Summa Th. II-II EN Qu.64 a.8

Whether one is guilty of murder through killing someone by chance?

Objection: 1. It would seem that one is guilty of murder through killing someone by chance. For we read (Gn 4,23-24) that Lamech slew a man in mistake for a wild beast [*The text of the Bible does not say so, but this was the Jewish traditional commentary on Gn 4,23], and that he was accounted guilty of murder. Therefore one incurs the guilt of murder through killing a man by chance.
2. Further, it is written (Ex 21,22): "If . . . one strike a woman with child, and she miscarry indeed . . . if her death ensue thereupon, he shall render life for life." Yet this may happen without any intention of causing her death. Therefore one is guilty of murder through killing someone by chance.
3. Further, the Decretals [*Dist. 1] contain several canons prescribing penalties for unintentional homicide. Now penalty is not due save for guilt. Therefore he who kills a man by chance, incurs the guilt of murder.

On the contrary Augustine says to Publicola (Ep. xlvii): "When we do a thing for a good and lawful purpose, if thereby we unintentionally cause harm to anyone, it should by no means be imputed to us." Now it sometimes happens by chance that a person is killed as a result of something done for a good purpose. Therefore the person who did it is not accounted guilty.
I answer that According to the Philosopher (Phys. ii, 6) "chance is a cause that acts beside one's intention." Hence chance happenings, strictly speaking, are neither intended nor voluntary. And since every sin is voluntary, according to Augustine (De Vera Relig. xiv) it follows that chance happenings, as such, are not sins.Nevertheless it happens that what is not actually and directly voluntary and intended, is voluntary and intended accidentally, according as that which removes an obstacle is called an accidental cause. Wherefore he who does not remove something whence homicide results whereas he ought to remove it, is in a sense guilty of voluntary homicide. This happens in two ways: first when a man causes another's death through occupying himself with unlawful things which he ought to avoid: secondly, when he does not take sufficient care. Hence, according to jurists, if a man pursue a lawful occupation and take due care, the result being that a person loses his life, he is not guilty of that person's death: whereas if he be occupied with something unlawful, or even with something lawful, but without due care, he does not escape being guilty of murder, if his action results in someone's death.

Reply to Objection: 1. Lamech did not take sufficient care to avoid taking a man's life: and so he was not excused from being guilty of homicide.
2. He that strikes a woman with child does something unlawful: wherefore if there results the death either of the woman or of the animated fetus, he will not be excused from homicide, especially seeing that death is the natural result of such a blow.
3. According to the canons a penalty, is inflicted on those who cause death unintentionally, through doing something unlawful, or failing to take sufficient care.


We must now consider other sinful injuries committed on the person. Under this head there are four points of inquiry:

(1) The mutilation of members;

(2) Blows;

(3) Imprisonment;

(4) Whether the sins that consist in inflicting such like injuries are aggravated through being perpetrated on persons connected with others?

Whether in some cases it may be lawful to maim anyone?

Objection: 1. It would seem that in no case can it be lawful to maim anyone. For Damascene says (De Fide Orth. iv, 20) that "sin consists in departing from what is according to nature, towards that which is contrary to nature." Now according to nature it is appointed by God that a man's body should be entire in its members, and it is contrary to nature that it should be deprived of a member. Therefore it seems that it is always a sin to maim a person.
2. Further, as the whole soul is to the whole body, so are the parts of the soul to the parts of the body (De Anima ii, 1). But it is unlawful to deprive a man of his soul by killing him, except by public authority. Therefore neither is it lawful to maim anyone, except perhaps by public authority.
3. Further, the welfare of the soul is to be preferred to the welfare of the body. Now it is not lawful for a man to maim himself for the sake of the soul's welfare: since the council of Nicea [*P. I, sect. 4, can. i] punished those who castrated themselves that they might preserve chastity. Therefore it is not lawful for any other reason to maim a person.

