Opinion
CORRIGAN, J.
A killing committed because of an unreasonable belief in the need for self-defense is voluntary manslaughter, not murder. “Unreasonable self-defense, also called imperfect self-defense, ‘obviates malice because [130]*130that most culpable of mental states “cannot coexist” with an actual belief that the lethal act was necessary to avoid one’s own death or serious injury at the victim’s hand.’ (People v. Rios (2000) 23 Cal.4th 450, 461 [97 Cal.Rptr.2d 512, 2 P.3d 1066].)” (People v. Beltran (2013) 56 Cal.4th 935, 951 [157 Cal.Rptr.3d 503, 301 P.3d 1120].)
The question here is whether the doctrine of unreasonable self-defense is available when belief in the need to defend oneself is entirely delusional. We conclude it is not. No state, it appears, recognizes “delusional self-defense” as a theory of manslaughter. We have noted that unreasonable self-defense involves a mistake of fact. (In re Christian S. (1994) 7 Cal.4th 768, 779, fn. 3 [30 Cal.Rptr.2d 33, 872 P.2d 574] (Christian S.).) A purely delusional belief in the need to act in self-defense may be raised as a defense, but that defense is insanity. Under our statutory scheme, a claim of insanity is reserved for a separate phase of trial. At a trial on the question of guilt, the defendant may not claim unreasonable self-defense based on insane delusion.
I. BACKGROUND
The relevant facts are undisputed. Defendant was, by all accounts, mentally ill. He had repeatedly been institutionalized and diagnosed as psychotic. On the day of the killing, he was living in a rehabilitation center. While visiting his grandmother’s house that morning, he became fidgety and anxious. At one point, he began to crawl under cars as his family and a friend tried to speak with him. He left his grandmother’s home around 12:30 p.m.
Meanwhile, 53-year-old Ella Suggs was doing her weekend shopping. She wore a necklace with a charm in the shape of a turtle, which had a magnifying glass in place of the shell. She also wore reading glasses on a chain around her neck. About 1:00 p.m., Brandon Wilson looked out a restaurant window and noticed Suggs sitting at a bus stop across the street. He saw defendant walk past Suggs, stop, look in both directions, and return to confront her. Defendant did not seem to be talking to himself.
Defendant grabbed Suggs and appeared to pull on something around her neck. Suggs raised her hands defensively, stood, and tried to walk away. Defendant pushed her back to a seated position, brought his hands together over his head, and plunged them toward Suggs’s chest. Then he fled, looking around as he ran. Suggs stood for a moment before falling. She had been stabbed with a paintbrush handle sharpened to a point. The weapon penetrated six or seven inches, through a lung and into her heart. Neither the turtle necklace nor the reading glasses were found at the scene or among Suggs’s possessions.
[131]*131Within half an hour, Wilson saw defendant return and approach the bus stop. He appeared to be puzzled, and fled. After Wilson alerted a security officer, police apprehended defendant. It took four officers to subdue him. His behavior was sufficiently bizarre that he was referred for psychiatric evaluation.
Charged with murder, defendant pleaded both not guilty and not guilty by reason of insanity. At the guilt phase, forensic psychiatrists were called by both prosecution and defense. They agreed that defendant suffered from schizophrenia, but disputed whether he was actively psychotic when he stabbed Suggs.
Defendant testified, and gave a confused account of the killing. On direct examination, he repeatedly said, “something went wrong out there in the street.” When asked for detail, he said, “Somebody was saying something violent to me, and I didn’t really — it was something violent happening while I was out there.” Defense counsel pursued the question of who was violent. Defendant said, “Some person out there,” but could not say whether the person was a man or a woman. He claimed to have blacked out. Counsel asked, “Did you pick that paint brush off the ground?” Defendant said, “Yeah, I made an object.” “What was it?” counsel asked. “I made an object after I was out on the ground dazed somewhere. After I was on the ground or whatever. However it happened.” Defendant admitted using the object but refused to say how. Asked if he stabbed someone with it, he responded, “I suppose.” When asked why he stabbed Suggs, defendant answered, “Person said something and did something to me, I didn’t just go do it to be doing it.” Defendant refused to say what had been done to him, and denied taking anything from Suggs.
The prosecutor was no more successful at eliciting a coherent version of the events. Defendant said that when he was at the bus stop, “They said something to me.” He denied asking Suggs for money or being angry that she would not give him any. He admitted making the paintbrush into a weapon “after I got up. I was mad and scared.” He then said he did not know if he had made it and thought he picked it up in that condition.1 He admitted stabbing Suggs, but claimed the act was unintentional. He denied trying to steal anything.
