Criminal intent -- what someone charged with a crime knew or was trying to do -- matters. At least, it should. Just ask Ignacio Flores-Figueroa, whose case is now before the Supreme Court.

An illegal alien working at a steel plant in Indiana, Flores-Figueroa pled guilty in 2006 to two federal immigration offenses. Whatever the Court decides, he will serve more than four years in prison for those two offenses before being deported to Mexico.

The Court is reconsidering his conviction on a related charge, aggravated identity theft, that tacked an extra two years onto his sentence. The Court's decision will have important implications not only for Flores-Figueroa but for many Americans, guilty or innocent, who face federal criminal charges.

It boils down to criminal intent and the words Congress uses -- or, increasingly, fails to use -- to define what the government must prove about a defendant's intentions or state of mind at the time of the violation.

The aggravated identity theft statute under which Flores-Figueroa was convicted failed to make it clear whether a defendant must know that the number on the fake ID he was using for employment purposes belonged to someone else or was merely fake -- that is, a number not assigned to a real person. The law applies to anyone who "knowingly transfers, possesses, or uses, without lawful authority, a means of identification of another person." Everyone agrees that "knowingly" applies to "transfers, posses, or uses," but does it reach "of another person"?

Flores-Figueroa admitted that he intended to obtain and use phony IDs, and for crimes related to that intent he should and will pay the full price set by Congress. But even though the numbers on the Social Security card and Permanent Resident cards Flores-Figueroa used had been issued to someone else, even the government hasn't argued that he knew that the numbers actually belonged to someone. Instead, the government has argued that a jury should not even consider Flores-Figueroa's criminal intent, or lack thereof. So far, the trial and appeals courts have sided with the government.

So what did Congress mean when it passed the law? Many states wouldn't have a problem figuring this out. They have adopted key parts of a model criminal code crafted more than 50 years ago by leading criminal-law scholars and practitioners. That model code instructs courts how to interpret poorly drafted criminal offenses, including those that fail to spell out what criminal intent a defendant must have acted on in order to be convicted.

Well-crafted intent requirements are crucial to protecting innocent Americans from criminal punishment. They help courts distinguish borrowing from theft, mistake from fraud, and accident from murder.

But at the federal level in particular, criminal-intent requirements and similar traditional protections have been in decline for decades. Some legislators have actually stripped criminal-intent protections from existing criminal laws.

Prosecutors almost uniformly defend such efforts. It's easier to win a conviction when the government doesn't have to prove what the accused knew or intended to do. But the results of these efforts leave law-abiding citizens at risk of conviction.

With over 4,400 federal criminal offenses in federal law, and tens of thousands more in federal regulations and state law, an enormous amount of behavior is subject to criminal punishment. And Congress alone keeps manufacturing new criminal offenses at a brisk pace -- about 50 a year.

That's why the Flores-Figueroa case is especially important. If the government's interpretation prevails, it could accelerate the decline of this key protection. More individuals will be prosecuted and convicted for conduct that they had no reason to believe violated any law.

It is this unintended consequence that is really at stake in the case before the Supreme Court. While the federal government and others may want to "throw the book" at Flores-Figueroa, the Justices need to worry about future cases, where the would-be defendants may have had no intent to break the law. Otherwise, this case could lead to a perverse result: The government could "win" an extra two years in prison for Ignacio Flores-Figueroa -- but only at the expense of cheapening its process of prosecuting and punishing serious crimes.

Andrew M. Grossman is Senior Legal Policy Analyst, and Brian W. Walsh is Senior Legal Research Fellow, in the Center for Legal and Judicial Studies at The Heritage Foundation.