H. L. A. Hart's separability thesis holds that legal validity is determined by social sources — such as legislative enactment or judicial recognition — not by whether a law meets any moral standard. In his 1958 Harvard Law Review article, Hart defined legal positivism as "the simple contention that it is in no sense a necessary truth that laws reproduce or satisfy certain demands of morality." He called the view that something only counts as law if it satisfies a moral criterion a fundamental "confusion" between what law is and what law ought to be.
Hart was careful, however, to deny a necessary conceptual connection between law and morality — not to claim the two are wholly unrelated in practice. He acknowledged that law and morality overlap "at a thousand points" empirically, meaning historical and social forces routinely cause legal systems to reflect moral norms. The separability thesis is a conceptual claim, not an empirical one: even if a law happens to be moral, its legal validity rests on its social pedigree, not its moral content.
Scholars have debated the precise scope of Hart's thesis. The Cambridge Law Journal article "There Is No Such Thing as the Separability Thesis" argues the label is contested, while the NYU Law Review's "Positivism and the Inseparability of Law and Morals" contends Hart's thesis is false because many necessary connections between law and morality do exist. Nonetheless, Hart's own words — closely mirroring the claim that law's existence and its moral merit are separate questions — remain the canonical statement of legal positivism's core idea.