The prosecutor argued for first degree murder, relying on both malice aforethought and felony-murder theories. The defense requested jury instructions on unreasonable self-defense (CALCRIM No. 571), mistake of fact [132]*132(CALCRIM No. 3406), and the effect of hallucination on the degree of murder (CALCRIM No. 627). The court refused those requests, but did tell the jury to consider the evidence of defendant’s mental illness in deciding whether he had acted with malice or the intent to rob Suggs. (CALCRIM No. 3428.)
The jury returned a first degree murder conviction. After the guilt phase, against the advice of counsel, defendant withdrew his plea of not guilty by reason of insanity and was sentenced to 25 years to life in prison. On appeal, he challenged the court’s refusal to instruct on unreasonable self-defense and hallucination. The Court of Appeal summarily rejected his argument on unreasonable self-defense, relying on People v. Mejia-Lenares (2006) 135 Cal.App.4th 1437 [38 Cal.Rptr.3d 404] (Mejia-Lenares) for the rule that the doctrine does not apply when belief in the need for self-defense arises solely from the defendant’s delusional mental state. However, the court held that the refusal to instruct on hallucination was prejudicial error. It remanded with directions for retrial or a conviction of second degree murder, at the prosecutor’s election.2
In this court, defendant contends he was entitled to an instruction on unreasonable self-defense. He does not claim there was any factual basis for him to believe he had to defend himself. His argument is that unreasonable self-defense may be based solely on a defendant’s delusional mental state. The same question was raised in People v. Wright (2005) 35 Cal.4th 964 [28 Cal.Rptr.3d 708, 111 P.3d 973], but there we deemed it unnecessary to reach the issue. We granted defendant’s petition for review in order to settle the question.
II. DISCUSSION
A. The Law of Homicide and Unreasonable Self-defense
Homicide, the killing of one human being by another, is not always criminal. In certain circumstances, a killing may be excusable or justifiable. (Pen. Code, § 195 et seq.;3 see 1 Witkin & Epstein, Cal. Criminal Law (4th ed. 2012) Crimes Against the Person, § 96, pp. 885-886.) Murder and manslaughter are the forms of criminal homicide. “Murder is the unlawful killing of a human being . . . with malice aforethought.” (§ 187, subd. (a).) Malice aforethought may be express or implied. (§ 188.)
The Penal Code defines express malice as “a deliberate intention unlawfully to take away the life of a fellow creature.” (§ 188.) In Christian S., we [133]*133explained that this “inartful language” means the defendant must intend to act unlawfully, or in other words, have a “wrongful intent.” (Christian S., supra, 7 Cal.4th at p. 778.) We rejected the argument that malice turns on whether the killing is deemed lawful, without regard to the defendant’s intent. (Id. at pp. 778-779.) However, the defendant need not intend to break the law or commit a crime. Rather, malice requires an intent to kill that is “unlawful” because the law deems it so. “ ‘The adverb “unlawfully” in the express malice definition means simply that there is no justification, excuse, or mitigation for the killing recognized by the law.’ ” (People v. Saille (1991) 54 Cal.3d 1103, 1115 [2 Cal.Rptr.2d 364, 820 P.2d 588].)
Malice is implied when an unlawful killing results from a willful act, the natural and probable consequences of which are dangerous to human life, performed with conscious disregard for that danger. (§ 188; People v. Gonzalez (2012) 54 Cal.4th 643, 653 [142 Cal.Rptr.3d 893, 278 P.3d 1242]; People v. Knoller (2007) 41 Cal.4th 139, 151-152 [59 Cal.Rptr.3d 157, 158 P.3d 731].)
“A killing with express malice formed willfully, deliberately, and with premeditation constitutes first degree murder.” (People v. Beltran, supra, 56 Cal.4th at p. 942.) “Second degree murder is the unlawful killing of a human being with malice aforethought but without the additional elements, such as willfulness, premeditation, and deliberation, that would support a conviction of first degree murder.” (People v. Knoller, supra, 41 Cal.4th at p. 151.) Thus, the mens rea required for murder is malice, express or implied. (Beltran, at p. 942.)
Manslaughter, a lesser included offense of murder, is an unlawful killing without malice. (§ 192; People v. Thomas (2012) 53 Cal.4th 771, 813 [137 Cal.Rptr.3d 533, 269 P.3d 1109].) Section 192 establishes three kinds of manslaughter: voluntary, involuntary, and vehicular. Only voluntary manslaughter is at issue here. Punishment is mitigated for this offense, which the law deems less blameworthy than murder because of the attendant circumstances and their impact on the defendant’s mental state. Two factors may preclude the formation of malice and reduce murder to voluntary manslaughter: heat of passion and unreasonable self-defense. (People v. Beltran, supra, 56 Cal.4th at pp. 942, 951; People v. Blakeley (2000) 23 Cal.4th 82, 87-88 [96 Cal.Rptr.2d 451, 999 P.2d 675].) Heat of passion is recognized by statute as a mitigating factor. (§ 192, subd. (a).) Unreasonable self-defense is founded on both statute and the common law. (People v. Anderson (2002) 28 Cal.4th 767, 782 [122 Cal.Rptr.2d 587, 50 P.3d 368].)
