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; 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.
Source: Rerum Novarum (Vatican.va)