Natural Law and Possession According to the Catholic Church: the Summa Theologica of Thomas Aquinas

  Now according to the natural law all things are common property ...
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  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 (Q. LVII, AA. 2, 3). Hence the ownership of possessions is not contrary to the natural law, but an addition thereto devised by human reason.

Summa Theologica, IIa IIae q. 66 a. 2.

(Aquinas, Thomas, Summa Theologica, trans. Fathers of the English Dominican Province, New York: Benziger Brothers, 1918, pt. 2, 2nd pt., q. 47-79, pp. 223-225.)