Thesis An unjust law is not really law: legal positivism is mistaken
Where it stands
H. L. A. Hart's The Concept of Law (1961) defends the separation of law and morality: what the law is is one question, what it ought to be is another. Lon Fuller argued against this in their famous 1958 exchange in the Harvard Law Review and later in The Morality of Law (1964), where he describes eight principles of legality — generality, publicity, non-retroactivity and so on — as an "inner morality" of law.
After the war, Gustav Radbruch argued that statutes reaching an intolerable level of injustice lose their character as law, a principle German courts actually applied. Ronald Dworkin, in Law's Empire (1986), argued that legal interpretation always involves moral judgment.
My thesis: positivism fails to capture what law is.