This Chapter examines existing legal theories regarding the legitimation of criminal justice. As explained in Chapter I, legitimacy in criminal law relates to the definition of permissible use of coercive power. This Chapter analyses three different standards of criminal law legitimacy. In particular, the challenges and features of the legality principle as identified in the previous Chapter and the various interpretations of its theoretical bases (Chapter III) will be analysed in the context of three theoretical models of criminal justice.

One of the conclusions drawn in the previous Chapter regarding the practical operation of the legality principle is that there is tension between the two conceptions of the character of criminal liability. On one hand, criminal norms should be stable and relatively slow to change; the law is a fixed body of rules with a specific content. Conversely, it is important for criminal liability to retain an open texture, as it is a means to bring about social change, and must adapt in line with societal developments. This antithetical account of the legitimate nature of criminal liability highlights a deeper conflict regarding the purpose of law.

There is an on-going dialogue within legal philosophy regarding the nature and purpose of law. Over the years, there have been two influential, yet conflicting schools of thought: formalism and realism. Formalism involves a conventional understanding of the law; law is a convention, with an independent structure and nature, static and unchanging. Realism, on the other hand, rejects a static concept of law and builds upon the idea that law should reflect social change and the common good; by definition law derives its content from its factual circumstances and social need.

Various theories have been developed in legal philosophy, outlining and extolling the virtues of formalism and realism, and, on occasion, providing for a third way. Examples are Hart's discussion on formalism and rule-scepticism, Dworkin's models of conventionalism, pragmatism and theory of integrity and Habermas’ discourse theory. In criminal law, various theories have also been formulated, for example, Damaška's activist and reactive state models, Claes’ conventionalism, pragmatic and ethical models of criminal law, and 't Hart and Foqué's relational theory of (criminal) law.