After Clark (defendant) shot and killed a police officer who had pulled him over for a traffic stop, he was charged with first-degree murder for “intentionally and knowingly” killing the officer in the line of duty. At a bench trial, Clark pleaded not guilty by reason of insanity and thereafter sought to introduce evidence by lay and expert witnesses of his undisputed mental illness, paranoid schizophrenia, to show that he did not intend to shoot the officer and did not know what he was doing. The trial judge ruled that Clark’s evidence of mental disease could not be considered on the issue of mens rea, but allowed it to be introduced on the issue of insanity. Clark’s insanity defense was rejected by the trial judge who determined that Clark’s schizophrenia did not prevent him from knowing that his actions were wrong, a required element for an insanity defense. The trial judge found Clark guilty of first-degree murder and sentenced him to life imprisonment. Clark appealed and the Arizona Court of Appeals affirmed the judgment. The state supreme court denied review and the U.S. Supreme Court granted certiorari to determine whether Arizona violated Due Process in barring consideration of defense evidence of mental illness on the issue of the mental element of the crime.