The US Supreme Court will kick off a new term on Oct. 7 with arguments about a classic topic, the knowledge of good and evil.

Specifically, the justices will be considering the constitutionality of a Kansas statute that abolished the insanity defense. It’s being challenged by Kraig Kahler, a man convicted by a Kansas jury for the 2009 murders of his two daughters, ex-wife, and former mother-in-law, plus burglary, and sentenced to death.

Kahler’s attorneys argue that the state’s approach to mental illness in criminal culpability violates the US Constitution’s eighth amendment prohibition on cruel and unusual punishment, as well as the 14th amendment guarantee of due process of law. They say their client’s sentence is a mistake because Kahler lacked the moral capacity to tell right from wrong when he killed his family. The attorneys say that this was because Kahler suffered from severe depression and personality disorders, and he should have been allowed to present an insanity defense.

Kansas argues in its brief that Kahler is not insane. Before he killed his family on Thanksgiving weekend 2009, he was a successful public official. He was reportedly obsessed with seeming to have the perfect life and family, and for a while he did. However, when his wife left him for a woman, Kahler was shattered. He lost his job and moved in with his parents.

Not long after, Kahler drove to his former mother-in-law’s house and shot everyone but his son. The state says Kahler’s actions—driving, sparing his son, aiming at his targets and hitting them, pursuing the victims from room to room—are evidence that he had the requisite intent to commit the murders. The state also contends that the expert testimony presented by the defendant didn’t show he was insane anyway, even had Kansas allowed the desired defense.

Most notably, the state argues that its statute is constitutional. It has not abolished the insanity defense but redefined it, Kansas claims. “While Kansas no longer has an affirmative defense called insanity, evidence of mental disease or defect is still admissible to show a lack of mens rea, thus exempting certain mentally ill individuals from criminal liability,“ the state writes.

The elements of a crime

Kansas law considers mental defects in the context of intent. Mens rea is one of two elements prosecutors must prove along with criminal conduct, or an actus reus. State law allows defendants to argue that they lacked the mental state—or mens rea—for criminal culpability, and to present evidence of a mental illness that prevented them from forming the requisite intent. But they can’t argue that they weren’t guilty by virtue of insanity.

An amicus brief filed by law professors and philosophers “for neither party” argues that the Kansas law is unjust because a mentally ill defendant could form the intent to commit a crime but still lack the capacity to judge between right and wrong. And a fair society doesn’t punish someone who can’t understand the consequences of their actions or any subsequent punishment. “Sanity is a precondition of responsibility,” they write. “The lack of an insanity defense, as in Kansas, will ensure that the state punishes some defendants in the absence of responsibility for their crimes.”

In other words, Kahler may have had the intent to kill. Yet, he might still not have been able to judge the morality of his act. That failure—not knowing in the moment of action that killing was wrong—would make Kahler not responsible for the crime by virtue of his mental illness.

The brief distinguishes between excuses, such as self-defense, and justifications that negate wrongdoing, such as infancy, duress, and insanity. The justification defenses exist because society recognizes that to be responsible for one’s acts a person has to be able to judge what they are doing.

In the beginning

Kahler argues that the insanity defense has a long and venerable history in humanity’s many legal traditions. His brief cites the first book of the Old Testament, Genesis, for support. “Ancient civilizations recognized the distinction between the insane and those capable of understanding the moral implications of their actions,” Kahler’s attorneys write. “In the early Jewish tradition, ‘madness’ was an excuse for otherwise punishable crimes. The first pages of the Torah introduce ‘knowledge of good and evil’ as a central reality of the human condition.”

In Islam, Christianity, and Greek philosophy, too, they argue, the ability to distinguish between right and wrong is a prerequisite to culpability. It’s been similarly established in the Anglo-Saxon tradition since the 18th century, according to Kahler’s counsel. That’s why 48 American jurisdictions—45 states, the federal criminal justice system, the military justice system, and the District of Columbia—currently provide an affirmative insanity defense that considers a defendant’s lack of moral culpability.

Kansas begs to differ. The state writes, “Many ancient references to insanity are at best ambiguous and consistent with the mens rea approach.” It dismisses claims that ancient courts considered insanity exculpatory. On the contrary, the state argues, insanity was a sign of intent. Neither the framers of the constitution nor ancient Greek philosophers would be bothered about exculpating the insane based on a “good and evil” test, the state says. “Kansas has reasonably determined that individuals who voluntarily and intentionally kill another human being are culpable, even if they do not recognize their actions are morally wrong,” the state concludes.

This is what most offends the philosophers. They argue that there is no basis for punishment if a person can’t make moral judgments, and a statute that fails to recognize as much is fundamentally unjust. “It is crucial for our society to tolerate a diversity of ethical views…but toleration and deference cannot go so far as to allow a government to perpetrate injustice of this sort. The Court should correct Kansas’s error,” they urge.

Kansas flips the script on opponents of the statute, saying its approach is actually more “evolved” than the old insanity defense. The state argues that by “redefining” insanity’s role in culpability, it severs the link between mental illness and criminality, minimizing societal stigma for the afflicted.

It’ll be months before we learn where the justices ultimately stand on the knowledge of good and evil and its role in crime and punishment. But by Monday, their most pressing concerns should at least become apparent.