Self-defense, when based on a reasonable belief that killing is necessary to avert an imminent threat of death or great bodily injury, is a complete [134]*134justification, and such a killing is not a crime. (§ 197, subd. 3; see 1 Witkin & Epstein, Cal. Criminal Law, supra, Defenses, § 67 et seq., p. 507 et seq.) A killing committed when that belief is unreasonable is not justifiable. Nevertheless, “one who holds an honest but unreasonable belief in the necessity to defend against imminent peril to life or great bodily injury does not harbor malice and commits no greater offense than manslaughter.” (People v. Flannel (1979) 25 Cal.3d 668, 672 [160 Cal.Rptr. 84, 603 P.2d 1] (Flannel).) We have also described this mental state as an “unreasonable but good faith belief’ in the need for self-defense. (E.g., People v. Barton (1995) 12 Cal.4th 186, 199 [47 Cal.Rptr.2d 569, 906 P.2d 531].) However, it is most accurately characterized as an actual but unreasonable belief.4
“ ‘A person who actually believes in the need for self-defense necessarily believes he is acting lawfully.’ [Citation.] Because express malice requires an intent to kill unlawfully, a killing in the belief that one is acting lawfully is not malicious.[5] The statutory definition of implied malice does not contain similar language, but we have extended the imperfect self-defense rationale to any killing that would otherwise have malice, whether express or implied.” (People v. Anderson, supra, 28 Cal.4th at p. 782.) “A defendant who acts with the requisite actual belief in the necessity for self-defense does not act with the base motive required for implied malice . . . .” (Christian S., supra, 7 Cal.4th at p. 780, fn. 4; accord, People v. Blakeley, supra, 23 Cal.4th at p. 88.)6 Unreasonable self-defense is “not a true defense; rather, it is a shorthand description of one form of voluntary manslaughter.” (People v. Barton, supra, 12 Cal.4th at p. 200.) Whenever there is substantial evidence that the defendant killed in unreasonable self-defense, the trial court must instruct on this theory of manslaughter. (Id. at p. 201.)
Here, defendant claims his request for an instruction on unreasonable self-defense should have been granted, even though his perception of a threat was entirely delusional. The claim fails, under both case law and statute. California cases reflect the understanding that unreasonable self-defense involves a misperception of objective circumstances, not a reaction produced by mental disturbance alone. And the statutory scheme, though it permits [135]*135evidence of mental illness to show that the defendant did not harbor malice, reserves the issue of legal insanity for a separate phase of trial. As shall be seen, a belief in the need for self-defense that is purely delusional is a paradigmatic example of legal insanity.
B. Case Law
The difference between unreasonable self-defense and a claim that mental deficiency prevented the formation of malice was made clear in Christian S., supra, 7 Cal.4th 768. There we considered whether the Legislature intended to do away with unreasonable self-defense when it abolished the defense of diminished capacity, in 1981. Diminished capacity was a judicially created concept. It allowed defendants to argue that because of mental infirmity, they lacked “awareness of the obligation to act within the general body of laws regulating society,” and therefore were incapable of acting with malice. (People v. Conley (1966) 64 Cal.2d 310, 322 [49 Cal.Rptr. 815, 411 P.2d 911] (Conley); see Christian S., supra, 1 Cal.4th at p. 774.)
In Christian S., we concluded that the Legislature did not mean to abrogate unreasonable self-defense along with diminished capacity. (Christian S., supra, 7 Cal.4th at p. 783.) In reaching that conclusion, we firmly distinguished the two theories. “The doctrine of imperfect self-defense had a lineage independent of the notion of mental capacity set forth in Conley, supra, 64 Cal.2d 310. In Flannel . . . , supra, 25 Cal.3d 668, we traced the long development of the doctrine in California courts. (People v. Wells (1949) 33 Cal.2d 330 [202 P.2d 53]; People v. Lewis (1960) 186 Cal.App.2d 585 [9 Cal.Rptr. 263]; Roads v. Superior Court (1969) 275 Cal.App.2d 593 [80 Cal.Rptr. 169]; People v. Sedeno (1974) 10 Cal.3d 703 [112 Cal.Rptr. 1, 518 P.2d 913].) Indeed, 30 years before the diminished-capacity defense was allowed, a California court approved the imperfect self-defense doctrine: ‘ “[I]f the act is committed under the influence of an uncontrollable fear of death or great bodily harm, caused by the circumstances, but without the presence of all the ingredients necessary to excuse the act on the ground of self-defense, the killing is manslaughter” [citation].’ (People v. Best (1936) 13 Cal.App.2d 606, 610 [57 P.2d 168].)” (Christian S, at p. 776.)
