Six hundred years later, the growth of capitalism has called forth a militant socialism in reaction. Its call for the abolition of all private property by the state incites Leo XIII to respond with the encyclical Rerum Novarum, inaugurating the tradition of Catholic Social Teaching. At the center of this document is a full-blown attack on socialism, based on a sturdy defense of the right of private property, a right that Leo feels the need to affirm more strongly than Aquinas did. While seeking (and no doubt perceiving himself) to be in line with Thomist teaching, Leo comes close to simply rooting the right of property in nature, in a way that Thomas never does. He does this by importing a Lockean metaphysical account of property, suggesting that a right to private property simply arises out of one’s labor upon that property. It is worth attending carefully to how he constructs his justification and how it differs from Aquinas’s.
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. (9)
The fact that God has given the earth for the use and enjoyment of the whole human race can in no way be a bar to the owning of private property. For God has granted the earth to mankind in general, not in the sense that all without distinction can deal with it as they like, but rather that no part of it was assigned to any one in particular, and that the limits of private possession have been left to be fixed by man's own industry, and by the laws of individual races.