51 These lesser societies and the larger society differ in many respects, because their immediate purpose and aim are different. Civil society exists for the common good, and hence is concerned with the interests of all in general, albeit with individual interests also in their due place and degree. It is therefore called a public society, because by its agency, as St. Thomas of Aquinas says, "Men establish relations in common with one another in the setting up of a commonwealth."(36) But societies which are formed in the bosom of the commonwealth are styled private, and rightly so, since their immediate purpose is the private advantage of the associates. "Now, a private society," says St. Thomas again, "is one which is formed for the purpose of carrying out private objects; as when two or three enter into partnership with the view of trading in common."(37) Private societies, then, although they exist within the body politic, and are severally part of the commonwealth, cannot nevertheless be absolutely, and as such, prohibited by public authority. For, to enter into a "society" of this kind is the natural right of man; and the State has for its office to protect natural rights, not to destroy them; and, if it forbid its citizens to form associations, it contradicts the very principle of its own existence, for both they and it exist in virtue of the like principle, namely, the natural tendency of man to dwell in society.
Source: Rerum Novarum (Vatican.va)