Aquinas on warfare and self-defense

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Thomas Aquinas
(1225-1274 CE)

texts in the Summa Theologica
related to self-defense and warfare

ST I-II Q. 94 Art. 2

Whether the natural law contains several precepts, or only one?

Objection 1. It would seem that the natural law contains, not several
precepts, but one only. For law is a kind of precept, as stated above
(92, 2). If therefore there were many precepts of the natural law,
it would follow that there are also many natural laws.

Objection 2. Further, the natural law is consequent to human nature. But human
nature, as a whole, is one; though, as to its parts, it is manifold.
Therefore, either there is but one precept of the law of nature, on
account of the unity of nature as a whole; or there are many, by reason
of the number of parts of human nature. The result would be that even
things relating to the inclination of the concupiscible faculty belong to
the natural law.

Objection 3. Further, law is something pertaining to reason, as stated above
(90, 1). Now reason is but one in man. Therefore there is only one
precept of the natural law.

On the contrary, The precepts of the natural law in man stand in
relation to practical matters, as the first principles to matters of
demonstration. But there are several first indemonstrable principles.
Therefore there are also several precepts of the natural law.

I answer that, As stated above (91, 3), the precepts of the
natural law are to the practical reason, what the first principles of
demonstrations are to the speculative reason; because both are
self-evident principles. Now a thing is said to be self-evident in two
ways: first, in itself; secondly, in relation to us. Any proposition is
said to be self-evident in itself, if its predicate is contained in the
notion of the subject: although, to one who knows not the definition of
the subject, it happens that such a proposition is not self-evident. For
instance, this proposition, “Man is a rational being,” is, in its very
nature, self-evident, since who says “man,” says “a rational being”: and
yet to one who knows not what a man is, this proposition is not
self-evident. Hence it is that, as Boethius says (De Hebdom.), certain
axioms or propositions are universally self-evident to all; and such are
those propositions whose terms are known to all, as, “Every whole is
greater than its part,” and, “Things equal to one and the same are equal
to one another.” But some propositions are self-evident only to the wise,
who understand the meaning of the terms of such propositions: thus to one
who understands that an angel is not a body, it is self-evident that an
angel is not circumscriptively in a place: but this is not evident to the
unlearned, for they cannot grasp it.

Now a certain order is to be found in those things that are apprehended
universally. For that which, before aught else, falls under apprehension,
is “being,” the notion of which is included in all things whatsoever a
man apprehends. Wherefore the first indemonstrable principle is that “the
same thing cannot be affirmed and denied at the same time,” which is
based on the notion of “being” and “not-being”: and on this principle all
others are based, as is stated in Metaph. iv, text. 9. Now as “being” is
the first thing that falls under the apprehension simply, so “good” is
the first thing that falls under the apprehension of the practical
reason, which is directed to action: since every agent acts for an end
under the aspect of good. Consequently the first principle of practical
reason is one founded on the notion of good, viz. that “good is that
which all things seek after.” Hence this is the first precept of law,
that “good is to be done and pursued, and evil is to be avoided.” All
other precepts of the natural law are based upon this: so that whatever
the practical reason naturally apprehends as man’s good (or evil) belongs
to the precepts of the natural law as something to be done or avoided.

Since, however, good has the nature of an end, and evil, the nature of a
contrary, hence it is that all those things to which man has a natural
inclination, are naturally apprehended by reason as being good, and
consequently as objects of pursuit, and their contraries as evil, and
objects of avoidance. Wherefore according to the order of natural
inclinations, is the order of the precepts of the natural law. Because in
man there is first of all an inclination to good in accordance with the
nature which he has in common with all substances: inasmuch as every
substance seeks the preservation of its own being, according to its
nature: and by reason of this inclination, whatever is a means of
preserving human life, and of warding off its obstacles, belongs to the
natural law. Secondly, there is in man an inclination to things that
pertain to him more specially, according to that nature which he has in
common with other animals: and in virtue of this inclination, those
things are said to belong to the natural law, “which nature has taught to
all animals” [Pandect. Just. I, tit. i], such as sexual intercourse,
education of offspring and so forth. Thirdly, there is in man an
inclination to good, according to the nature of his reason, which nature
is proper to him: thus man has a natural inclination to know the truth
about God, and to live in society: and in this respect, whatever pertains
to this inclination belongs to the natural law; for instance, to shun
ignorance, to avoid offending those among whom one has to live, and other
such things regarding the above inclination.

