It’s unconstitutional to execute people with intellectual disabilities, that much the U.S. Supreme Court has made clear. But things get fuzzy when individual states try to legally determine “intellectual disability,” and that ambiguity is leading Texas to its latest hearing before the high court.

On Tuesday morning, the eight justices will hear arguments surrounding Texas’ method of determining the condition, ultimately deciding if the state’s approach fits within past rulings specifying who can be put to death. The case is brought forth by Bobby Moore, a death row inmate of more than 36 years.

In April 1980, Moore, then 20, walked into a Houston supermarket with two other men, wearing a wig and holding a shotgun, according to Texas’ brief to the high court. He approached the clerks’ counter and shot 73-year-old James McCarble once in the head, killing him. Decades after receiving the death sentence, the 57-year-old man still sits in prison.

Moore’s appeals have been exhaustive; he was even granted a second sentencing hearing in 2001 and again handed the death penalty. In his latest appeal, Moore’s attorneys claim he is intellectually disabled, which would make him ineligible for execution. But courts have disagreed on how to legally determine the disability.

Two previous Supreme Court rulings have addressed executing intellectually disabled people. In 2002, Atkins v. Virginia ruled that executing those with the disability violates the Eighth Amendment’s ban on cruel and unusual punishment, but left it up to the states to legally define the condition. In 2014, Hall v. Florida specified that an IQ higher than 70 did not alone eliminate the condition, and that legal determination “is distinct from a medical diagnosis but is informed by the medical community’s diagnostic framework.”

“I’m very concerned of this propensity to push the envelope and define more and more people as intellectually disabled, including people that regular folks would not for a moment consider to have that condition.”

— Kent Scheidegger, legal director of the Criminal Justice Legal Fund

After Atkins, Texas’ highest criminal court set the state’s definition of intellectual disability in Ex parte Briseno as those with a low IQ and poor adaptive functioning since childhood. In setting the standard, the Texas Court of Criminal Appeals controversially referred to Lennie, a character from John Steinbeck’s Of Mice and Men, in describing how to define the condition.

“Most Texas citizens might agree that Steinbeck’s Lennie should, by virtue of his lack of reasoning ability and adaptive skills, be exempt,” the ruling stated. “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?”

The same year Hall was decided, a state district court determined that Moore was indeed intellectually disabled, but the ruling was overturned by the Court of Criminal Appeals. Judge Cheryl Johnson wrote that the lower court erred by using current medical standards to determine the condition, instead of the framework set up by the criminal appeals court, which adheres to the 1992 definition set by the American Association on Intellectual and Developmental Disabilities (AAIDD).

“Although 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,” Johnson wrote in the opinion. “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, which we have repeatedly asked it to do.”

On Tuesday, attorney Cliff Sloan, representing Moore, will argue that the CCA violated the constitution by requiring courts to use medically outdated standards to determine if a death row inmate is intellectually disabled and ineligible for execution.

Moore’s brief claims that courts should be using current medical standards to determine intellectual disability, and that the Briseno definition is unjust and only qualifies certain people with intellectual disability as exempt from execution, instead of all of them.

“In Briseno, the CCA — while ostensibly adopting a 1992 clinical definition of intellectual disability — criticized the medical community’s diagnostic framework as ‘exceedingly subjective,’ and fashioned its own additional ‘factors’ for intellectual disability derived from lay stereotypes and lacking any clinical foundation,” Moore’s brief to the high court said.

“There really isn’t any confusion in the medical and the scholarly environments about what is intellectual disability. This is a condition that’s been around since as long as there’s been people.”

— Margaret Nygren, executive director and CEO of AAIDD

Solicitor General Scott Keller will argue for Texas, claiming that the state’s way of defining intellectual disability is in line with the Atkins and Hall decisions, and that the CCA ruled correctly, since Hall distinguishes that the legal definition of intellectual disability is different than a medical diagnosis.

“The CCA actually considered the clinical definitions that petitioner labels ‘current medical standards’… and relied on them to apply certain concepts,” the brief states. “What the CCA did not do is adopt those definitions wholesale as Texas’s legal standard for Atkins claims.”

In regards to the “Lennie” discussions, the brief said it is a “strained effort” by Moore and the ACLU to taint the Briseno test.

“The ‘Lennie’ comment should be understood for what it actually was: an aside,” the brief states.

Kent Scheidegger, legal director of the pro-death penalty Criminal Justice Legal Foundation, filed a brief in support of Texas. He said in a phone interview that he doesn’t have a problem with the Atkins ruling, but is concerned about the “creep” in defining intellectual disability.

“I’m very concerned of this propensity to push the envelope and define more and more people as intellectually disabled, including people that regular folks would not for a moment consider to have that condition,” Scheidegger said.

A push that doesn’t exist, according to Margaret Nygren, the executive director and CEO for AAIDD.

“There really isn’t any confusion in the medical and the scholarly environments about what is intellectual disability,” she said. “This is a condition that’s been around since as long as there’s been people.”

In the petition, Moore is asking the court to reverse the CCA’s ruling and toss out his sentence. If the court rules in his favor, it could set even further restrictions on how states can legally determine intellectual disability, as it did in Hall.

Nygren hopes the court will further intertwine the medical definition with the legal one.

“What I’d like to see happen is a recognition that statutory definition should really mirror the diagnostic criteria supported by research,” she said.

Texas is asking that the court affirm the state court’s ruling, or, at the very least remand the case back to the CCA to further review current medical standards, instead of setting national precedents.

“If the Court concludes that the CCA did not sufficiently consult these sources, then the appropriate remedy would not be to install part or all of those definitions as a national Atkins standard,” the state’s brief wrote. “The redress for that claimed injury would be a remand with instructions to give due ‘consideration’ to [the current diagnostic framework].”

Other death penalty news:

• Moore will be the second Texas death row inmate to have his case heard by the U.S. Supreme Court this fall. In October, Duane Buck argued for a new sentencing trial since a psychologist at his original trial testified that black men are more dangerous than whites.

• This year, Texas and the nation will see the fewest number of executions in 20 years.