"Property systems are in general not completely self-enforcing. They depend for their definition upon a constellation of legal procedures, both civil and criminal. The course of the law itself cannot be regarded as subject to the price system. The judges and the police may indeed be paid, but the system itself would disappear if on each occasion they were to sell their services and decisions. Thus the definition of property rights based on the price system depends precisely on the lack of universality of private property and of the price system. This ties in with the third hypothesis put forward in section I. The price system is not, and perhaps in some basic sense cannot be, universal. To the extent that it is incomplete, it must be supplemented by an implicit or explicit social contract. Thus one might loosely say that the categorical imperative and the price system are essential complements."