Reply to Objection 1. All these precepts of the law of nature have the character
of one natural law, inasmuch as they flow from one first precept.

Reply to Objection 2. All the inclinations of any parts whatsoever of human
nature, e.g. of the concupiscible and irascible parts, in so far as they
are ruled by reason, belong to the natural law, and are reduced to one
first precept, as stated above: so that the precepts of the natural law
are many in themselves, but are based on one common foundation.

Reply to Objection 3. Although reason is one in itself, yet it directs all things
regarding man; so that whatever can be ruled by reason, is contained
under the law of reason.

ST II-II Q. 40 Art. 1

Whether it is always sinful to wage war?

Objection 1. It would seem that it is always sinful to wage war. Because
punishment is not inflicted except for sin. Now those who wage war are
threatened by Our Lord with punishment, according to Mt. 26:52: “All that
take the sword shall perish with the sword.” Therefore all wars are

Objection 2. Further, whatever is contrary to a Divine precept is a sin. But
war is contrary to a Divine precept, for it is written (Mt. 5:39): “But I
say to you not to resist evil”; and (Rm. 12:19): “Not revenging
yourselves, my dearly beloved, but give place unto wrath.” Therefore war
is always sinful.

Objection 3. Further, nothing, except sin, is contrary to an act of virtue.

But war is contrary to peace. Therefore war is always a sin.

Objection 4. Further, the exercise of a lawful thing is itself lawful, as is
evident in scientific exercises. But warlike exercises which take place
in tournaments are forbidden by the Church, since those who are slain in
these trials are deprived of ecclesiastical burial. Therefore it seems
that war is a sin in itself.

On the contrary, Augustine says in a sermon on the son of the centurion
[Ep. ad Marcel. cxxxviii]: “If the Christian Religion forbade war
altogether, those who sought salutary advice in the Gospel would rather
have been counselled to cast aside their arms, and to give up soldiering
altogether. On the contrary, they were told: ‘Do violence to no man . . .
and be content with your pay’ [Lk. 3:14. If he commanded them to be
content with their pay, he did not forbid soldiering.”

I answer that, In order for a war to be just, three things are
necessary. First, the authority of the sovereign by whose command the war
is to be waged. For it is not the business of a private individual to
declare war, because he can seek for redress of his rights from the
tribunal of his superior. Moreover it is not the business of a private
individual to summon together the people, which has to be done in
wartime. And as the care of the common weal is committed to those who are
in authority, it is their business to watch over the common weal of the
city, kingdom or province subject to them. And just as it is lawful for
them to have recourse to the sword in defending that common weal against
internal disturbances, when they punish evil-doers, according to the
words of the Apostle (Rm. 13:4): “He beareth not the sword in vain: for
he is God’s minister, an avenger to execute wrath upon him that doth
evil”; so too, it is their business to have recourse to the sword of war
in defending the common weal against external enemies. Hence it is said
to those who are in authority (Ps. 81:4): “Rescue the poor: and deliver
the needy out of the hand of the sinner”; and for this reason Augustine
says (Contra Faust. xxii, 75): “The natural order conducive to peace
among mortals demands that the power to declare and counsel war should be
in the hands of those who hold the supreme authority.”

Secondly, a just cause is required, namely that those who are attacked,
should be attacked because they deserve it on account of some fault.
Wherefore Augustine says (QQ. in Hept., qu. x, super Jos.): “A just war
is wont to be described as one that avenges wrongs, when a nation or
state has to be punished, for refusing to make amends for the wrongs
inflicted by its subjects, or to restore what it has seized unjustly.”

Thirdly, it is necessary that the belligerents should have a rightful
intention, so that they intend the advancement of good, or the avoidance
of evil. Hence Augustine says (De Verb. Dom. [The words quoted are to be
found not in St. Augustine’s works, but Can. Apud. Caus. xxiii, qu. 1):
“True religion looks upon as peaceful those wars that are waged not for
motives of aggrandizement, or cruelty, but with the object of securing
peace, of punishing evil-doers, and of uplifting the good.” For it may
happen that the war is declared by the legitimate authority, and for a
just cause, and yet be rendered unlawful through a wicked intention.
Hence Augustine says (Contra Faust. xxii, 74): “The passion for
inflicting harm, the cruel thirst for vengeance, an unpacific and
relentless spirit, the fever of revolt, the lust of power, and such like
things, all these are rightly condemned in war.”

