In People v. Barton (1995), 12 Cal. 4th 186, the Supreme Court observed: "The sole difference between true self-defense and 'unreasonable self-defense' is that the former applies only when the defendant acts in response to circumstances that cause the defendant to fear, and would lead a reasonable person to fear, the imminent infliction of death or great bodily injury ( 197, 198); unreasonable self-defense, on the other hand, does not require the defendant's fear to be reasonable." (Id. at pp. 199-200, italics in original.) The Supreme Court observed further that unreasonable self-defense is "not a true defense; rather, it is a shorthand description of one form of voluntary manslaughter. And voluntary manslaughter, whether it arises from unreasonable self-defense or from a killing during a sudden quarrel or heat of passion, is not a defense but a crime; more precisely, it is a lesser offense included in the crime of murder. Accordingly, when a defendant is charged with murder the trial court's duty to instruct sua sponte, or on its own initiative, on unreasonable self-defense is the same as its duty to instruct on any other lesser included offense: this duty arises whenever the evidence is such that a jury could reasonably conclude that the defendant killed the victim in the unreasonable but good faith belief in having to act in self-defense." (Id. at pp. 200-201.)