On the contrary It is written (Ex 21,24): "Eye for eye, tooth for tooth, hand for hand, foot for foot."
I answer that Since a member is part of the whole human body, it is for the sake of the whole, as the imperfect for the perfect. Hence a member of the human body is to be disposed of according as it is expedient for the body. Now a member of the human body is of itself useful to the good of the whole body, yet, accidentally it may happen to be hurtful, as when a decayed member is a source of corruption to the whole body. Accordingly so long as a member is healthy and retains its natural disposition, it cannot be cut off without injury to the whole body. But as the whole of man is directed as to his end to the whole of the community of which he is a part, as stated above (Question [61], Article [1]; Question [64], Articles [2],5), it may happen that although the removal of a member may be detrimental to the whole body, it may nevertheless be directed to the good of the community, in so far as it is applied to a person as a punishment for the purpose of restraining sin. Hence just as by public authority a person is lawfully deprived of life altogether on account of certain more heinous sins, so is he deprived of a member on account of certain lesser sins. But this is not lawful for a private individual, even with the consent of the owner of the member, because this would involve an injury to the community, to whom the man and all his parts belong. If, however, the member be decayed and therefore a source of corruption to the whole body, then it is lawful with the consent of the owner of the member, to cut away the member for the welfare of the whole body, since each one is entrusted with the care of his own welfare. The same applies if it be done with the consent of the person whose business it is to care for the welfare of the person who has a decayed member: otherwise it is altogether unlawful to maim anyone.

Reply to Objection: 1. Nothing prevents that which is contrary to a particular nature from being in harmony with universal nature: thus death and corruption, in the physical order, are contrary to the particular nature of the thing corrupted, although they are in keeping with universal nature. In like manner to maim anyone, though contrary to the particular nature of the body of the person maimed, is nevertheless in keeping with natural reason in relation to the common good.
2. The life of the entire man is not directed to something belonging to man; on the contrary whatever belongs to man is directed to his life. Hence in no case does it pertain to a person to take anyone's life, except to the public authority to whom is entrusted the procuring of the common good. But the removal of a member can be directed to the good of one man, and consequently in certain cases can pertain to him.
3. A member should not be removed for the sake of the bodily health of the whole, unless otherwise nothing can be done to further the good of the whole. Now it is always possible to further one's spiritual welfare otherwise than by cutting off a member, because sin is always subject to the will: and consequently in no case is it allowable to maim oneself, even to avoid any sin whatever. Hence Chrysostom, in his exposition on Mt 19,12 (Hom. lxii in Matth.), "There are eunuchs who have made themselves eunuchs for the kingdom of heaven," says: "Not by maiming themselves, but by destroying evil thoughts, for a man is accursed who maims himself, since they are murderers who do such things." And further on he says: "Nor is lust tamed thereby, on the contrary it becomes more importunate, for the seed springs in us from other sources, and chiefly from an incontinent purpose and a careless mind: and temptation is curbed not so much by cutting off a member as by curbing one's thoughts."

Whether it is lawful for parents to strike their children, or masters their slaves?

Objection: 1. It would seem unlawful for parents to strike their children, or masters their slaves. For the Apostle says (Ep 6,4): "You, fathers, provoke not your children to anger"; and further on (Ep 9,6): "And you, masters, do the same thing to your slaves [Vulg.: 'to them'] forbearing threatenings." Now some are provoked to anger by blows, and become more troublesome when threatened. Therefore neither should parents strike their children, nor masters their slaves.
2. Further, the Philosopher says (Ethic. x, 9) that "a father's words are admonitory and not coercive." Now blows are a kind of coercion. Therefore it is unlawful for parents to strike their children.
3. Further, everyone is allowed to impart correction, for this belongs to the spiritual almsdeeds, as stated above (Question [32], Article [2]). If, therefore, it is lawful for parents to strike their children for the sake of correction, for the same reason it will be lawful for any person to strike anyone, which is clearly false. Therefore the same conclusion follows.

On the contrary It is written (Pr 13,24): "He that spareth the rod hateth his son," and further on (Pr 23,13): "Withhold not correction from a child, for if thou strike him with the rod, he shall not die. Thou shalt beat him with the rod, and deliver his soul from hell." Again it is written (Si 33,28): "Torture and fetters are for a malicious slave."
I answer that Harm is done a body by striking it, yet not so as when it is maimed: since maiming destroys the body's integrity, while a blow merely affects the sense with pain, wherefore it causes much less harm than cutting off a member. Now it is unlawful to do a person a harm, except by way of punishment in the cause of justice. Again, no man justly punishes another, except one who is subject to his jurisdiction. Therefore it is not lawful for a man to strike another, unless he have some power over the one whom he strikes. And since the child is subject to the power of the parent, and the slave to the power of his master, a parent can lawfully strike his child, and a master his slave that instruction may be enforced by correction.