Reply to Objection 1. As Augustine says (Contra Faust. xxii, 70): “To take the
sword is to arm oneself in order to take the life of anyone, without the
command or permission of superior or lawful authority.” On the other
hand, to have recourse to the sword (as a private person) by the
authority of the sovereign or judge, or (as a public person) through zeal
for justice, and by the authority, so to speak, of God, is not to “take
the sword,” but to use it as commissioned by another, wherefore it does
not deserve punishment. And yet even those who make sinful use of the
sword are not always slain with the sword, yet they always perish with
their own sword, because, unless they repent, they are punished eternally
for their sinful use of the sword.

Reply to Objection 2. Such like precepts, as Augustine observes (De Serm. Dom. in
Monte i, 19), should always be borne in readiness of mind, so that we be
ready to obey them, and, if necessary, to refrain from resistance or
self-defense. Nevertheless it is necessary sometimes for a man to act
otherwise for the common good, or for the good of those with whom he is
fighting. Hence Augustine says (Ep. ad Marcellin. cxxxviii): “Those whom
we have to punish with a kindly severity, it is necessary to handle in
many ways against their will. For when we are stripping a man of the
lawlessness of sin, it is good for him to be vanquished, since nothing is
more hopeless than the happiness of sinners, whence arises a guilty
impunity, and an evil will, like an internal enemy.”

Reply to Objection 3. Those who wage war justly aim at peace, and so they are not
opposed to peace, except to the evil peace, which Our Lord “came not to
send upon earth” (Mt. 10:34). Hence Augustine says (Ep. ad Bonif.
clxxxix): “We do not seek peace in order to be at war, but we go to war
that we may have peace. Be peaceful, therefore, in warring, so that you
may vanquish those whom you war against, and bring them to the prosperity
of peace.”

Reply to Objection 4. Manly exercises in warlike feats of arms are not all
forbidden, but those which are inordinate and perilous, and end in
slaying or plundering. On olden times warlike exercises presented no such
danger, and hence they were called “exercises of arms” or “bloodless
wars,” as Jerome states in an epistle [Reference incorrect: cf. Veget.,
De Re Milit. i].

ST II-II Q. 40 Art. 2

Whether it is lawful for clerics and bishops to fight?

Objection 1. It would seem lawful for clerics and bishops to fight. For, as
stated above (1), wars are lawful and just in so far as they protect
the poor and the entire common weal from suffering at the hands of the
foe. Now this seems to be above all the duty of prelates, for Gregory
says (Hom. in Ev. xiv): “The wolf comes upon the sheep, when any unjust
and rapacious man oppresses those who are faithful and humble. But he who
was thought to be the shepherd, and was not, leaveth the sheep, end
flieth, for he fears lest the wolf hurt him, and dares not stand up
against his injustice.” Therefore it is lawful for prelates and clerics
to fight.

Objection 2. Further, Pope Leo IV writes (xxiii, qu. 8, can. Igitur): “As
untoward tidings had frequently come from the Saracen side, some said
that the Saracens would come to the port of Rome secretly and covertly;
for which reason we commanded our people to gather together, and ordered
them to go down to the seashore.” Therefore it is lawful for bishops to

Objection 3. Further, apparently, it comes to the same whether a man does a
thing himself, or consents to its being done by another, according to Rm.
1:32: “They who do such things, are worthy of death, and not only they
that do them, but they also that consent to them that do them.” Now
those, above all, seem to consent to a thing, who induce others to do it.

But it is lawful for bishops and clerics to induce others to fight: for
it is written (xxiii, qu. 8, can. Hortatu) that Charles went to war with
the Lombards at the instance and entreaty of Adrian, bishop of Rome.
Therefore they also are allowed to fight.

Objection 4. Further, whatever is right and meritorious in itself, is lawful
for prelates and clerics. Now it is sometimes right and meritorious to
make war, for it is written (xxiii, qu. 8, can. Omni timore) that if “a
man die for the true faith, or to save his country, or in defense of
Christians, God will give him a heavenly reward.” Therefore it is lawful
for bishops and clerics to fight.

