The legal philosopher Lon Fuller once invented an earnest monarch named Rex who discovered many wrong ways to make law. First, Rex wrote a detailed code of laws, but, to avoid confusing the public, kept it secret. “To Rex’s surprise this sensible plan was deeply resented by his subjects. They declared it was very unpleasant to have one’s case decided by rules when there was no way of knowing what those rules were,” Fuller wrote. So Rex refined his code even further and made it public. But its detail and precision made it “a masterpiece of obscurity.” Soon “a picket appeared before the royal palace carrying a sign that read, ‘How can anybody follow a rule that nobody can understand?’”

Next week the Supreme Court will look at cases in which two criminal defendants make similar pleas. On Monday, a violent neo-Nazi contends that he is facing 15 years in prison under a law that not only he but some of the most learned judges in the country find incomprehensible; the next day, a dealer in “designer drugs,” claims that he is facing prison under a law so complex that its prohibitions are effectively ​secret from anyone except skilled chemists.

The neo-Nazi, Samuel Johnson, faces a 15-year minimum sentence under the Armed Career Criminal Act. ACCA provides that any person convicted in federal court of a firearms offense will receive a minimum 15-year sentence if he or she has previously been convicted three times in state or federal court of “a violent felony or a serious drug offense.” As originally passed in 1984, the Act limited the “violent felonies” to crimes in which force was actually used or threatened, or to any robbery or burglary; two years later, Congress made the law even “tougher.” It now specifies that any offense is a “violent felony” if it “is burglary, arson, or extortion, involves use of explosives, or otherwise involves conduct that presents a serious potential risk of physical injury to another.”

The last part is called the “residual clause.” With admirable restraint, Justice Antonin Scalia wrote a decade ago that the clause “is, to put it mildly, not a model of clarity.” In fact, it has tied the federal courts in knots. In the past decade, the Supreme Court has had to settle disputes over whether “violent felony” applies to attempted burglary (no), driving under the influence (no), failure to report for incarceration (no), and intentional flight from law enforcement in a motor vehicle (yes). But confusion persists, with different standards prevailing in different appellate-court jurisdictions. For example, in the Fifth Circuit, reckless assault is “violent,” while in the Sixth, reckless homicide is not. In the Fourth Circuit, battery of a police officer is not “violent,” in the Tenth it is. In the Fifth, Sixth, Seventh, and Tenth Circuits, fleeing law enforcement on foot is “violent”; in the Eighth, Ninth, and Eleventh it is not.

As these crazy results piled up, the Court’s cries for help have grown louder. In 2006, Scalia, dissenting in the attempted burglary case, argued that the Act “violates ... the constitutional prohibition against vague criminal laws.” In 2008, Justice Alito wrote that “only Congress can rescue the federal courts from the mire into which ACCA’s draftsmanship” has thrust it. In 2011, Scalia again urged the Court to admit that ACCA “is a drafting failure and declare it void for vagueness.”

Against this backdrop, Johnson v. United States reached the Court last November. As the head of something called the Aryan Liberation Movement, Johnson boasted to FBI informants that he had, and planned to use, napalm, explosives, an AK-47, 1,100 rounds of ammunition, and silencers. He was convicted of being a “felon in possession” of firearms and ammunition; the district court promoted him to career status because of two previous convictions of robbery and one of possession of a short-barrel (“sawed-off”) shotgun. Before the Court, his federal defender argued that mere “possession” of an illegal weapon was not “purposeful, violent, and aggressive” enough to qualify as a “violent felony.” She asked the Court to add short-barrel possession to the list of felonies that aren’t “violent”; if it did so, she said, “this Court need not get into whether [ACCA] is unconstitutionally vague and the baby should go out with the bath water.”

Two months later, however, the justices, without explanation, threw up their hands. They set the case for reargument on the sole issue of whether ACCA is “unconstitutionally vague.”

