The general safety, peace and order and the general health are secured for the most part by police and administrative agencies. Property and contract, security of acquisitions and security of transactions are the domain in which law is most effective and is chiefly invoked. As the Romanist puts it, in the case of natural possession the law secures the relation of the physical person to the object; in juristic possession the law secures the relation of the will to the object. Upon the downfall of authority, seventeenth- and eighteenth-century jurists sought to put natural reason behind private property as behind all other institutions. Theories by which men have sought to give a rational account of private property as a social and legal institution may be arranged conveniently in six principal groups, each including many forms. These groups may be called: Natural-law theories, metaphysical theories, historical theories, positive theories, psychological theories and sociological theories.