The case for calling the rule of recognition ‘law’ is that the rule providing criteria for the identification of other rules of the system may well be thought a defining feature of a legal system, and so itself worth calling ‘law’; the case for calling it ‘fact’ is that to assert that such a rule exists is indeed to make an external statement of an actual fact concerning the manner in which the rules of an ‘efficacious’ system are identified.

H. L. A Hart, The Concept of Law, loc. 343