How do you define whether someone’s sufficiently mentally disabled to avoid the death penalty?

That’s a question that could be resolved by the Supreme Court, which on Monday morning agreed to take up a new case that could limit how states set death penalty policies. The issue at hand is whether Florida’s approach to determining mental disability in death penalty cases violates an earlier Supreme Court ruling on the issue.

“This is really one of the most important death penalty cases that the court has undertaken in years,” says Brian Stull, a senior staff attorney at the American Civil Liberties Union’s Capital Punishment Project, which advocates for the abolition of the death penalty.

The case at hand involves a Florida man, Freddie Lee Hall, sentenced to death for the 1978 murder of Karol Hurst, a 21-year-old pregnant woman.

His lawyers argue that he is mentally disabled and should not be put to death. In 2002, the U.S. Supreme Court ruled that executing the mentally disabled constitutes “cruel and unusual punishment,” which is banned by the Constitution, but left it up to states to define mental disability. Florida uses a 70-point IQ score cutoff, but Hall’s lowest score was a 71. His lawyers say that in order to comply with the 2002 ruling, his IQ should be read as somewhere between 67 and 75, to account for what scientists call a “standard error of measurement.”

But, in December, the Florida Supreme Court reiterated it didn’t buy the argument.

“We again decline to adopt this line of reasoning,” the court wrote in its opinion.

If the U.S. Supreme Court rules in favor of Hall—determining how mental disability should be defined—it could strip states of some power in setting such policies.