This is the penultimate draft of a paper originally presented at the Hart-Fuller at 50 conference, held at the NYU Law School in February 2008. A revised version will appear in the NYU Law Review. The paper seeks to clarify and assess HLA Hart's famous claim that legal positivism somehow involves a 'separation of law and morals.' The paper contends that Hart's 'separability thesis should not be confused with the 'social thesis,' with the 'sources thesis,' or with a methodological thesis (...) about jurisprudence. Hart's thesis denies the existence of necessary (conceptual) connections between law and morality. But that thesis is false: there are many necessary connections between law and morality, some of them conceptually significant. Among these is an important negative connection: law is of its nature morally fallible and morally risky. Lon Fuller emphasised the 'internal morality of law,' the 'morality that makes law possible'. Hart stressed that there is also an immorality that law makes possible. Law's nature is seen not only in its internal virtues, in legality, but also in its internal vices, in legalism. (shrink)