Reply to Objection: 1. Since anger is a desire for vengeance, it is aroused chiefly when a man deems himself unjustly injured, as the Philosopher states (Rhet. ii). Hence when parents are forbidden to provoke their children to anger, they are not prohibited from striking their children for the purpose of correction, but from inflicting blows on them without moderation. The command that masters should forbear from threatening their slaves may be understood in two ways. First that they should be slow to threaten, and this pertains to the moderation of correction; secondly, that they should not always carry out their threats, that is that they should sometimes by a merciful forgiveness temper the judgment whereby they threatened punishment.
2. The greater power should exercise the greater coercion. Now just as a city is a perfect community, so the governor of a city has perfect coercive power: wherefore he can inflict irreparable punishments such as death and mutilation. On the other hand the father and the master who preside over the family household, which is an imperfect community, have imperfect coercive power, which is exercised by inflicting lesser punishments, for instance by blows, which do not inflict irreparable harm.
3. It is lawful for anyone to impart correction to a willing subject. But to impart it to an unwilling subject belongs to those only who have charge over him. To this pertains chastisement by blows.

Whether it is lawful to imprison a man?

Objection: 1. It would seem unlawful to imprison a man. An act which deals with undue matter is evil in its genus, as stated above (FS, Question [18], Article [2]). Now man, having a free-will, is undue matter for imprisonment which is inconsistent with free-will. Therefore it is unlawful to imprison a man.
2. Further, human justice should be ruled by Divine justice. Now according to Si 15,14, "God left man in the hand of his own counsel." Therefore it seems that a man ought not to be coerced by chains or prisons.
3. Further, no man should be forcibly prevented except from doing an evil deed; and any man can lawfully prevent another from doing this. If, therefore, it were lawful to imprison a man, in order to restrain him from evil deeds, it would be lawful for anyone to put a man in prison; and this is clearly false. Therefore the same conclusion follows.

On the contrary We read in Lv 24 that a man was imprisoned for the sin of blasphemy.
I answer that In the goods three things may be considered in due order. First, the substantial integrity of the body, and this is injured by death or maiming. Secondly, pleasure or rest of the senses, and to this striking or anything causing a sense of pain is opposed. Thirdly, the movement or use of the members, and this is hindered by binding or imprisoning or any kind of detention.Therefore it is unlawful to imprison or in any way detain a man, unless it be done according to the order of justice, either in punishment, or as a measure of precaution against some evil.

Reply to Objection: 1. A man who abuses the power entrusted to him deserves to lose it, and therefore when a man by sinning abuses the free use of his members, he becomes a fitting matter for imprisonment.
2. According to the order of His wisdom God sometimes restrains a sinner from accomplishing a sin, according to Jb 5,12: "Who bringeth to nought the designs of the malignant, so that their hand cannot accomplish what they had begun, while sometimes He allows them to do what they will." In like manner, according to human justice, men are imprisoned, not for every sin but for certain ones.
3. It is lawful for anyone to restrain a man for a time from doing some unlawful deed there and then: as when a man prevents another from throwing himself over a precipice, or from striking another. But to him alone who has the right of disposing in general of the actions and of the life of another does it belong primarily to imprison or fetter, because by so doing he hinders him from doing not only evil but also good deeds.

Whether the sin is aggravated by the fact that the aforesaid injuries are perpetrated on those who are connected with others?

