What is established by statute or common law regarding criminal acts?

The assertion that no act can be a crime unless it has been previously established is rooted in fundamental legal principles known as "nullum crimen, nulla poena sine lege," which translates to "no crime, no punishment without a law." This principle signifies that individuals cannot be prosecuted for actions that were not classified as criminal at the time they were committed.

This ensures that people have a clear understanding of what constitutes a criminal act and that they cannot be convicted of crimes based on arbitrary or retrospective decisions. It safeguards the rule of law by preventing ex post facto laws, which would allow the government to punish individuals for acts that were not deemed illegal at the time they occurred.

Understanding this principle is vital for ensuring justice and fairness within the legal system, as it places a necessary limit on governmental power over individuals and establishes a transparent framework for defining criminal behavior.