St. Thomas raises the question of private property in the sixty-sixth question of the Secunda Secundæ. The question under which the subsequent articles are organized purports to deal with the issues of theft and robbery. As always, Thomas recognizes that he must start right at the beginning and ask first: is it possible for people to possess things at all?
To this, Thomas answers with a resounding yes. God has what Thomas calls “sovereign dominion” over all created things according to his Will, but God has given over to man the stewardship of those things which man needs in order to pursue his good life, or, in Thomas’s rather austere words, those things which are necessary for “the sustenance of man’s body.” From this, we can say that man has what Thomas calls “natural dominion” over things (in relation to God’s sovereign dominion) and it is natural for man to possess things external to himself so that he can use them and labor over them in order to sustain his body.
It is important to remember the different meanings that Thomas acknolwedges in his understanding of natural right just a few pages prior, in II-II Q57 A3:
[T]he natural right or just is that which by its very nature is adjusted to or commensurate with another person. Now this may happen in two ways; first, according as it is considered absolutely: thus a male by its very nature is commensurate with the female to beget offspring by her, and a parent is commensurate with the offspring to nourish it. Secondly a thing is naturally commensurate with another person, not according as it is considered absolutely, but according to something resultant from it, for instance the possession of property. For if a particular piece of land be considered absolutely, it contains no reason why it should belong to one man more than to another, but if it be considered in respect of its adaptability to cultivation, and the unmolested use of the land, it has a certain commensuration to be the property of one and not of another man, as the Philosopher shows (Polit. ii, 2).
It is clear that the natural right with respect to the possession of property is meant to be understood in the second sense, as Thomas says.
Returning to II-II Q66, Thomas narrows his inquiry from the broad – whether it is natural for man in general to possess things in general – to the specific: “whether it is lawful for a man to possess a thing as his own?” Here Thomas is responding to those who would claim that because all things are God’s property, no man can really have ownership over anything. In support of this he quotes Basil and Ambrose, who condemn the rich who seize as their own things which rightfully belong as common goods to all. First, Thomas replies that private possession of things serves both practical ends (it induces men to labor with more care over things which are their own, it prevents the confusion of ownership and responsibility that is present when all things are owned in common) and spiritual/moral ends (the possession of private property allows people to be generous and to develop the virtues of giving to those in need).
Thomas then gives what is probably his most controversial statement in the question, and what might reasonably be considered a summary of his position on the nature of private property:
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. Hence the ownership of possessions is not contrary to the natural law, but an addition thereto devised by human reason.
What Thomas is doing here is delineating the status of private property in relation to the laws (differentiating between the natural law and the human law), and making clear the significance of property both as a social or practical institution (which he derives from Aristotle and from several Roman jurists) and as a thing understood within the idealistic Christian-theological (which he derives from multiple Christian sources, but primarily the Gospel writers and the early Church fathers) worldview. Speaking again on the same topic, in another section of the Summa Theologica which is concerning the natural law, Thomas writes that
[W]e might say that for man to be naked is of the natural law, because nature did not give him clothes, but art invented them. In this sense, “the possession of all things in common and universal freedom” are said to be of the natural law, because, to wit, the distinction of possessions […] [was] not brought in by nature, but devised by human reason for the benefit of human life.
This helps to clarify for us what it means for Thomas for a thing to be a consequence of the natural law or to be an invention of the human law. Notice the contingent status of private property under this understanding – there is no absolute guarantee to one’s particular possessions derived from the natural law, according to Thomas, but that which is provided by the human laws. Moreover, the human laws which provide those guarantees do so “for the benefit of human life.”