Objection: 1. It would seem that the sin is not aggravated by the fact that the aforesaid injuries are perpetrated on those who are connected with others. Such like injuries take their sinful character from inflicting an injury on another against his will. Now the evil inflicted on a man's own person is more against his will than that which is inflicted on a person connected with him. Therefore an injury inflicted on a person connected with another is less grievous.
2. Further, Holy Writ reproves those especially who do injuries to orphans and widows: hence it is written (Si 35,17): "He will not despise the prayers of the fatherless, nor the widow when she poureth out her complaint." Now the widow and the orphan are not connected with other persons. Therefore the sin is not aggravated through an injury being inflicted on one who is connected with others.
3. Further, the person who is connected has a will of his own just as the principal person has, so that something may be voluntary for him and yet against the will of the principal person, as in the case of adultery which pleases the woman but not the husband. Now these injuries are sinful in so far as they consist in an involuntary commutation. Therefore such like injuries are of a less sinful nature.

On the contrary It is written (Dt 28,32) as though indicating an aggravating circumstance: "Thy sons and thy daughters shall be given to another people, thy eyes looking on [*Vulg.: 'May thy sons and thy daughters be given,' etc.]."
I answer that Other things being equal, an injury is a more grievous sin according as it affects more persons; and hence it is that it is a more grievous sin to strike or injure a person in authority than a private individual, because it conduces to the injury of the whole community, as stated above (FS, Question [73], Article [9]). Now when an injury is inflicted on one who is connected in any way with another, that injury affects two persons, so that, other things being equal, the sin is aggravated by this very fact. It may happen, however, that in view of certain circumstances, a sin committed against one who is not connected with any other person, is more grievous, on account of either the dignity of the person, or the greatness of the injury.

Reply to Objection: 1. An injury inflicted on a person connected with others is less harmful to the persons with whom he is connected, than if it were perpetrated immediately on them, and from this point of view it is a less grievous sin. But all that belongs to the injury of the person with whom he is connected, is added to the sin of which a man is guilty through injuring the other one in himself.
2. Injuries done to widows and orphans are more insisted upon both through being more opposed to mercy, and because the same injury done to such persons is more grievous to them since they have no one to turn to for relief.
3. The fact that the wife voluntarily consents to the adultery, lessens the sin and injury, so far as the woman is concerned, for it would be more grievous, if the adulterer oppressed her by violence. But this does not remove the injury as affecting her husband, since "the wife hath not power of her own body; but the husband" (1Co 7,4). The same applies to similar cases. of adultery, however, as it is opposed not only to justice but also to chastity, we shall speak in the treatise on Temperance (Question [154], Article [8]).


We must now consider the sins opposed to justice, whereby a man injures his neighbor in his belongings; namely theft and robbery.

Under this head there are nine points of inquiry:

(1) Whether it is natural to man to possess external things?

(2) Whether it is lawful for a man to possess something as his own?

(3) Whether theft is the secret taking of another's property?

(4) Whether robbery is a species of sin distinct from theft?

(5) Whether every theft is a sin?

(6) Whether theft is a mortal sin?

(7) Whether it is lawful to thieve in a case of necessity?

(8) Whether every robbery is a mortal sin?

(9) Whether robbery is a more grievous sin than theft?

Whether it is natural for man to possess external things?

Objection: 1. It would seem that it is not natural for man to possess external things. For no man should ascribe to himself that which is God's. Now the dominion over all creatures is proper to God, according to Ps 23,1, "The earth is the Lord's," etc. Therefore it is not natural for man to possess external things.
2. Further, Basil in expounding the words of the rich man (Lc 12,18), "I will gather all things that are grown to me, and my goods," says [*Hom. in Luc. xii, 18]: "Tell me: which are thine? where did you take them from and bring them into being?" Now whatever man possesses naturally, he can fittingly call his own. Therefore man does not naturally possess external things.
3. Further, according to Ambrose (De Trin. i [*De Fide, ad Gratianum, i, 1]) "dominion denotes power." But man has no power over external things, since he can work no change in their nature. Therefore the possession of external things is not natural to man.

On the contrary It is written (Ps 8,8): "Thou hast subjected all things under his feet."
I answer that External things can be considered in two ways. First, as regards their nature, and this is not subject to the power of man, but only to the power of God Whose mere will all things obey. Secondly, as regards their use, and in this way, man has a natural dominion over external things, because, by his reason and will, he is able to use them for his own profit, as they were made on his account: for the imperfect is always for the sake of the perfect, as stated above (Question [64], Article [1]). It is by this argument that the Philosopher proves (Polit. i, 3) that the possession of external things is natural to man. Moreover, this natural dominion of man over other creatures, which is competent to man in respect of his reason wherein God's image resides, is shown forth in man's creation (Gn 1,26) by the words: "Let us make man to our image and likeness: and let him have dominion over the fishes of the sea," etc.

