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 reconstruct 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. In contrast to Fuller’s view, this view looks arbitrary, and - a point on which I focus - completely undefended. And yet, despite the many years since Fuller presented his arguments against legal positivism, this argument has not been adequately answered. I conclude that even if Fuller’s arguments are answerable, they are yet to be answered.