H.L.A Hart made a famous claim that legal positivism somehow involves a “separation of law and morals.” This Article seeks to clarify and assess this claim, contending that Hart’s separability thesis should not be confused with the social thesis, the sources thesis, or a methodological thesis about jurisprudence. In contrast, Hart’s separability thesis denies the existence of any necessary conceptual connections between law and morality. That thesis, however, is false: There are many necessary connections between law and morality, some of them conceptually significant. Among them is an important negative connection: Law is, of its nature, morally fallible and morally risky. Lon Fuller emphasized what he called the “internal morality of law,” the “morality that makes law possible.” This Article argues that Hart’s most important message is 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.