On the contrary, It was said to Peter as representing bishops and
clerics (Mt. 16:52): “Put up again thy sword into the scabbard [Vulg.:
‘its place’] [“Scabbard” is the reading in Jn. 18:11.” Therefore it is
not lawful for them to fight.

I answer that, Several things are requisite for the good of a human
society: and a number of things are done better and quicker by a number
of persons than by one, as the Philosopher observes a(Polit. i, 1), while
certain occupations are so inconsistent with one another, that they
cannot be fittingly exercised at the same time; wherefore those who are
deputed to important duties are forbidden to occupy themselves with
things of small importance. Thus according to human laws, soldiers who
are deputed to warlike pursuits are forbidden to engage in commerce
[Cod. xii, 35, De Re Milit.].

Now warlike pursuits are altogether incompatible with the duties of a
bishop and a cleric, for two reasons. The first reason is a general one,
because, to wit, warlike pursuits are full of unrest, so that they hinder
the mind very much from the contemplation of Divine things, the praise of
God, and prayers for the people, which belong to the duties of a cleric.
Wherefore just as commercial enterprises are forbidden to clerics,
because they unsettle the mind too much, so too are warlike pursuits,
according to 2 Tim. 2:4: “No man being a soldier to God, entangleth
himself with secular business.” The second reason is a special one,
because, to wit, all the clerical Orders are directed to the ministry of
the altar, on which the Passion of Christ is represented sacramentally,
according to 1 Cor. 11:26: “As often as you shall eat this bread, and
drink the chalice, you shall show the death of the Lord, until He come.”
Wherefore it is unbecoming for them to slay or shed blood, and it is more
fitting that they should be ready to shed their own blood for Christ, so
as to imitate in deed what they portray in their ministry. For this
reason it has been decreed that those who shed blood, even without sin,
become irregular. Now no man who has a certain duty to perform, can
lawfully do that which renders him unfit for that duty. Wherefore it is
altogether unlawful for clerics to fight, because war is directed to the
shedding of blood.

Reply to Objection 1. Prelates ought to withstand not only the wolf who brings
spiritual death upon the flock, but also the pillager and the oppressor
who work bodily harm; not, however, by having recourse themselves to
material arms, but by means of spiritual weapons, according to the saying
of the Apostle (2 Cor. 10:4): “The weapons of our warfare are not carnal,
but mighty through God.” Such are salutary warnings, devout prayers, and,
for those who are obstinate, the sentence of excommunication.

Reply to Objection 2. Prelates and clerics may, by the authority of their
superiors, take part in wars, not indeed by taking up arms themselves,
but by affording spiritual help to those who fight justly, by exhorting
and absolving them, and by other like spiritual helps. Thus in the Old
Testament (Joshua 6:4) the priests were commanded to sound the sacred
trumpets in the battle. It was for this purpose that bishops or clerics
were first allowed to go to the front: and it is an abuse of this
permission, if any of them take up arms themselves.

Reply to Objection 3. As stated above (23, 4, ad 2) every power, art or
virtue that regards the end, has to dispose that which is directed to the
end. Now, among the faithful, carnal wars should be considered as having
for their end the Divine spiritual good to which clerics are deputed.
Wherefore it is the duty of clerics to dispose and counsel other men to
engage in just wars. For they are forbidden to take up arms, not as
though it were a sin, but because such an occupation is unbecoming their

Reply to Objection 4. Although it is meritorious to wage a just war, nevertheless
it is rendered unlawful for clerics, by reason of their being deputed to
works more meritorious still. Thus the marriage act may be meritorious;
and yet it becomes reprehensible in those who have vowed virginity,
because they are bound to a yet greater good.

ST II-II Q. 64 Art. 3

Whether it is lawful for a private individual to kill a man who has

Objection 1. It would seem lawful for a private individual to kill a man who
has sinned. For nothing unlawful is commanded in the Divine law. Yet, on
account of the sin of the molten calf, Moses commanded (Ex. 32:27): “Let
every man kill his brother, and friend, and neighbor.” Therefore it is
lawful for private individuals to kill a sinner.

Objection 2. Further, as stated above (2, ad 3), man, on account of sin, is
compared to the beasts. Now it is lawful for any private individual to
kill a wild beast, especially if it be harmful. Therefore for the same
reason, it is lawful for any private individual to kill a man who has

Objection 3. Further, a man, though a private individual, deserves praise for
doing what is useful for the common good. Now the slaying of evildoers is
useful for the common good, as stated above (2). Therefore it is
deserving of praise if even private individuals kill evil-doers.