Flannel, we noted, had rejected the idea that “ ‘the doctrine of unreasonable belief is necessarily bound up with or limited by the concepts of either heat of passion or diminished capacity.’ ” (Christian S., supra, 7 Cal.4th at p. 777, quoting Flannel, supra, 25 Cal.3d at p. 677.) “The two doctrines relate to the concept of malice, but the similarity ends there. Unlike diminished capacity, imperfect self-defense is not rooted in any notion of mental capacity or awareness of the need to act lawfully.” (Christian S., supra, 1 Cal.4th at pp. 777-778.)
[136]*136Thus, unreasonable self-defense is not premised on considerations of mental disorder. From its earliest appearance in California law, unreasonable self-defense has been deemed to apply when the defendant’s act was “ ‘caused by the circumstances,’ ” rather than by cognitive defects alone. (People v. Best, supra, 13 Cal.App.2d at p. 610, italics added; see Christian S., supra, 7 Cal.4th at p. 776; Flannel, supra, 25 Cal.3d at p. 676; People v. Lewis, supra, 186 Cal.App.2d at p. 598.) As we said in Christian S., unreasonable self-defense “is based on a defendant’s assertion that he lacked malice . . . because he acted under an unreasonable mistake of fact — that is, the need to defend himself against imminent peril of death or great bodily harm.” (Christian S., supra, 1 Cal.4th at p. 779, fn. 3, italics added.)
In Mejia-Lenares, supra, 135 Cal.App.4th 1437, the Court of Appeal relied on Christian S. in holding that purely delusional acts are excluded from the scope of unreasonable self-defense. The court noted that California cases involving delusional defendants tended to be treated under the rubric of diminished capacity, when that doctrine was viable, whereas cases involving factual misperceptions were analyzed in terms of unreasonable self-defense. (Mejia-Lenares, at pp. 1448-1449.) Mejia-Lenares reasoned that because unreasonable self-defense is “a species of mistake of fact (see [Christian S., supra, 7 Cal.4th] at p. 779, fn. 3) ... it cannot be founded on delusion. In our view, a mistake of fact is predicated upon a negligent perception of facts, not, as in the case of a delusion, a perception of facts not grounded in reality.[7] A person acting under a delusion is not negligently interpreting actual facts; instead, he or she is out of touch with reality.” (Mejia-Lenares, at pp. 1453-1454.)
We agree with the Mejia-Lenares court that unreasonable self-defense, as a form of mistake of fact, has no application when the defendant’s actions are [137]*137entirely delusional. A defendant who makes a factual mistake misperceives the objective circumstances. A delusional defendant holds a belief that is divorced from the circumstances. The line between mere misperception and delusion is drawn at the absence of an objective correlate. A person who sees a stick and thinks it is a snake is mistaken, but that misinterpretation is not delusional. One who sees a snake where there is nothing snakelike, however, is deluded. Unreasonable self-defense was never intended to encompass reactions to threats that exist only in the defendant’s mind.
Defendant claims this limitation is inconsistent with our decision in People v. Wells, supra, 33 Cal.2d 330 (Wells). It is not; delusion was not a factor in Wells. Unlike most cases of unreasonable self-defense, Wells was not a homicide case but the prosecution of a prison inmate for assault with malice aforethought under section 4500.8 (Wells, at p. 334.) Wells had been ejected from a disciplinary hearing for disrupting the proceedings. In the hall outside, he encountered the guard who had brought charges against him. Wells “seized a heavy crockery cuspidor and threw it” at the guard, severely injuring him. (Id. at p. 338.)
Wells testified that he had no intent to hit the guard, but only picked up the cuspidor to defend himself from another guard who struck him with a baton. The blow caused him to fall and release the cuspidor. (Wells, supra, 33 Cal.2d at p. 339.) The defense also offered testimony from prison physicians that Wells suffered from an abnormal physical and mental condition, not amounting to insanity. According to the doctors, he was in a state of tension that rendered him highly sensitive to external stimuli and abnormally fearful for his personal safety. As a result, he reacted to apparent threats more violently and unpredictably than an average person would. (Id. at pp. 344 — 345.) This court reasoned that if Wells had “acted only under the influence of fear of bodily harm, in the belief, honest though unreasonable, that he was defending himself from such harm by the use of a necessary amount of force, then . . . the essential element of ‘malice aforethought’ would be lacking.” (Id. at p. 345; see § 4500.)
