Antiquity was content to maintain the economic and social _status quo_ or at least to idealize it and maintain it in an ideal form. The Middle Ages were content to accept _suum cuique tribuere_ as conclusive. It was enough that acquisition of land and movables and private ownership of them were part of the existing social system. Upon the downfall of authority, seventeenth-and eighteenth-century jurists sought to put natural reason behind private property as behind all other institutions. When Kant had undermined this foundation, the nineteenth-century philosophical jurists sought to deduce property from a fundamental metaphysical datum; the historical jurists sought to record the unfolding of the idea of private property in human experience, thus showing the universal idea; the utilitarian demonstrated private property by his fundamental test and the positivist established its validity and necessity by observation of human institutions and their evolution. In other words, here as elsewhere, when eighteenth-century natural law broke down, jurists sought to put new foundations under the old structure of natural rights, just as natural rights had been put as a new foundation to support institutions which theretofore had found a sufficient basis in authority. 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: (1) Natural-law theories, (2) metaphysical theories, (3) historical theories, (4) positive theories, (5) psychological theories and (6) sociological theories. Of the natural-law theories, some proceed on a conception of principles of natural reason derived from the nature of things, some on conceptions of human nature. The former continue the ideas of the Roman lawyers. They start with a definite principle found as the explanation of a concrete case and make it a universal foundation for a general law of property. As it has been put, they find a postulate of property and derive property therefrom by deduction. Such theories usually start either from the idea of occupation or from the idea of creation through labor. Theories purporting to be based on human nature are of three forms. Some proceed on a conception of natural rights, taken to be qualities of human nature reached by reasoning as to the nature of the abstract man. Others proceed upon the basis of a social contract expressing or guaranteeing the rights derived by reason from the nature of man in the abstract. In recent thinking a third form has arisen which may be called an economic natural law. In this form of theory, a general foundation for property is derived from the economic nature of man or from the nature of man as an economic entity. These are modern theories of natural law on an economic instead of an ethical basis.