Reply to Objection: 1. God has sovereign dominion over all things: and He, according to His providence, directed certain things to the sustenance of man's body. For this reason man has a natural dominion over things, as regards the power to make use of them.
2. The rich man is reproved for deeming external things to belong to him principally, as though he had not received them from another, namely from God.
3. This argument considers the dominion over external things as regards their nature. Such a dominion belongs to God alone, as stated above.

Whether it is lawful for a man to possess a thing as his own?

Objection: 1. It would seem unlawful for a man to possess a thing as his own. For whatever is contrary to the natural law is unlawful. Now according to the natural law all things are common property: and the possession of property is contrary to this community of goods. Therefore it is unlawful for any man to appropriate any external thing to himself.
2. Further, Basil in expounding the words of the rich man quoted above (Article [1], Objection [2]), says: "The rich who deem as their own property the common goods they have seized upon, are like to those who by going beforehand to the play prevent others from coming, and appropriate to themselves what is intended for common use." Now it would be unlawful to prevent others from obtaining possession of common goods. Therefore it is unlawful to appropriate to oneself what belongs to the community.
3. Further, Ambrose says [*Serm. lxiv, de temp.], and his words are quoted in the Decretals [*Dist. xlvii., Can. Sicut hi.]: "Let no man call his own that which is common property": and by "common" he means external things, as is clear from the context. Therefore it seems unlawful for a man to appropriate an external thing to himself.

On the contrary Augustine says (De Haeres., haer. 40): "The 'Apostolici' are those who with extreme arrogance have given themselves that name, because they do not admit into their communion persons who are married or possess anything of their own, such as both monks and clerics who in considerable number are to be found in the Catholic Church." Now the reason why these people are heretics was because severing themselves from the Church, they think that those who enjoy the use of the above things, which they themselves lack, have no hope of salvation. Therefore it is erroneous to maintain that it is unlawful for a man to possess property.
I answer that Two things are competent to man in respect of exterior things. One is the power to procure and dispense them, and in this regard it is lawful for man to possess property. Moreover this is necessary to human life for three reasons. First because every man is more careful to procure what is for himself alone than that which is common to many or to all: since each one would shirk the labor and leave to another that which concerns the community, as happens where there is a great number of servants. Secondly, because human affairs are conducted in more orderly fashion if each man is charged with taking care of some particular thing himself, whereas there would be confusion if everyone had to look after any one thing indeterminately. Thirdly, because a more peaceful state is ensured to man if each one is contented with his own. Hence it is to be observed that quarrels arise more frequently where there is no division of the things possessed.The second thing that is competent to man with regard to external things is their use. In this respect man ought to possess external things, not as his own, but as common, so that, to wit, he is ready to communicate them to others in their need. Hence the Apostle says (1Tm 6,17-18): "Charge the rich of this world . . . to give easily, to communicate to others," etc.

Reply to Objection: 1. Community of goods is ascribed to the natural law, not that the natural law dictates that all things should be possessed in common and that nothing should be possessed as one's own: but because the division of possessions is not according to the natural law, but rather arose from human agreement which belongs to positive law, as stated above (Question [57], Articles [2],3). Hence the ownership of possessions is not contrary to the natural law, but an addition thereto devised by human reason.
2. A man would not act unlawfully if by going beforehand to the play he prepared the way for others: but he acts unlawfully if by so doing he hinders others from going. In like manner a rich man does not act unlawfully if he anticipates someone in taking possession of something which at first was common property, and gives others a share: but he sins if he excludes others indiscriminately from using it. Hence Basil says (Hom. in Luc. xii, 18): "Why are you rich while another is poor, unless it be that you may have the merit of a good stewardship, and he the reward of patience?"
3. When Ambrose says: "Let no man call his own that which is common," he is speaking of ownership as regards use, wherefore he adds: "He who spends too much is a robber."

Whether the essence of theft consists in taking another's thing secretly?

