The principle of legality is a concept that developed during, and in the aftermath of the French Revolution and Enlightenment. Noted philosophers and writers of that era, such as Beccaria, Montesquieu and Feuerbach, adopted this principle into domestic criminal law as a means of rationalising the criminal justice system. The legality principle, alongside other important legal principles, was developed as a means of minimising the potential arbitrariness of criminal law and to prevent abuse by the state. It is now one of the cornerstones of modern criminal justice systems, featuring in criminal codes, national constitutions and international legal instruments. The principle of legality is made up of four sub-principles as explained above.

However, the legality principle is much more than these four prohibitions as it has intrinsic value, adding legitimacy to criminal law. Certain aspects, such as the prohibitions of customary criminal liability and vagueness were also features of legal systems before the French Revolution and also exist in the Chinese and Islamic legal systems. However, there are qualitative differences between the primitive criminal justice systems of the medieval period and the western criminal systems of today. The source of its legitimising function lies in its theoretical rationales.

This Chapter examines the theoretical rationales and core values of the legality principle. Its purpose and relationship to the values of individual autonomy and liberty, democracy, separation of powers and the Rule of Law will be analysed, in addition to the principles of legal certainty and guilt.

The purpose of such analysis is two-fold. First, this Chapter aims at identifying the core values of the legality principle. The principle is approached not only as a set of rules for law-making and adjudication, but as a principle that generates legitimacy. Criminal behaviour and the abuse of power are on-going social facts and balancing criminal sanctions with due process and the rights of the individual are core functions of the state. The legality principle is a mechanism for balancing this dual function, enabling the state to maintain legitimacy while imposing penal sanctions.

Secondly, the goal is to delve deeper into the interpretation of these theoretical rationales.