provoke us to deeds. In all our dealings we should observe two things, namely, justice and mercy; but anger hinders us in both: “For the anger of a man worketh not the justice of God.”[43] For such a one may indeed be willing but his anger prevents him. A certain philosopher once said to a man who had offended him: “I would punish you, were I not angry.” “Anger hath no mercy, nor fury when it breaketh forth.”[45] And: “In their fury they slew a man.”[46]

It is for all this that Christ taught us not only to beware of murder but also of anger. The good physician removes the external symptoms of a malady; and, furthermore, he even removes the very root of the illness, so that there will be no relapse. So also the Lord wishes us to avoid the beginnings of sins; and anger is thus to be avoided because it is the beginning of murder.

(For “Questions for Discussion” see Chapter 6.)


1. St. Thomas also treats of this Commandment in “Summa Theol.,” II-II, Q. lxix. art. 2, 3; Q. cxii, art. 6. “The Lord points out (Matt., v. 21) the twofold force of this Commandment. The one is prohihitory and forbids us to kill; the other is mandatory and commands us to cultivate charity, peace, and friendship towards our enemies, to have peace with all men, and finally to suffer all things with patience” (“Roman Catechism,” “Fifth Commandment,” 2).

2. Gen., ix. 3

3. Aristotle, “Politics,” I.

4. I Cor., x. 25.

5. Deut., xxxii. 39.

6. Prov., viii. 15.

7. Rom., xiii. 4.

8. Exod., xxii. 18.

9. Rom. vi. 23.

10. Killing in a just war and killing by accident are among the other exceptions to this Commandment. The soldier is guiltless who in a just war takes the life of an enemy, provided that he is not actuated by motives of ambition or cruelty, but by a pure desire to serve the interests of his country. . . . Again, death caused, not by intent or design, but by accident, is not

murder” (“Roman Catechism,” “loc. cit.,” 5-6).

11. Book I, xxvii.

12. “Ibid.”

13.–“It is not lawful to take one-s own life. No man possesses such power over his own life as to be free to put himself to death. We find that the Commandment does not say, ‘Thou shalt not kill another,’ but simply, ‘Thou shalt not kill’ ” (“Roman Catechism,” “loc. cit.,” 10).

14. Isa., i. 15.

15. John, iii. 15.

16. Ecclus., xiii. 19.

17. Exod., xxi. 12.

18. “De Animal.,” IV.

19. Ps. lvi. 5.

20. Prov., i. 15-16.

21. Rom., i. 32.

22. Prov., xxiv. 11.

23. John, viii. 44.

24. II Cor., ix. 6.

25. Isa., i. 19.

26. Matt., v. 21-22.

27. Exod., xxi. 14. “The Gospel has taught us that it is unlawful even to be angry with anyone. . . . From these words [of Christ, cited above] it clearly follows that he who is angry with his brother is not free from sin, even though he does not display his wrath. So also he who gives indication of his anger sins grievously; and he who treats another with great harshness and hurls insults at him, sins even more grievously. This, however, is to be understood of cases in which no just cause of anger exists. God and His laws permit us to be angry when we correct the faults of those who are subject to us. But even in these cases the anger of a Christian should spring from stern duty and not from the impulse of passion, for we are temples of the Holy Ghost in which Jesus Christ may dwell” (“Roman Catechism,” “loc cit.,” 12).

28. James, i. 19.

29. Mic., vii. 9.

30. Prov., xxvii. 4.

31. “Ibid.,” 3.

32. Ps. iv. 5.

33. Eph., iv. 26.

34. Matt., v. 25, 26.

35. I John, iii. 15.

36. “Epist.,” cxi.

37. Prov., xv. 18.

38. Gen., xlix. 7.

39. Prov., xii. 16.

40. Matt., v. 22.

41. “Ibid.”

42. Prov., xv. 1.

43. James, i. 20.

44. Prov., xxvii. 4.

45. Gen., xlix. 6.

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