 Most conceptions of the common good identify a set of facilities that citizens have a special obligation to maintain in virtue of the fact that these facilities serve certain common interests. The relevant facilities may be part of the natural environment (e.g., the atmosphere, a freshwater aquifer, etc.) or human artifacts (e.g., hospitals, schools, etc.). But the most important facilities in the literature are social institutions and practices. For example, a scheme of private property exists when members of a community conform to rules that assign individuals certain forms of authority over external objects. Private property, as a social institution, serves a common interest of citizens in being able to assert private control over their physical environment, and so many conceptions include this institution as part of the common good.