Later in the 20th century, H. L. A. Hart attacked Austin for his simplifications and Kelsen for his fictions in The Concept of Law .  Hart argued law is a system of rules, divided into primary (rules of conduct) and secondary ones (rules addressed to officials to administer primary rules). Secondary rules are further divided into rules of adjudication (to resolve legal disputes), rules of change (allowing laws to be varied) and the rule of recognition (allowing laws to be identified as valid). Two of Hart's students continued the debate: In his book Law's Empire , Ronald Dworkin attacked Hart and the positivists for their refusal to treat law as a moral issue. Dworkin argues that law is an " interpretive concept",  that requires judges to find the best fitting and most just solution to a legal dispute, given their constitutional traditions. Joseph Raz , on the other hand, defended the positivist outlook and criticised Hart's "soft social thesis" approach in The Authority of Law .  Raz argues that law is authority, identifiable purely through social sources and without reference to moral reasoning. In his view, any categorisation of rules beyond their role as authoritative instruments in mediation are best left to sociology , rather than jurisprudence.