Last spring, the Supreme Court of the United States made a little-remarked constitutional ruling in Kahler v. Kansas. Upon casual inspection, Kahler looks like a doctrinal dead-end. The petitioner asked the Supreme Court to recognize a due process right for mentally ill defendants to raise the M’Naghten right-and-wrong test of insanity, and the Court said, “No.” The petitioner’s failure notwithstanding, Kahler is not a barren vine. On the contrary, it is heavy-laden with new doctrinal insights for criminal law scholars.

The case deserves a thorough look—not for what it can teach us about constitutional contentions that the Court has rendered unviable for the foreseeable future—but for what the creative arguments written by the petitioner’s attorneys can teach us about criminal law. Specifically, the briefing in Kahler unmasks a cached moral blameworthiness requirement in criminal law doctrine, on the same plane as the canonical requirements of voluntariness, action, and mens rea, but buried over the last two centuries in the rules governing the insanity defense.