Defendant argues that Wells’s attack on a prison guard was completely unprovoked, with no objective circumstances supporting the claim of unreasonable self-defense. He misreads the case. As noted in Mejia-Lenares, Wells held a “belief which, although skewed by mental illness, was nevertheless factually based.” (Mejia-Lenares, supra, 135 Cal.App.4th at p. 1449.) There was no evidence that Wells’s perception of a threat was delusional. To the [138]*138contrary, he claimed his actions were an attempt to defend himself from an actual baton-wielding guard. (Wells, supra, 33 Cal.2d at p. 339.) The expert testimony was that Wells was abnormally sensitive to external stimuli. (Id. at pp. 344-345.) Wells does not support defendant’s claim that unreasonable self-defense requires no objective basis.
Defendant also places great weight on this statement in Flannel, supra, 25 Cal.3d 668: “No matter how the mistaken assessment is made, an individual cannot genuinely perceive the need to repel imminent peril or bodily injury and simultaneously be aware that society expects conformity to a different standard.” (Id. at p. 679, italics added.) Defendant contends delusions are necessarily included in Flannel’s expansive characterization of the misperceptions that may motivate an act of unreasonable self-defense. However, the comment on which he relies was predicated on a concept of malice that was central to the diminished capacity defense. The Legislature has specifically repudiated that concept. Section 188, as amended in 1981 and revised in 1982, declares that “[n]either an awareness of the obligation to act within the general body of laws regulating society nor acting despite such awareness is included within the definition of malice.” (See Stats. 1981, ch. 404, § 6, p. 1593; Stats. 1982, ch. 893, § 4, p. 3318.)
In Christian S., we observed that Flannel’s discussion of unreasonable self-defense “had two independent premises: (1) the notion of mental capacity set forth in Conley, supra, 64 Cal.2d 310, 322 [i.e., diminished capacity], and (2) a grounding in both well-developed common law and in the statutory requirement of malice (. . . § 187). The 1981 amendments make clear the Legislature intended to eliminate the notion of diminished capacity. Thus, that part of the reasoning in Flannel ... is no longer valid. But, Flannel’s other premise was not affected by the amendments.” (Christian S., supra, 7 Cal.4th at p. 777-, see Mejia-Lenares, supra, 135 Cal.App.4th at p. 1454.) Defendant seeks support from the part of Flannel’s discussion that is now invalid. In the surviving aspect of its analysis, based on the common law and statutory roots of unreasonable self-defense, Flannel incorporated the observation made in People v. Best, supra, 13 Cal.App.2d at page 610, that unreasonable self-defense entails a reaction that is “ ‘ “caused by the circumstances.” ’ ” (Flannel, supra, 25 Cal.3d at p. 676; see Christian S., at p. 776.)
The phrase “caused by the circumstances” denotes a motivation arising from objective facts, not delusions. Accordingly, Flannel comports with our view that purely delusional perceptions of threats to personal safety cannot be [139]*139relied upon to claim unreasonable self-defense. This understanding of the doctrine is consistently reflected in the decisions of other state courts.9
C. Statute
Defendant asserts a statutory basis for his claim in section 28, subdivision (a) (hereafter, section 28(a)). This provision states that evidence of mental disorders is admissible “on the issue of whether or not the accused actually formed a required specific intent, premeditated, deliberated, or harbored malice aforethought, when a specific intent crime is charged,” a theory sometimes referred to as “diminished actuality.” (See People v. Mills (2012) 55 Cal.4th 663, 671 [147 Cal.Rptr.3d 833, 286 P.3d 754] (Mills).) Section 28(a) bars evidence of the defendant’s capacity to form a required mental state, consistent with the abolition of the diminished capacity defense.10
Defendant contends the plain language of section 28(a) permits him to introduce evidence of the mental disorder that gave rise to his belief in the need for self-defense, and precluded him from actually harboring malice. If section 28(a) is viewed in isolation, this construction is logically defensible. However, it is unsustainable when the provision is considered in light of the statutory scheme governing evidence of mental illness, and the legislative history of section 28.
“ ‘[T]he “plain meaning” rule does not prohibit a court from determining whether the literal meaning of a measure comports with its purpose or [140]*140whether such a construction of one provision is consistent with other provisions of the statute.’ [Citation.] [¶] The language is construed in the context of the statute as a whole and the overall statutory scheme, and we give ‘significance to every word, phrase, sentence, and part of an act in pursuance of the legislative purpose. [Citation.]’ [Citations.] The intent of the law prevails over the letter of the law, and ‘ “the letter will, if possible, be so read as to conform to the spirit of the act.” ’ ” (People v. Canty (2004) 32 Cal.4th 1266, 1276-1277 [14 Cal.Rptr.3d 1, 90 P.3d 1168].)