Objection: 1. It would seem that it is not essential to theft to take another's thing secretly. For that which diminishes a sin, does not, apparently, belong to the essence of a sin. Now to sin secretly tends to diminish a sin, just as, on the contrary, it is written as indicating an aggravating circumstance of the sin of some (Is 3,9): "They have proclaimed abroad their sin as Sodom, and they have not hid it." Therefore it is not essential to theft that it should consist in taking another's thing secretly.
2. Further, Ambrose says [*Serm. lxiv, de temp., Article [2], Objection [3], Can. Sicut hi.]: and his words are embodied in the Decretals [*Dist. xlvii]: "It is no less a crime to take from him that has, than to refuse to succor the needy when you can and are well off." Therefore just as theft consists in taking another's thing, so does it consist in keeping it back.
3. Further, a man may take by stealth from another, even that which is his own, for instance a thing that he has deposited with another, or that has been taken away from him unjustly. Therefore it is not essential to theft that it should consist in taking another's thing secretly.

On the contrary Isidore says (Etym. x): "'Fur' [thief] is derived from 'furvus' and so from 'fuscus' [dark], because he takes advantage of the night."
I answer that Three things combine together to constitute theft. The first belongs to theft as being contrary to justice, which gives to each one that which is his, so that it belongs to theft to take possession of what is another's. The second thing belongs to theft as distinct from those sins which are committed against the person, such as murder and adultery, and in this respect it belongs to theft to be about a thing possessed: for if a man takes what is another's not as a possession but as a part (for instance, if he amputates a limb), or as a person connected with him (for instance, if he carry off his daughter or his wife), it is not strictly speaking a case of theft. The third difference is that which completes the nature of theft, and consists in a thing being taken secretly: and in this respect it belongs properly to theft that it consists in "taking another's thing secretly."

Reply to Objection: 1. Secrecy is sometimes a cause of sin, as when a man employs secrecy in order to commit a sin, for instance in fraud and guile. In this way it does not diminish sin, but constitutes a species of sin: and thus it is in theft. In another way secrecy is merely a circumstance of sin, and thus it diminishes sin, both because it is a sign of shame, and because it removes scandal.
2. To keep back what is due to another, inflicts the same kind of injury as taking a thing unjustly: wherefore an unjust detention is included in an unjust taking.
3. Nothing prevents that which belongs to one person simply, from belonging to another in some respect: thus a deposit belongs simply to the depositor, but with regard to its custody it is the depositary's, and the thing stolen is the thief's, not simply, but as regards its custody.

Whether theft and robbery are sins of different species?

Objection: 1. It would seem that theft and robbery are not sins of different species. For theft and robbery differ as "secret" and "manifest": because theft is taking something secretly, while robbery is to take something violently and openly. Now in the other kinds of sins, the secret and the manifest do not differ specifically. Therefore theft and robbery are not different species of sin.
2. Further, moral actions take their species from the end, as stated above (FS, Question [1], Article [3]; Question [18], Article [6]). Now theft and robbery are directed to the same end, viz. the possession of another's property. Therefore they do not differ specifically.
3. Further, just as a thing is taken by force for the sake of possession, so is a woman taken by force for pleasure: wherefore Isidore says (Etym. x) that "he who commits a rape is called a corrupter, and the victim of the rape is said to be corrupted." Now it is a case of rape whether the woman be carried off publicly or secretly. Therefore the thing appropriated is said to be taken by force, whether it be done secretly or publicly. Therefore theft and robbery do not differ.

On the contrary The Philosopher (Ethic. v, 2) distinguishes theft from robbery, and states that theft is done in secret, but that robbery is done openly.
I answer that Theft and robbery are vices contrary to justice, in as much as one man does another an injustice. Now "no man suffers an injustice willingly," as stated in Ethic. v, 9. Wherefore theft and robbery derive their sinful nature, through the taking being involuntary on the part of the person from whom something is taken. Now the involuntary is twofold, namely, through violence and through ignorance, as stated in Ethic. iii, 1. Therefore the sinful aspect of robbery differs from that of theft: and consequently they differ specifically.

