But the theory of natural law, devised for a society organized on the basis of kinship and developed for a society organized on the basis of relations, did not suffice for a society which conceived of itself as an aggregate of individuals and was reorganizing on the basis of competitive self-assertion. Again the convenient ambiguity of _ius_, which could mean not only right and law but "a right," was pressed into service and _ius naturale_ gave us natural rights. The ultimate thing was not natural law as before, not merely principles of eternal validity, but natural rights, certain qualities inherent in man and demonstrated by reason, which natural law exists to secure and to which positive law ought to give effect. Later these natural rights came to be the bane of juristic thinking. Yet they achieved great things in their day. Under the influence of this theory jurists worked out a scheme of "legal rights" that effectively secures almost the whole field of individual interests of personality and individual interests of substance. It put a scientific foundation under the medieval scheme of the claims and duties involved in the relation of king to tenants in chief, out of which the judges had developed the immemorial rights of Englishmen, and enabled the common-law rights of Englishmen to become the natural rights of man, intrenched as such in our bills of rights. Thus it served as a needed check upon the exuberance of growth stimulated by the theory of natural law. It kept a certain needed rigidity in a time when law threatened to become wholly fluid. And this steadying influence was strengthened from another quarter. The Roman jurisconsult was teacher, philosopher and practitioner in one. As a lawyer he had the exigencies of the general security ever before him in that he felt the imperative need of being able to advise with assurance what tribunals would do on a given state of facts. The seventeenth- and eighteenth-century jurists were chiefly teachers and philosophers. Happily they had been trained to accept the Roman law as something of paramount authority and so were able to give natural law a content by assuming its identity with an ideal form of the law which they knew and in which they had been trained. As the Roman jurisconsult built in the image of the old law of the city, they built on idealized Roman lines. If Roman law could no longer claim to be embodied authority, they assumed that, corrected in its details by a juristic-philosophical critique, it was embodied reason.