Under California’s statutory scheme, “[p]ersons who are mentally incapacitated” are deemed unable to commit a crime as a matter of law. (§ 26, par. Two.) Mental ■ incapacity under section 26 is determined by the M’Naghten test for legal insanity provided in section 25, subdivision (b). (M’Naghten’s Case (1843) 8 Eng.Rep. 718, 722; People v. Phillips (2000) 83 Cal.App.4th 170, 173 [99 Cal.Rptr.2d 448]; see Stats. 2007, ch. 31, § 5, pp. 138-139.) Under M’Naghten, insanity is established if the defendant was unable either to understand the nature and quality of the criminal act, or to distinguish right from wrong when the act was committed. (§ 25, subd. (b); Mills, supra, 55 Cal.4th at p. 671; People v. Skinner (1985) 39 Cal.3d 765, 775-777 [217 Cal.Rptr. 685, 704 P.2d 752] (Skinner).)11
A claim of unreasonable self-defense based solely on delusion is quintessentially a claim of insanity under the M’Naghten standard of inability to distinguish right from wrong. Its rationale is that mental illness caused the defendant to perceive an illusory threat, form an actual belief in the need to kill in self-defense, and act on that belief without wrongful intent. (People v. Anderson, supra, 28 Cal.4th at p. 782; Christian S., supra, 7 Cal.4th at pp. 778, 780, fn. 4.) In M’Naghten’s Case itself, the judges observed; “[I]f under the influence of [a] delusion [the defendant] supposes another man to be in the act of attempting to take away his life, and he kills that man, as he supposes, in self-defence, he would be exempt from punishment.” (M’Naghten’s Case, supra, 8 Eng.Rep. at p. 723; see Skinner, supra, 39 Cal.3d at p. 781, fn. 13.) We noted in Skinner that this example “applies the right/wrong prong of the M’Naghten test to an insane delusion in the same manner as it is applied to other forms of insanity. The delusion . . . results in an inability to appreciate that the act is wrong. The defendant believes he is defending himself.” (Skinner, at p. 781, fn. 13.)
Thus, what defendant attempted here was to assert a claim of legal insanity at the guilt phase of his trial. That is not allowed under our statutes. Section 1026 sets out the applicable procedure when, as in this case, the defendant pleads both not guilty and not guilty by reason of insanity. The trial [141]*141is bifurcated, with the question of guilt tried first. The defendant is presumed innocent, of course, but in order to reserve the issue of sanity for the second phase of trial the defendant is also conclusively presumed to have been legally sane at the time of the offense. (§ 1026, subd. (a); Mills, supra, 55 Cal.4th at p. 681.)12 Evidence of the defendant’s mental state may not be admitted at the guilt phase to prove insanity. (§ 1020; Mills, at p. 672.)13 If the defendant is found guilty, the trial proceeds to the sanity phase, where the defendant bears the burden of proof by a preponderance of the evidence. (§§ 1026, subd. (a), 25, subd. (b).) “The separation of the two stages of the bifurcated trial is solely for the purpose of keeping the issues of guilt and sanity distinct; for other purposes, the trial is regarded as single and continuing.” (5 Witkin & Epstein, Cal. Criminal Law, supra, Criminal Trial, § 795, p. 1227; see Wells, supra, 33 Cal.2d at p. 349.)
In Mills, we held that the jury should not be instructed on the presumption of sanity at the guilt phase, because the question of legal sanity is then irrelevant. (Mills, supra, 55 Cal.4th at pp. 680-681.) “The Legislature’s intent in providing for bifurcation when a defendant pleads both not guilty and not guilty by reason of insanity was to simplify the issues before the jury, by ‘removjmg] entirely from the first stage of the trial any issue as to legal sanity.’ (Wells, supra, 33 Cal.2d at p. 352.)” (Mills, at p. 681.) Therefore, it is improper for the jury to weigh the presumption of sanity during deliberations on the question of guilt. (Ibid.) However, it is equally improper for the jury to consider whether the defendant was legally insane under M’Naghten. Whether mental disease or defect prevented the defendant from understanding the nature and quality of the criminal act, or appreciating its wrongfulness, are questions relevant only at a sanity trial. (§§ 25, subd. (b), 1016, 1020, 1026, subd. (a); Mills, at pp. 671-672.)
