With the constant deepening of the rule of law in China,how to explain and justify the differentiation between abstract legal norms and specific case facts in judicial adjudication has become an important issue in legal theory.Traditional legal theory mainly explains the rationality of this distinction through Hume's axiom,but faces huge challenges.System theory of law abandons this approach of a priori theory,viewing the legal system as an operationally closed and cognitively open social system,and the differentiation between legal norms and case facts is regarded as a cognitive schema within the legal system.This schema executes both self-reference and external-reference within the legal system and coordinates the relationship between the legal system and the environment,contributing to the realization of the social function of the legal system to safeguard normative expectations.