The Armed Career Criminal Act (ACCA) increases the minimum criminal penalty for defendants convicted of illegal firearm possession who also have three prior violent crime convictions. While the Act lists many crimes as qualifying as “violent”—such as burglary, arson, and extortion—it also contains a catch-all provision, a “residual clause,”that includes crimes that “otherwise involve conduct that presents a serious potential risk of physical injury to another.” While that language may seem clear, its precise meaning has bedeviled courts for decades. In fact, Johnson v. United States represents the fifth time since 2007 that the Supreme Court has been asked to clarify what the residual clause means. For example, does drunk driving count? How about fleeing from officers in a high-speed chase? Even though the high court only hears about 75 cases per year—and it rarely revisits a law within such a short time-span—the ACCA’s residual clause keeps coming back. As Justice Antonin Scalia quipped in the last such case, “We try to include an ACCA residual-clause case in about every second or third volume of the United States Reports.” Justice Scalia’s comment came in a dissent in which he argued that the residual clause is unconstitutionally vague, and it seems that the rest of his colleagues paid attention. This is the second time this term that this case will be argued before the Court. Last November, the issue was whether merely (illegally) possessing a short-barreled shotgun is a crime that fits into the residual clause. In January, however, the Court ordered that the case be re-argued on the larger question of whether the residual clause is itself unconstitutionally vague. Apparently, in discussing the law for the fifth time, the justices got tired of trying to answer questions that Congress should have addressed by writing a clearer law. Cato now joins the National Association of Criminal Defense Lawyers, the National Association of Federal Defenders, and Families Against Mandatory Minimums in arguing that the clause should indeed be void for vagueness. Despite four previous attempts to clarify the law, lower courts are as confused as ever about how the ACCA interacts with, among other offenses, attempted crimes, physical attacks on police officers, and statutory rape cases. This vagueness is not just a problem for defendants like Mr. Johnson here; it raises concerns about the separation of powers. The Supreme Court has said that overly vague statutes impermissibly draft judges into a legislative role. Quite so: vague language forces the judiciary, not the legislature, to define criminal offenses and establish their penalties. Legislation—especially when it implicates individual liberty—must be clear and understandable enough that the general public can ascertain the conduct it prohibits. If trained and experienced judges can’t even figure out what a law means, clearly it’s too vague for an average person to understand. If at first, and second, and third, and fourth you don’t succeed in clarifying vague language, perhaps it’s time to throw out the legal text and try again. As another Samuel Johnson might say, injuries are revenged, crimes are avenged, and vague laws are rewritten.