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For several years, the Supreme Court’s Second Amendment jurisprudence has revolved around the deceptively simple idea that modern gun laws must be consistent with the nation’s historical tradition of firearm regulation. In practice, however, that command has often generated more heat than light. Courts and litigants have spent years searching for historical analogs, debating how close is close enough and struggling to determine when a modern regulation meaningfully resembles something from the founding era.

In United States v. Hemani, handed down earlier this month, the court offered one of its clearest answers yet. The case involved a federal law that makes it a crime for an “unlawful user” of a controlled substance to possess a firearm. Ali Hemani admitted that he used marijuana several times a week and kept a gun in his home. The government did not allege that he was intoxicated while possessing the firearm, that he was addicted to marijuana, that he had ever threatened anyone, or that his gun ownership had resulted in any danger. His regular marijuana use alone was enough, the government argued, to place him outside the Second Amendment’s protection.

Writing for the majority, Justice Neil Gorsuch rejected that argument. But the significance of the decision extends well beyond marijuana use. Hemani is really a case about the discipline that constitutional analogical reasoning requires. The government attempted to justify the statute by invoking historical laws directed at so-called “habitual drunkards.” Yet, as the court explained, those laws regulated a very different class of people for very different reasons and through very different mechanisms.

The historical “habitual drunkard” was not simply someone who drank regularly. Early American........

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