Reply to Objection: 1. In the other kinds of sin the sinful nature is not derived from something involuntary, as in the sins opposed to justice: and so where there is a different kind of involuntary, there is a different species of sin.
2. The remote end of robbery and theft is the same. But this is not enough for identity of species, because there is a difference of proximate ends, since the robber wishes to take a thing by his own power, but the thief, by cunning.
3. The robbery of a woman cannot be secret on the part of the woman who is taken: wherefore even if it be secret as regards the others from whom she is taken, the nature of robbery remains on the part of the woman to whom violence is done.

Whether theft is always a sin?

Objection: 1. It would seem that theft is not always a sin. For no sin is commanded by God, since it is written (Si 15,21): "He hath commanded no man to do wickedly." Yet we find that God commanded theft, for it is written (Ex 12,35-36): "And the children of Israel did as the Lord had commanded Moses [Vulg.: 'as Moses had commanded']. . . and they stripped the Egyptians." Therefore theft is not always a sin.
2. Further, if a man finds a thing that is not his and takes it, he seems to commit a theft, for he takes another's property. Yet this seems lawful according to natural equity, as the jurists hold. [*See loc. cit. in Reply.] Therefore it seems that theft is not always a sin.
3. Further, he that takes what is his own does not seem to sin, because he does not act against justice, since he does not destroy its equality. Yet a man commits a theft even if he secretly take his own property that is detained by or in the safe-keeping of another. Therefore it seems that theft is not always a sin.

On the contrary It is written (Ex 20,15): "Thou shalt not steal."
I answer that If anyone consider what is meant by theft, he will find that it is sinful on two counts. First, because of its opposition to justice, which gives to each one what is his, so that for this reason theft is contrary to justice, through being a taking of what belongs to another. Secondly, because of the guile or fraud committed by the thief, by laying hands on another's property secretly and cunningly. Wherefore it is evident that every theft is a sin.

Reply to Objection: 1. It is no theft for a man to take another's property either secretly or openly by order of a judge who has commanded him to do so, because it becomes his due by the very fact that it is adjudicated to him by the sentence of the court. Hence still less was it a theft for the Israelites to take away the spoils of the Egyptians at the command of the Lord, Who ordered this to be done on account of the ill-treatment accorded to them by the Egyptians without any cause: wherefore it is written significantly (Sg 10,19): "The just took the spoils of the wicked."
2. With regard to treasure-trove a distinction must be made. For some there are that were never in anyone's possession, for instance precious stones and jewels, found on the seashore, and such the finder is allowed to keep [*Dig. I, viii, De divis. rerum: Inst. II, i, De rerum divis.]. The same applies to treasure hidden underground long since and belonging to no man, except that according to civil law the finder is bound to give half to the owner of the land, if the treasure trove be in the land of another person [*Inst. II, i, 39: Cod. X, xv, De Thesauris]. Hence in the parable of the Gospel (Mt 13,44) it is said of the finder of the treasure hidden in a field that he bought the field, as though he purposed thus to acquire the right of possessing the whole treasure. On the other Land the treasure-trove may be nearly in someone's possession: and then if anyone take it with the intention, not of keeping it but of returning it to the owner who does not look upon such things as unappropriated, he is not guilty of theft. In like manner if the thing found appears to be unappropriated, and if the finder believes it to be so, although he keep it, he does not commit a theft [*Inst. II, i, 47]. In any other case the sin of theft is committed [*Dig. XLI, i, De acquirend, rerum dominio, 9: Inst. II, i, 48]: wherefore Augustine says in a homily (Serm. clxxviii; De Verb. Apost.): "If thou hast found a thing and not returned it, thou hast stolen it" (Dig. xiv, 5, can. Si quid invenisti).
3. He who by stealth takes his own property which is deposited with another man burdens the depositary, who is bound either to restitution, or to prove himself innocent. Hence he is clearly guilty of sin, and is bound to ease the depositary of his burden. On the other hand he who, by stealth, takes his own property, if this be unjustly detained by another, he sins indeed; yet not because he burdens the retainer, and so he is not bound to restitution or compensation: but he sins against general justice by disregarding the order of justice and usurping judgment concerning his own property. Hence he must make satisfaction to God and endeavor to allay whatever scandal he may have given his neighbor by acting this way.

Summa Th. II-II EN Qu.64 a.8