The purpose of this essay is to offer a reconstruction of Lon Fuller’s critique of Hart’s legal positivism. I show that contrary to the claims of Fuller’s many critics, one can derive from his work a clear and powerful argument against legal positivism, at least in the guise found in the work of H.L.A. Hart. The essence of the argument is that Fuller’s principles of legality posit that the same considerations that count for law’s excellence are relevant also for the determining what counts as law. I contrast this view with Hart’s legal positivism, which acknowledged that the principles of legality are relevant for law’s excellence, but considered them irrelevant for determining the question what counts as law. I argue that the positivist position is arbitrary, and - a point on which I focus - completely undefended. I draw from this point a more general challenge to Hart’s theory of law (as well as that of many of his followers), namely that though claimed to be a true theory of law, it has no resources to explain why this is so. I argue that Fuller’s theory does not suffer from this problem, because Fuller rejected a staple of contemporary jurisprudence - the separation of conceptual and normative jurisprudence.