It is very interesting to read Pope Leo XIII’s defense of private property in Rerum Novarum. Rather than focusing on the classic defenses of private property (1. men are more attentive with their own possessions, 2. human affairs are more orderly conducted, 3. a more peaceful state results), he address specifically man’s labor and acting upon nature.
9. Here, again, we have further proof that private ownership is in accordance with the law of nature. Truly, that which is required for the preservation of life, and for life’s well-being, is produced in great abundance from the soil, but not until man has brought it into cultivation and expended upon it his solicitude and skill. Now, when man thus turns the activity of his mind and the strength of his body toward procuring the fruits of nature, by such act he makes his own that portion of nature’s field which he cultivates – that portion on which he leaves, as it were, the impress of his personality [SA – It belongs to a man because it bears an imprint of his personality]; and it cannot but be just that he should possess that portion as his very own, and have a right to hold it without any one being justified in violating that right.
10. So strong and convincing are these arguments that it seems amazing that some should now be setting up anew certain obsolete opinions in opposition to what is here laid down. They assert that it is right for private persons to have the use of the soil and its various fruits, but that it is unjust for any one to possess outright either the land on which he has built or the estate which he has brought under cultivation. But those who deny these rights do not perceive that they are defrauding man of what his own labor has produced. For the soil which is tilled and cultivated with toil and skill utterly changes its condition; it was wild before, now it is fruitful; was barren, but now brings forth in abundance. That which has thus altered and improved the land becomes so truly part of itself as to be in great measure indistinguishable and inseparable from it. [SA – Even stronger, that which man acts upon becomes “part of” him and “inseparable from” him.] Is it just that the fruit of a man’s own sweat and labor should be possessed and enjoyed by any one else? As effects follow their cause, so is it just and right that the results of labor should belong to those who have bestowed their labor. –Rerum Novarum (empahsis mine)
Whereas the classical defense focuses upon the common good, i.e. that men in general care for goods well and the peace of society, His Holiness takes the approach of addressing the man himself in his acting upon nature.