In his 1958 Harvard Law Review exchange with Lon Fuller, Professor H.L.A. Hart of Oxford University wrote:
It is surely not arguable (without some desperate extension of the word "sanction" or artificial meaning of the word "law") that every law in a municipal system must have a sanction, yet it is at least plausible to argue that a legal system must, to be a legal system, provide sanctions for certain of its rules.
Three years later Professor Hart published his book, The Concept of Law. Widely acclaimed as a significant and important contribution to twentieth-century jurisprudential thought, the book in large part represents an elaboration of the simple idea expressed above. For Hart makes clear that, though he shares with John Austin, Hans Kelsen and legal positivism in general an insistence upon the separation of "is" and "ought" and a high regard for linguistic analysis of legal terms, he rejects their basically common view that no part of law can be understood without reference to the monopolization of coercive power in the hands of an overriding political authority. Whether formulated in Austin's terms that law is essentially and exclusively a system of habitually obeyed commands of the sovereign addressed to his subjects, violation of which will lead to the imposition of sanctions; or in Kelsen's, that law is to be understood as a system of depersonalized directions to officials to impose given sanctions upon the occurrence of certain events (conditions), Hart spurns the imperative analysis as being inadequate to describe the essential features of a developed legal system. For Hart, "law without sanctions is perfectly conceivable."
The Place of Sanctions in Professor H.L.A. Hart's Concept of Law,
Duq. L. Rev.
Available at: https://dsc.duq.edu/dlr/vol5/iss1/3