Imagine a world in which the moral code and the penal code were identical. At one time, that scenario existed. Centuries ago in England, there were only nine felonies, including murder, rape and robbery. Everyone knew what constituted criminal behavior.

My, how things have changed. Today, there are perhaps 4,500 federal offenses and more than 300,000 relevant regulations on the books. (No one knows exactly how many. The Justice Department and the American Bar Association each tried to identify every federal crime and failed.) No reasonable person, not even a judge or lawyer, could possibly know all of these legal prohibitions, although criminal penalties are attached to each.

Nor does any moral rule of thumb, such as the Ten Commandments, offer any guidance. That’s because many statutes outlaw conduct that is not inherently blameworthy. Consequently, average people are on their own.

How did this happen? Partly because passing criminal legislation makes for good politics. Politicians can avoid being labeled “soft on crime” by enacting criminal laws or upping the penalties for existing laws. It doesn’t matter that existing statutes outlaw the same conduct, that the new statutes are vague when it comes to defining the conduct that is criminal, or that the new penalties are unduly harsh. No politician ever lost an election by being too “tough” on crime.

Congress also has fallen into the habit of using the criminal law to enforce complex regulatory regimes. That’s a recipe for confusion and injustice. The penal code forbids identified conduct altogether, but a regulatory program assumes that some conduct is permissible but places limitations on when and how it can be done. When the two approaches are mixed, the upshot is that no average citizen can readily distinguish lawful from unlawful conduct which means that people can be arrested, convicted and imprisoned for conduct that a reasonable person would not even have known was a crime.

That problem can ruin lives. Abner Schoenwetter spent six years in prison for importing Honduran lobsters packed in plastic, not paper, and for supposedly violating a Honduran regulation (later declared invalid by the Honduran attorney general) that made his lobsters marginally too small. Lawrence Lewis was charged with a felony and ultimately pleaded guilty to a misdemeanor for following the standard procedure to clean up toilet overflows at a military retirement home, which, unbeknownst to him, diverted the refuse into the Potomac River. Sadly, they are not alone.

There is a remedy: Allow a party to raise a “mistake of law” defense. This essentially would let a defendant argue that, although he may have erred, he tried to remain law-abiding and violated a law that he didn’t know and no reasonable person would have known applied to the conduct in which he is engaged. The law still clings to the maxim that “ignorance of the law is no excuse,” at a time when ignorance of some laws is inevitable, particularly as to statutes and regulations that outlaw conduct that is not inherently harmful or blameworthy.

Permitting a “mistake of law” defense sensibly balances society’s strong interest in law enforcement with society’s even more powerful interest in not punishing morally blameless parties. Allowing courts and juries to filter out phony from legitimate claims will separate the blameworthy from the blameless and protect the latter. The cost of making that distinction likely will prove minimal and, in any event, is worth it. Punishing someone who is blameless is unjust and reduces the public’s respect for our legal system, and those costs must be weighed, too.

In 1958, Harvard Law professor Henry Hart lamented, “What sense does it make to insist upon procedural safeguards in criminal prosecutions if anything whatever can be made a crime in the first place?” Today, 55 years later, there still is no good answer to his question, but there is a good remedy for the overcriminalization problem he recognized.

-Paul J. Larkin Jr. is a senior research fellow in the Edwin Meese Center for Legal and Judicial Studies at the Heritage Foundation who has extensively studied the subject of overcriminalization.