In his 1958 Harvard Law Review article “Positivism and the Separation of Law and Morals,” H. L. A. Hart argues that it is “in no sense a necessary truth that laws reproduce or satisfy certain demands of morality.” This is a conceptual claim about what makes something law: legal existence/validity is determined by social facts (such as recognition by legal officials), not by whether the rule is morally good.
Hart’s point is not that law and morality never overlap in practice. He explicitly acknowledges that law and morals are intertwined “at a thousand points,” meaning that legal systems often incorporate moral ideas and are influenced by moral criticism. The separability thesis targets necessity, not frequency: moral merit may coincide with legality, but it is not a required condition for something to count as law.
Later discussions of Hart’s position (e.g., treatments in the Cambridge Law Journal and NYU Law Review) frame this as denying a necessary conceptual connection between law and morality, while leaving room for empirical and institutional connections. That is why Hart can simultaneously describe immoral laws as legally valid and still insist that moral evaluation of law remains crucial.