In ST II-II, Q. 66, article 5, “Whether theft is always a sin?”, reply to objection 2, Aquinas touches on the question of previously unappropriated goods in a way that anticipates John Locke’s later “homesteading” or “labor” theory of property or ownership (from his Second Treatise on Government). First, the objection, then Aquinas’s reply:
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.
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 hand 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).
The Natural Law Libertarian 