Unconstitutional vagueness is a three-legged doctrine. Courts throw out statutes they can’t understand because, first, if a judge can’t understand it then God help the citizen who is trying not to break it; second, if the language is loose then cops and prosecutors can pick and choose cases on arbitrary, and possibly discriminatory, grounds; and, third and probably most important, if courts have to struggle with the language then the legislature has shifted its work—writing the law—to the judiciary, in violation of the separation of powers. The last leg is probably the most important to the Court; the reargument is a sign that, since Congress has done nothing despite repeated urgings, at least four members of the Court are ready to strike down the clause altogether.

The second case, McFadden v. United States, concerns the regrettably named Controlled Substances Analogue Enforcement Act of 1986, or the “Analogue Act.” The Controlled Substances Act allows the Department of Justice to designate specific drugs as “controlled,” then punish those who make, sell, or possess them illegally. But in the 80s, Congress began to worry about the rise of “designer drugs.” These are chemically similar to “controlled substances,” but, because they are new and underground, not yet listed by DOJ as “controlled.”  The Analogue Act makes it a crime to manufacture or distribute a compound whose “chemical structure” and “stimulant, depressant, or hallucinogenic effect” are both “substantially similar” to that of a controlled substance. The aim is to stay ahead of the drug designers; thus, there isn’t a public list of “analogues.” (DEA has a list of those it has identified—but that list is secret, and a new drug doesn’t have to be on the list to be a punishable “analogue.”)

Stephen McFadden of Staten Island was a designer of “bath salts,” crystals openly sold in stores but used more often for smoking or snorting than for actual bathing. He was convicted of one count of conspiracy to distribute drug analogues and eight counts of distributing them. At trial, the government introduced wiretap conversations in which McFadden compared his products’ effects to both cocaine and meth. A trial court instructed the jury that the government had to prove that the “salts” had “substantially similar” structures to controlled substances—but that McFadden didn’t need to have known that. His guilt would be established if the jury found that he intended the “salts” to be consumed by humans and to have “stimulant, depressant, or hallucinogenic” effect.  

On appeal, McFadden argued that the statute was unconstitutionally vague because there is no public list of analogues, but the Fourth Circuit said such a requirement “would undermine the very purpose of the Act, which is to prevent individuals from creating slightly modified versions of controlled substances.”  McFadden then asked the Supreme Court to decide whether the government should have been required to prove that he knew that his products were chemical “analogues.”

The “substantially similar” requirement, which must be proved with expert scientific testimony, has vexed trial courts. In an amicus brief, a group of forensic scientists argue that the term has “no quantifiable meaning” and thus requires experts to testify “based on little more than subjective feelings.” The appeals courts are split over what kind of evidence proves “substantial similarity.”

If scientists don’t know what it means, and courts can’t agree on how to prove it, how can an ordinary citizen figure it out? Courts are split on this too: The Seventh Circuit, in a leading case, has held a “defendant must know that the substance at issue has a chemical structure substantially similar to that of a controlled substance.” The Fourth Circuit rejected that rule, holding that the government need only show that the defendant intended the drug to have the forbidden effect on humans.

The Supreme Court might choose to issue a “scientific” definition of “substantial similarity”; it could refine the intent and knowledge requirement; or it could junk the Analogue Act altogether. That last option seems unlikely; but there is something a bit disturbing about a law that prohibits unknown substances and then punishes those who don’t know about them. The rule proposed by McFadden would in essence limit the government to enforcing the act against actual chemists.

Both of these laws are products of the 1980s. Back then, Congress was in a panic over about supposed soulless “superpredators” and a seemingly losing “war on drugs.” Politicians were like Fuller’s fictional monarch Rex, passing one law after another to fix past and future problems—and to show that they were “tough on crime.” We are slowly awakening from the resulting nightmare of criminalization and mass incarceration. This Court has been quite critical of federal criminal overreach; one or both of these laws may not survive review.