What are the motifs in The Concept of Law by H. L. A. Hart?

Natural law and legal positivism is a recurring idea in the book. Hart is one of the first to reintroduce in explicit terms the contrast between natural law theories of law and legal positivism in modern philosophy of law. Legal positivism holds that laws, their existence and form, are based not in morality but on institutions and social facts. Whether a law exists and whether it is good or bad are separate questions. This is not to say that laws cannot be judged as good or bad, but only that their existence does not depend on whether they are good or bad. Law is what is positive and ordained. Hart was among the most prominent legal positivists of the twentieth century.