On the contrary, Augustine says (De Civ. Dei i) [Can. Quicumque
percutit, caus. xxiii, qu. 8: “A man who, without exercising public
authority, kills an evil-doer, shall be judged guilty of murder, and all
the more, since he has dared to usurp a power which God has not given

I answer that, As stated above (2), it is lawful to kill an evildoer
in so far as it is directed to the welfare of the whole community, so
that it belongs to him alone who has charge of the community’s welfare.
Thus it belongs to a physician to cut off a decayed limb, when he has
been entrusted with the care of the health of the whole body. Now the
care of the common good is entrusted to persons of rank having public
authority: wherefore they alone, and not private individuals, can
lawfully put evildoers to death.

Reply to Objection 1. The person by whose authority a thing is done really does
the thing as Dionysius declares (Coel. Hier. iii). Hence according to
Augustine (De Civ. Dei i, 21), “He slays not who owes his service to one
who commands him, even as a sword is merely the instrument to him that
wields it.” Wherefore those who, at the Lord’s command, slew their
neighbors and friends, would seem not to have done this themselves, but
rather He by whose authority they acted thus: just as a soldier slays the
foe by the authority of his sovereign, and the executioner slays the
robber by the authority of the judge.

Reply to Objection 2. A beast is by nature distinct from man, wherefore in the
case of a wild beast there is no need for an authority to kill it;
whereas, in the case of domestic animals, such authority is required, not
for their sake, but on account of the owner’s loss. On the other hand a
man who has sinned is not by nature distinct from good men; hence a
public authority is requisite in order to condemn him to death for the
common good.

Reply to Objection 3. It is lawful for any private individual to do anything for
the common good, provided it harm nobody: but if it be harmful to some
other, it cannot be done, except by virtue of the judgment of the person
to whom it pertains to decide what is to be taken from the parts for the
welfare of the whole.

ST II-II Q. 64 Art. 5

Whether it is lawful to kill oneself?

Objection 1. It would seem lawful for a man to kill himself. For murder is a
sin in so far as it is contrary to justice. But no man can do an
injustice to himself, as is proved in Ethic. v, 11. Therefore no man sins
by killing himself.

Objection 2. Further, it is lawful, for one who exercises public authority, to
kill evil-doers. Now he who exercises public authority is sometimes an
evil-doer. Therefore he may lawfully kill himself.

Objection 3. Further, it is lawful for a man to suffer spontaneously a lesser
danger that he may avoid a greater: thus it is lawful for a man to cut
off a decayed limb even from himself, that he may save his whole body.
Now sometimes a man, by killing himself, avoids a greater evil, for
example an unhappy life, or the shame of sin. Therefore a man may kill

Objection 4. Further, Samson killed himself, as related in Judges 16, and yet
he is numbered among the saints (Heb. 11). Therefore it is lawful for a
man to kill himself.

Objection 5. Further, it is related (2 Mach. 14:42) that a certain Razias
killed himself, “choosing to die nobly rather than to fall into the hands
of the wicked, and to suffer abuses unbecoming his noble birth.” Now
nothing that is done nobly and bravely is unlawful. Therefore suicide is
not unlawful.

On the contrary, Augustine says (De Civ. Dei i, 20): “Hence it follows
that the words ‘Thou shalt not kill’ refer to the killing of a man–not
another man; therefore, not even thyself. For he who kills himself, kills
nothing else than a man.”

I answer that, It is altogether unlawful to kill oneself, for three
reasons. First, because everything naturally loves itself, the result
being that everything naturally keeps itself in being, and resists
corruptions so far as it can. Wherefore suicide is contrary to the
inclination of nature, and to charity whereby every man should love
himself. Hence suicide is always a mortal sin, as being contrary to the
natural law and to charity. Secondly, because every part, as such,
belongs to the whole. Now every man is part of the community, and so, as
such, he belongs to the community. Hence by killing himself he injures
the community, as the Philosopher declares (Ethic. v, 11). Thirdly,
because life is God’s gift to man, and is subject to His power, Who kills
and makes to live. Hence whoever takes his own life, sins against God,
even as he who kills another’s slave, sins against that slave’s master,
and as he who usurps to himself judgment of a matter not entrusted to
him. For it belongs to God alone to pronounce sentence of death and life,
according to Dt. 32:39, “I will kill and I will make to live.”

