The distinction between murder and manslaughter developed to prevent individuals who killed another in a drunken brawl from receiving an automatic death penalty. Intoxication itself is no excuse for committing a crime. Since ancient times debate has existed as to whether intoxication should be partially excusing or whether it should increase the gravity of the crime. Saint Thomas Aquinas believed that intoxication should be partially excusing. In contrast, Aristotle thought the penalty should be doubled, since in addition to the crime, the intoxicated individual was setting a bad example for others.
Settled insanity does qualify for an insanity defense.
This concept in California dates back to the late 1800s. Diminished capacity should not be confused with diminished responsibility whereby a sentence is reduced after a defendant has been found guilty of a crime. In federal jurisdictions, mental illness, but not drug and alcohol intoxication, can be a basis for a downward departure in sentencing. Most importantly, the expanded definitions of premeditation and malice required for murder were eliminated with a return to common law definitions. The expanded definitions involved gradations of guilt that led to conviction for different crimes according to degree of blameworthiness, using aspects usually reserved for ALI insanity defenses.