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St. Thomas Aquinas
Catechetical Instructions

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  • THE FIFTH COMMANDMENT: "Thou Shalt Not Kill."
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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



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.


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