Reply to Objection 1. Murder is a sin, not only because it is contrary to
justice, but also because it is opposed to charity which a man should
have towards himself: in this respect suicide is a sin in relation to
oneself. On relation to the community and to God, it is sinful, by reason
also of its opposition to justice.

Reply to Objection 2. One who exercises public authority may lawfully put to
death an evil-doer, since he can pass judgment on him. But no man is
judge of himself. Wherefore it is not lawful for one who exercises public
authority to put himself to death for any sin whatever: although he may
lawfully commit himself to the judgment of others.

Reply to Objection 3. Man is made master of himself through his free-will:
wherefore he can lawfully dispose of himself as to those matters which
pertain to this life which is ruled by man’s free-will. But the passage
from this life to another and happier one is subject not to man’s
free-will but to the power of God. Hence it is not lawful for man to take
his own life that he may pass to a happier life, nor that he may escape
any unhappiness whatsoever of the present life, because the ultimate and
most fearsome evil of this life is death, as the Philosopher states
(Ethic. iii, 6). Therefore to bring death upon oneself in order to
escape the other afflictions of this life, is to adopt a greater evil in
order to avoid a lesser. On like manner it is unlawful to take one’s own
life on account of one’s having committed a sin, both because by so doing
one does oneself a very great injury, by depriving oneself of the time
needful for repentance, and because it is not lawful to slay an evildoer
except by the sentence of the public authority. Again it is unlawful for
a woman to kill herself lest she be violated, because she ought not to
commit on herself the very great sin of suicide, to avoid the lesser sir;
of another. For she commits no sin in being violated by force, provided
she does not consent, since “without consent of the mind there is no
stain on the body,” as the Blessed Lucy declared. Now it is evident that
fornication and adultery are less grievous sins than taking a man’s,
especially one’s own, life: since the latter is most grievous, because
one injures oneself, to whom one owes the greatest love. Moreover it is
most dangerous since no time is left wherein to expiate it by repentance.
Again it is not lawful for anyone to take his own life for fear he should
consent to sin, because “evil must not be done that good may come” (Rm.
3:8) or that evil may be avoided especially if the evil be of small
account and an uncertain event, for it is uncertain whether one will at
some future time consent to a sin, since God is able to deliver man from
sin under any temptation whatever.

Reply to Objection 4. As Augustine says (De Civ. Dei i, 21), “not even Samson is
to be excused that he crushed himself together with his enemies under the
ruins of the house, except the Holy Ghost, Who had wrought many wonders
through him, had secretly commanded him to do this.” He assigns the same
reason in the case of certain holy women, who at the time of persecution
took their own lives, and who are commemorated by the Church.

Reply to Objection 5. It belongs to fortitude that a man does not shrink from
being slain by another, for the sake of the good of virtue, and that he
may avoid sin. But that a man take his own life in order to avoid penal
evils has indeed an appearance of fortitude (for which reason some, among
whom was Razias, have killed themselves thinking to act from fortitude),
yet it is not true fortitude, but rather a weakness of soul unable to
bear penal evils, as the Philosopher (Ethic. iii, 7) and Augustine (De
Civ. Dei 22,23) declare.

ST II-II Q. 64 Art. 7

Whether it is lawful to kill a man in self-defense?

Objection 1. It would seem that nobody may lawfully kill a man in
self-defense. For Augustine says to Publicola (Ep. xlvii): “I do not
agree with the opinion that one may kill a man lest one be killed by him;
unless one be a soldier, exercise a public office, so that one does it
not for oneself but for others, having the power to do so, provided it be
in keeping with one’s person.” Now he who kills a man in self-defense,
kills him lest he be killed by him. Therefore this would seem to be

Objection 2. Further, he says (De Lib. Arb. i, 5): “How are they free from sin
in sight of Divine providence, who are guilty of taking a man’s life for
the sake of these contemptible things?” Now among contemptible things he
reckons “those which men may forfeit unwillingly,” as appears from the
context (De Lib. Arb. i, 5): and the chief of these is the life of the
body. Therefore it is unlawful for any man to take another’s life for the
sake of the life of his own body.

