At its all-too-rare best, American law involves the careful application of serious principles to concrete facts. At its all-too-common worst, it involves the proclamation of worthy bromides followed by obfuscation, injustice, and self-congratulation.

The battle between facts and verbiage is being intensely fought right now in one specialized area—the application of the death penalty to defendants with intellectual disability (ID). (ID is the current diagnostic term for what used to be called “mental retardation.”) A case scheduled for argument Tuesday before the United States Supreme Court is the latest skirmish.

The principle at stake—serious, humane, and, one would think, all but unexceptionable morally—is that executing an individual with ID violates the Eighth Amendment’s ban on “cruel and unusual punishments.” The Court announced the Eighth Amendment rule in 2002 in a case called Atkins v. Virginia. Since then, states in the death belt have been suggesting the Justices can’t really be serious about the rule. Moore v. Texas, Tuesday’s case, tests a definition used by Texas courts that essentially prevents the state’s legal definition of ID from moving past the state courts’ estimate of what it was in 1992, even though current psychiatric understanding has changed. Texas assures the Court that they support the rule of Atkins—but they ask the Court to let the state apply its own definition of ID.

That’s despite the fact that in a 2014 case, Hall v. Florida, the Supreme Court overturned a Florida rule that barred any defendant with an IQ score of 70 or higher from even presenting other evidence that he or she had ID. The 70-IQ cutoff was a legal one; clinicians don’t have a firm rule about scores. The Court voided the rule. It told the state its rules need not precisely conform to clinical standards, but must be “informed” by the medical and psychiatric consensus about ID.

Which brings us to Bobby James Moore, who was convicted of shooting a grocery clerk to death during a botched robbery in 1980. After the Atkins decision, Moore filed a habeas corpus claim in Texas state court arguing that he was intellectually disabled and thus could not be executed.

Let’s understand something: ID is a psychiatric and scientific term, not something given to the care of lawyers. It’s not self-explicating (any more than are legal terms like “due process” or “cruel and unusual”) but it’s not meaningless, and it is used by a large trained clinical community. The definition has changed over the years as knowledge has grown. The current definition was set forth in 2013 in the fifth edition of American Psychiatric Association’s Diagnostic and Statistical Manual of Mental Disorders, known as the “DSM-5.” Under that definition, the condition must have developed during early life. Second, the individual must have “deficits in intellectual functions,” as measured by “clinical assessment and individualized, standardized intelligence testing.” And finally, the individual must also have “deficits in adaptive functioning” that “limit functioning in one or more areas of daily life.”

Lawyers and judges don’t like to give up the idea that there can be a separate “legal ID” that doesn’t depend on a lot of double-talking Ph.D.s. There’s some history there—“insanity,” as used in most state legal systems for criminal law purposes, isn’t a psychiatric term at all, but one that came out of the common law. Looking at that history, some judges wonder why they can’t just invent a new meaning for ID as well. But there is no definition of “medical insanity.” Only lawyers use the term in any serious sense.

In Moore’s case, the trial judge heard evidence for two days and held that Moore had the condition. The Texas Court of Criminal Appeals overturned that judgment and reinstated his death sentence. The appeals court said that the trial judge had been wrong to rely on the 2013, DSM-5 definition of ID. Earlier Texas cases relied on a standard put forth by ID clinicians in 1992. The appeals court said Texas was going to stick with that no matter what current research showed: “[A]lthough the mental-health fields and opinions of mental-health experts inform the factual decision, they do not determine whether an individual is exempt from execution under Atkins. The decision to modify the legal standard for intellectual disability in the capital-sentencing context rests with this Court unless and until the Legislature acts . . . .” Applying the old standard, the Texas court held that Moore did not meet its standards for ID, and thus could be executed.

Though it cited the 1992 clinical standard, the Texas court also cited as law an idiosyncratic standard it had developed itself after Atkins. In that 2004 case, In re Briseno, the court had said it was all very well for mental-health professionals to “define mental retardation broadly to provide an adequate safety net for those who are at the margin and might well become mentally-unimpaired citizens if given additional social services support.” But that definition, it said, didn’t bind Texas courts, who are responsible to public opinion in the state, not psychiatric knowledge.

We, however, must define that level and degree of mental retardation at which a consensus of Texas citizens would agree that a person should be exempted from the death penalty. Most Texas citizens might agree that [John] Steinbeck’s Lennie [in his novel, Of Mice and Men,] should, by virtue of his lack of reasoning ability and adaptive skills, be exempt. But, does a consensus of Texas citizens agree that all persons who might legitimately qualify for assistance under the social services definition of mental retardation be exempt from an otherwise constitutional penalty? Put another way, is there a national or Texas consensus that all of those persons whom the mental health profession might diagnose as meeting the criteria for mental retardation are automatically less morally culpable than those who just barely miss meeting those criteria? Is there, and should there be, a “mental retardation” bright-line exemption from our state’s maximum statutory punishment?

The answer to the last question, as I read the Supreme Court’s caselaw, is somewhere between “Yes” and “Look, hoss, what part of ‘yes’ do you not understand?” Hall requires that legal judgments be “informed” by clinical ones—it doesn’t empower judges to decide that their voters find psychiatrists irrelevant.

Even more remarkable is the state court’s phrasing of the Eighth Amendment query as whether “a consensus of Texas citizens” would find a given defendant suitable for the application of the Atkins rule. The Eighth Amendment, last time I checked, was a part of the U.S. Constitution, and there’s no “That’s fine up there in Yankeeland but don’t signify down here” exception in that document.

This kind of “you’re not serious” case is a subtle but significant challenge to the Supreme Court’s authority. Last term’s abortion case, Whole Woman’s Health v. Hellerstedt, concerned the “undue burden” standard the Court has announced for regulations of abortion. The Texas legislature, and the Fifth Circuit Court of Appeals, had approved a sweeping set of restrictions that would have reduced abortion availability drastically in the state. They were labeled as “health regulations,” but the evidence showed the new rules would have had no significant health advantage. The appeals court had said, wink wink nudge nudge, that if a legislature called a law a health law, then it didn’t matter how little benefit it brought or how hard it made getting an abortion.

The Court smacked down that challenge. Will it do the same with the Of Mice and Men rule?