Section 28(a) allows defendants to introduce evidence of mental disorder to show they did not actually form a mental state required for guilt of a charged crime. But the scope of the diminished actuality defense is necessarily limited by the presumption of sanity, which operates at a trial on the question of guilt to bar the defendant from claiming he is not guilty because he is legally insane. This limitation was explored in Wells, supra, 33 Cal.2d 330. Although Wells predated the enactment of section 28, its analysis [142]*142established the distinction between actual formation of a mental state and capacity to form a mental state that is now found in section 28(a).14
“Whenever a particular mental state, such as a specific intent, is by statute made an essential element of a crime, that specific state must be proved like any other fact. [Citations.] Since, however, the mental capacity to commit the crime — insofar as legal sanity is concerned — is conclusively presumed at the first section of the trial, it follows that the . . . general rule . . . must be deemed to be qualified to this extent: That evidence tending to show lack of mental capacity to commit the crime because of legal insanity is barred at that stage. This simply means that legal sanity is not in issue at the first stage of the trial and that the evidence must be confined to that which is pertinent to issues then being litigated.” (Wells, supra, 33 Cal.2d at p. 350.)
Wells explained that evidence challenging the defendant’s actual formation of a mental state is admissible, but only so long as it does not go toward a claim of legal insanity: “As a general rule, on the not guilty plea, evidence . . . tending to show that the defendant, who at this stage is conclusively presumed sane, either did or did not, in committing the overt act, possess the specific essential mental state, is admissible, but evidence tending to show legal sanity or legal insanity is not admissible. Thus, if the proffered evidence tends to show not merely that he did or did not, but rather that because of legal insanity he could not, entertain the . . . essential mental state, then that evidence is inadmissible under the not guilty plea and is admissible only on the trial on the plea of not guilty by reason of insanity. . . . Evidence which tends to show legal insanity (likewise, sanity) is not admissible at the first stage of the trial because it is not pertinent to any issue then being litigated; but competent evidence, other than proof of sanity or insanity, which tends to show that a (then presumed) legally sane defendant either did or did not in fact possess the required specific intent or motive is admissible.” (Wells, supra, 33 Cal.2d at pp. 350-351; see § 1020.)
To understand how Wells’ s distinction between evidence of mental capacity and evidence of actual formation of a mental state came to be reflected in section 28(a), a brief historical review is necessary. The Wells distinction was eroded in subsequent cases. (Mills, supra, 55 Cal.4th at p. 671; see People v. Wetmore (1978) 22 Cal.3d 318, 324-326 [149 Cal.Rptr. 265, 583 P.2d 1308] (Wetmore).) In Wetmore, it was expressly disavowed: “Wells’ distinction [143]*143between evidence that defendant did not entertain the requisite intent, which is admissible, and evidence that he could not entertain that intent, which is inadmissible, cannot be supported. ‘[As] a matter of logic, any proof tending to show that a certain mental condition could not exist is relevant and should be admissible to show that it did not exist. And, of course, proof that something could not exist is the best possible evidence that it did not exist.’ (Louisell & Hazard, Insanity as a Defense: The Bifurcated Trial (1961) 49 Cal.L.Rev. 805, 819.)” (Wetmore, at p. 324.)
The Wetmore court recognized that its holding would “require the jury to hear the same evidence twice, once to determine diminished capacity and once to determine insanity.” (Wetmore, supra, 22 Cal.3d at p. 331.) To avoid such a “pointless waste of judicial time and resources,” the court invited the Legislature to “reconsider the wisdom of the statutes providing for bifurcated trial.” (Ibid.) It suggested that “[t]he evidentiary duplication inherent in the present procedure could be eliminated either by a unitary trial [citations], or by a new method of bifurcation in which issues of diminished capacity and insanity are tried together at the second phase of the trial [citations].” (Ibid.)
However, the Legislature roundly rejected Wetmore's invitation. It acted in 1981 to preserve the bifurcated trial statutes and eliminate the defense of diminished capacity. At the same time the Legislature codified, in section 28(a), the distinction between capacity and actuality that was drawn in Wells and disparaged in Wetmore. (Stats. 1981, ch. 404, § 4, p. 1592; see People v. Saille, supra, 54 Cal.3d at p. 1111.)15 The following year, the voters adopted an initiative measure that again abolished the diminished capacity defense, and went further in the restoration of former law by reinstating the M’Naghten test for legal insanity. That standard had prevailed in California [144]*144from the earliest days of our statehood until this court replaced it with an alternative formulation in People v. Drew (1978) 22 Cal.3d 333, 345, 149 Cal.Rptr. 275, 583 P.2d 1318. (See Skinner, supra, 39 Cal.3d at p. 768; Saille, supra, 54 Cal.3d at p. 1112.)
“As a result of these developments, the current state of California law on the insanity defense and proof of the defendant’s mental state is generally consistent with the principles set out in Wells.” (Mills, supra, 55 Cal.4th at p. 671.) Accordingly, the provisions of section 28(a) allowing evidence of diminished actuality are “qualified” by the caveat that at a trial on the question of guilt, “evidence tending to show lack of mental capacity to commit the crime because of legal insanity is barred . . . .” (Wells, supra, 33 Cal.2d at p. 350.) The Wells court squarely rejected the idea that defendants have the right to present evidence of insanity at the guilt phase to persuade the jury they did not entertain a required mental state. “The force which otherwise would attach to [that] argument,” said the court, “is fully obviated by the provisions for trial on the issue of not guilty by reason of insanity. We are satisfied that the enactments [providing for a bifurcated trial and a presumption of sanity at the guilt phase] ... are a valid exercise of legislative power and that the evidence to be received at the first stage of the trial is properly limited to the extent of excluding evidence tending to show either sanity or insanity.” (Wells, at p. 353.)