Objection 3. Further, Pope Nicolas [Nicolas I, Dist. 1, can. De his clericis]
says in the Decretals: “Concerning the clerics about whom you have
consulted Us, those, namely, who have killed a pagan in self-defense, as
to whether, after making amends by repenting, they may return to their
former state, or rise to a higher degree; know that in no case is it
lawful for them to kill any man under any circumstances whatever.” Now
clerics and laymen are alike bound to observe the moral precepts.
Therefore neither is it lawful for laymen to kill anyone in self-defense.

Objection 4. Further, murder is a more grievous sin than fornication or
adultery. Now nobody may lawfully commit simple fornication or adultery
or any other mortal sin in order to save his own life; since the
spiritual life is to be preferred to the life of the body. Therefore no
man may lawfully take another’s life in self-defense in order to save his
own life.

Objection 5. Further, if the tree be evil, so is the fruit, according to Mt.
7:17. Now self-defense itself seems to be unlawful, according to Rm.
12:19: “Not defending [Douay: ‘revenging’] yourselves, my dearly
beloved.” Therefore its result, which is the slaying of a man, is also

On the contrary, It is written (Ex. 22:2): “If a thief be found
breaking into a house or undermining it, and be wounded so as to die; he
that slew him shall not be guilty of blood.” Now it is much more lawful
to defend one’s life than one’s house. Therefore neither is a man guilty
of murder if he kill another in defense of his own life.

I answer that, Nothing hinders one act from having two effects, only one
of which is intended, while the other is beside the intention. Now moral
acts take their species according to what is intended, and not according
to what is beside the intention, since this is accidental as explained
above (43, 3; I-II, 12, 1). Accordingly the act of self-defense
may have two effects, one is the saving of one’s life, the other is the
slaying of the aggressor. Therefore this act, since one’s intention is to
save one’s own life, is not unlawful, seeing that it is natural to
everything to keep itself in “being,” as far as possible. And yet, though
proceeding from a good intention, an act may be rendered unlawful, if it
be out of proportion to the end. Wherefore if a man, in self-defense,
uses more than necessary violence, it will be unlawful: whereas if he
repel force with moderation his defense will be lawful, because according
to the jurists [Cap. Significasti, De Homicid. volunt. vel casual.], “it
is lawful to repel force by force, provided one does not exceed the
limits of a blameless defense.” Nor is it necessary for salvation that a
man omit the act of moderate self-defense in order to avoid killing the
other man, since one is bound to take more care of one’s own life than of
another’s. But as it is unlawful to take a man’s life, except for the
public authority acting for the common good, as stated above (3), it
is not lawful for a man to intend killing a man in self-defense, except
for such as have public authority, who while intending to kill a man in
self-defense, refer this to the public good, as in the case of a soldier
fighting against the foe, and in the minister of the judge struggling
with robbers, although even these sin if they be moved by private

Reply to Objection 1. The words quoted from Augustine refer to the case when one
man intends to kill another to save himself from death. The passage
quoted in the Second Objection is to be understood in the same sense.
Hence he says pointedly, “for the sake of these things,” whereby he
indicates the intention. This suffices for the Reply to the Second

Reply to Objection 3. Irregularity results from the act though sinless of taking
a man’s life, as appears in the case of a judge who justly condemns a man
to death. For this reason a cleric, though he kill a man in self-defense,
is irregular, albeit he intends not to kill him, but to defend himself.

Reply to Objection 4. The act of fornication or adultery is not necessarily
directed to the preservation of one’s own life, as is the act whence
sometimes results the taking of a man’s life.

Reply to Objection 5. The defense forbidden in this passage is that which comes
from revengeful spite. Hence a gloss says: “Not defending
yourselves–that is, not striking your enemy back.”

The Summa Theologica of St. Thomas Aquinas
Second and Revised Edition, 1920
Literally translated by Fathers of the English Dominican Province
Online Edition Copyright © 2000 by Kevin Knight
Nihil Obstat. F. Innocentius Apap, O.P., S.T.M., Censor. Theol.
Imprimatur. Edus. Canonicus Surmont, Vicarius Generalis. Westmonasterii.
Nihil Obstat. F. Raphael Moss, O.P., S.T.L. and F. Leo Moore, O.P., S.T.L.
Imprimatur. F. Beda Jarrett, O.P., S.T.L., A.M., Prior Provincialis Angliæ