British legal philosopher H. L. A. Hart undertakes to state the necessary and sufficient conditions for the existence of a legal system. He says that a legal system exists when certain institutional rules are accepted by officials whose rulings are by and large obeyed. The institutional rules constitute a legislative power for society plus courts of adjudication, and they include a master precept ‒ called a rule of recognition ‒ which identifies the valid laws. Hart's inventory of the constituents of a legal system is incomplete. He leaves out an essential feature of the process by which existing laws are brought to bear on cases. A crucial component of a legal system is shared acceptance of canons of argument. A legal system, properly so called, establishes criteria of good and sufficient legal argument. Other legal scholars define what's missing from Hart's account of a legal system. Owen Fiss speaks of "disciplining rules" that guide the performance of interpretation. Ronald Dworkin speaks of "ground rules" that state truth conditions for propositions of law. Such directives say how the activity of legal argument and legal judgment ought to be conducted.