203This Article challenges the traditional understanding of the principle of legality—nulla poena sine lege. Conventional wisdom concerning the principle fails to express the full range of values at stake in preserving the rule of law. In order to appreciate the deeper significance of these values, this Article considers an application of the principle of legality to a dispute about mens rea rather than to 4 controversy about actus reus. This discussion helps to reveal some of the larger issues that are involved in protecting the principle of legality. In addition, a focus on mens rea indicates the limitations of statutory solutions to some of the problems raised by the principle. Finally, this perspective demonstrates how the rule of law serves to protect law-abiding persons, and not only scoundrels. The most general conclusion drawn is that fidelity to law cannot be construed merely as fidelity to statutory law, but must be understood as fidelity to the principles of justice that underlie statutory law.