In this way Florida—and states like Georgia and Texas, too—flouted the Atkins rule by rendering its mandate almost unrecognizable. Can’t lawfully execute the mentally disabled? No problem, they concluded, we’ll just change the definition of disability. That will be much harder to do after Tuesday’s ruling. By a 5-4 vote, with Justice Anthony Kennedy again the swing vote, the Court has refined and broadened Atkins’s scope. Florida’s onerous rule “disregards established medical practices,” Kennedy wrote for the majority, and those practices require courts to consider the sort of evidence Florida refused to consider in Hall’s case. Will it be enough to spare his life for good? Stay tuned.

It is significant—fitting, even—that Kennedy, the great defender of states’ rights, would come down so squarely against the state’s interest here. It ought to signal how far afield the Florida Supreme Court went to endorse Hall's execution. It is also significant that the Court’s majority shored Atkins up not in clinical terms but by devoting significant space in its opinion to a description, in the plainest and most wrenching terms, of the scope of Hall’s cognitive disabilities and the impact they have had upon his life.

Hall could not assist in his own defense, for example, because his lawyer found him to have a "mental … level much lower than his age" at best comparable to the lawyer’s 4-year-old daughter.” Another attorney from another case came forward years ago to testify that he “couldn’t really understand anything [Hall] said” because of his disability. Kennedy noted that Hall’s “siblings testified that there was something ‘very wrong’ with him as a child,” which probably helps explain, in the words of Hall’s sentencing judge, why he was raised “under the most horrible family circumstances imaginable.”

This evidence was before Florida officials for years, was ignored by those officials even after Atkins, and now is part of a Supreme Court majority opinion which declares it not just relevant but perhaps dispositive to the outcome of the case. The Court thus has forced state officials and lower-court judges to back away from the dry formalism they have relied upon to execute the cognitively disabled. Florida’s IQ-test rule, Kennedy concluded

disregards established medical practice in two interrelated ways. It takes an IQ score as final and conclusive evidence of a defendant’s intellectual capacity, when experts in the field would consider other evidence. It also relies on a purportedly scientific measurement of the defendant’s abilities, his IQ score, while refusing to recognize that the score is, on its own terms, imprecise.

More than that, though, Hall signals to officials in other states that the Court meant what it said in Atkins. “If the States were to have complete autonomy to define intellectual disability as they wished, the Court’s decision in Atkins could become a nullity, and the Eight Amendment’s protection of human dignity would not become a reality.” Kennedy wrote. “This Court thus reads Atkins to provide substantial guidance on the definition of intellectual disability.” This clarification comes too late for Marvin Wilson, a severely mentally disabled man Texas killed two years ago. But it comes just in time, perhaps, for men like Hall and Warren Lee Hill in Georgia.