Objection 1: It would seem that theft is not always a sin. For no sin is commanded by God, since it is written (Ecclus. 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.
Obj. 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.
Obj. 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 Obj. 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 (Wis. 10:19): "The just took the spoils of the wicked."
Reply Obj. 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 (Matt. 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).
Reply Obj. 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.