6 What is of far greater moment, however, is the fact that the remedy they propose is manifestly against justice. For, every man has by nature the right to possess property as his own. This is one of the chief points of distinction between man and the animal creation, for the brute has no power of self direction, but is governed by two main instincts, which keep his powers on the alert, impel him to develop them in a fitting manner, and stimulate and determine him to action without any power of choice. One of these instincts is self preservation, the other the propagation of the species. Both can attain their purpose by means of things which lie within range; beyond their verge the brute creation cannot go, for they are moved to action by their senses only, and in the special direction which these suggest. But with man it is wholly different. He possesses, on the one hand, the full perfection of the animal being, and hence enjoys at least as much as the rest of the animal kind, the fruition of things material. But animal nature, however perfect, is far from representing the human being in its completeness, and is in truth but humanity's humble handmaid, made to serve and to obey. It is the mind, or reason, which is the predominant element in us who are human creatures; it is this which renders a human being human, and distinguishes him essentially from the brute. And on this very account - that man alone among the animal creation is endowed with reason - it must be within his right to possess things not merely for temporary and momentary use, as other living things do, but to have and to hold them in stable and permanent possession; he must have not only things that perish in the use, but those also which, though they have been reduced into use, continue for further use in after time.
Source: Rerum Novarum (Vatican.va)