(7) Parliament may by law prescribe (a) the circumstances under which, and the class or classes of cases in which, a person may be detained for a period longer than three months under any law providing for preven tive detention without obtaining the opinion of an Advisory Board in accordance with the provisions of sub clause (a) of clause (4); (b) the maximum period for which any person may in any class or classes of cases be detained under any law provid ing for preventive detention; and (c) the procedure to be followed by an Advisory Board in an inquiry under sub clause (a) of clause (4)." Mr. Nambiar urged that the word "law" in article 21 should be understood, not in the sense of an enactment but as signifying the immutable and universal principles of natural justice the jus naturale of the civil law and that the expression "procedure established by law" meant the same thing as that famous phrase "due process of law" in the American Constitution in its procedural aspect.