In the case of Eric Michael Clark v. Arizona (2005), the U.S. Supreme Court examined whether or not a state law could limit a defendant's ability to claim insanity as a defense in criminal cases. The case revolved around Eric Michael Clark, who was charged with first-degree murder but claimed he was insane at the time of his crime due to paranoid schizophrenia. However, under Arizona law, defendants were only allowed to use evidence of mental illness to argue they did not have specific intent required for certain crimes; it did not allow them to argue that their mental illness prevented them from knowing right from wrong - which is traditionally how insanity defenses work in most jurisdictions. The court ruled 6-3 in favor of upholding Arizona’s limited definition and application of an insanity defense, stating that states had significant leeway when defining legal terms like "insanity". This ruling effectively upheld restrictions on using claims about one's inability due to mental disease or defect from understanding right versus wrong as part of an insanity plea.

In the dissenting opinion for Eric Michael Clark v. Arizona, Justice Kennedy disagreed with the majority's decision to uphold Arizona's insanity defense statute, which only allowed defendants to prove they did not know their actions were wrong at the time of committing a crime. He argued that this narrow definition was inconsistent with common law principles and medical understanding of mental illness. Furthermore, he contended that it violated due process by preventing defendants from presenting evidence about their mental state as part of their defense. The justice also criticized the court’s ruling for its potential impact on mentally ill individuals who may lack capacity but still understand right from wrong in a legal sense; such people could be unfairly punished under this standard because they cannot demonstrate an inability to distinguish between right and wrong despite suffering severe impairment in other aspects related to cognition or control over behavior.