How does the principle of legality protect people from criminal punishment?

The principle of legality is a fundamental safeguard in criminal law that protects individuals from arbitrary state power by ensuring that criminal liability is governed by clear, established rules. It functions through four primary protections:12

• No Crime Without Law: Conduct cannot be punished as a crime unless it has been explicitly forbidden by a statute or established rule of law. Immoral or antisocial behavior that is not legally prohibited remains outside the scope of criminal punishment.1
• Strict Interpretation: Criminal statutes must be interpreted strictly to prevent overreach. If a law is ambiguous, it is typically construed narrowly in favor of the accused, ensuring that individuals are not punished for conduct that falls outside the clear intent of the legislation.1
• Prohibition of Retroactivity: The law forbids the application of criminal statutes to acts committed before those laws were enacted. An individual can only be convicted if their conduct was criminal at the time it occurred, a protection embodied in ex post facto prohibitions.134
• Fair Warning: Statutes must be clear and unambiguous. This ensures that potential lawbreakers have the opportunity to understand what conduct is prohibited, allowing them to guide their behavior according to the law.12

By requiring that laws be knowable, prospective, and clearly defined, the principle of legality upholds the ethical necessity of preventing the arbitrary use of state authority.12