by W Borman
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 acknowledges in his understanding of natural right, in II-II Q 57, 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 A2, 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 concerning the natural law, Thomas writes that
A thing is said to belong to the natural law in two ways. First, because nature inclines thereto: e.g. that one should not do harm to another. Secondly, because nature did not bring in the contrary: thus we 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 either a consequence of the natural law or to be an invention of the human law. Natural law as well as natural right have both a primary and a secondary sense, and it is only in the secondary sense that private property is natural. Notice the contingent status of private property under this understanding – there is no absolute guarantee to one’s particular possessions directly from the natural law, according to Thomas, but that which is provided by human reason. Moreover, the human laws which provide those guarantees do so “for the benefit of human life.” Thus, property is not a natural right considered absolutely, but only consequently and as a result of natural reason.
[Revised from a post at Incudi Reddere]
 Ius sive iustum naturale est quod ex sui natura est adaequatum vel commensuratum alteri. Hoc autem potest contingere dupliciter. Uno modo, secundum absolutam sui considerationem, sicut masculus ex sui ratione habet commensurationem ad feminam ut ex ea generet, et parens ad filium ut eum nutriat. Alio modo aliquid est naturaliter alteri commensuratum non secundum absolutam sui rationem, sed secundum aliquid quod ex ipso consequitur, puta proprietas possessionum. Si enim consideretur iste ager absolute, non habet unde magis sit huius quam illius, sed si consideretur quantum ad opportunitatem colendi et ad pacificum usum agri, secundum hoc habet quandam commensurationem ad hoc quod sit unius et non alterius, ut patet per philosophum, in II Polit. (II-II Q 57, A3)
 Communitas rerum attribuitur iuri naturali, non quia ius naturale dictet omnia esse possidenda communiter et nihil esse quasi proprium possidendum, sed quia secundum ius naturale non est distinctio possessionum, sed magis secundum humanum condictum, quod pertinet ad ius positivum, ut supra dictum est. Unde proprietas possessionum non est contra ius naturale; sed iuri naturali superadditur per adinventionem rationis humanae. (II-II Q 66, A 2, ad. 1)
 [A]liquid dicitur esse de iure naturali dupliciter. Uno modo, quia ad hoc natura inclinat, sicut non esse iniuriam alteri faciendam. Alio modo, quia natura non induxit contrarium, sicut possemus dicere quod hominem esse nudum est de iure naturali, quia natura non dedit ei vestitum, sed ars adinvenit. Et hoc modo communis omnium possessio, et omnium una libertas, dicitur esse de iure naturali, quia scilicet distinctio possessionum et servitus non sunt inductae a natura, sed per hominum rationem, ad utilitatem humanae vitae. Et sic in hoc lex naturae non est mutata nisi per additionem. (I-II Q 94, A 5, ad. 3)