The United States Supreme Court has confirmed that state law does not violate due process by “restricting consideration of defense evidence of mental illness and incapacity to its bearing on a claim of insanity, thus eliminating its significance directly on the issue of the mental element of the crime charged.” (Clark v. Arizona (2006) 548 U.S. 735, 742 [165 L.Ed.2d 842, 126 S.Ct. 2709].) It is clear that the scope of section 28(a) must be restricted as we have explained, given California’s statutory requirement that guilt and sanity be tried separately.
The bifurcated approach offers substantial benefits to the defense. At the guilt phase, the prosecution must prove beyond a reasonable doubt each element of the offense, including mens rea. The defendant has the opportunity to obtain an acquittal or a verdict on a lesser included offense, without having to claim insanity and risk the prospect of involuntary commitment for psychiatric treatment. The defense has available the panoply of strategies open to legally sane defendants, including unreasonable self-defense based on mistake of fact. It may choose to put on no evidence, or it may introduce any relevant and admissible evidence on the question of guilt. If the defendant is found not guilty, the trial is over. If there is a conviction, the trial moves to the second phase, devoted to the question of legal sanity. There the defendant bears the burden of proof by a preponderance of the evidence, and may be [145]*145found not guilty by reason of insanity. This process affords the defense two chances at a favorable verdict.
However, the defense may not litigate the question of legal sanity at both phases. The defendant is presumed sane at the guilt phase, and cannot introduce evidence of insanity to counter the prosecution’s showing of mens rea. A claim of self-defense based solely on delusion is more than a claim of unreasonable self-defense; as we have shown, it is a claim of legal insanity. If section 28(a) were applied to allow the defendant to make that claim at the guilt phase, the burden would shift to the prosecution to prove beyond a reasonable doubt that the defendant was not insane. The statutory scheme would be turned on its head.
We have recognized that the evidence of a defendant’s mental state at the guilt and sanity phases “may be overlapping.” (People v. Hernandez, supra, 22 Cal.4th at p. 520.) But the extent of the overlap is limited by the rule that “the defendant’s sanity is irrelevant at the guilt phase and evidence tending to prove insanity, as opposed to the absence of a particular mental element of the offense, is inadmissible.” (Mills, supra, 55 Cal.4th at p. 672.) The defendant may employ mental state evidence in different ways at the guilt and sanity phases, but may not make the same showing twice. (Ibid.) The Wetmore court acknowledged that the diminished capacity defense, once the limits imposed in Wells were removed, allowed the duplication of evidence at the guilt and sanity phases to “approach[] a totality.” (Wetmore, supra, 22 Cal.3d at p. 331.) As noted above, the statutory amendments of 1981 and 1982 abolished that practice and effectively endorsed the distinctions and restrictions delineated in Wells. Among those restrictions is the settled rule that “insanity ... is either a complete defense or it is no defense at all.” (Wells, supra, 33 Cal.2d at p. 349, and citing cases.)
Accordingly, a claim of delusional belief in the need for self-defense is reserved for the sanity phase, where it may result in complete exoneration from criminal liability. (M’Naghten’s Case, supra, 8 Eng.Rep. at p. 723; Skinner, supra, 39 Cal.3d at p. 781, fn. 13.) It may not be employed to reduce a defendant’s degree of guilt. When the Legislature enacted section 28, it certainly did not intend to allow defendants to do what the Wetmore court had permitted: argue first that their mental condition made them guilty of a lesser crime, and then that the same condition made them not guilty at all by reason of insanity.16
Our construction of section 28(a) has no effect on evidence of mental disorders that do not amount to legal insanity. As noted in Mills, “a [146]*146defendant may suffer from a diagnosable mental illness without being legally insane under the M’Naghten standard. (See 1 Witkin & Epstein, Cal. Criminal Law[, supra,] Defenses, § 17 et seq., p. 349 et seq.)” (Mills, supra, 55 Cal.4th at p. 672.) All relevant evidence of mental states short of insanity is admissible at the guilt phase under section 28(a), including evidence bearing on unreasonable self-defense, as in Mills and Wells. (Mills, at p. 678; Wells, supra, 33 Cal.2d at p. 346.) The M’Naghten test is a narrow one. It is only when “mental illness is manifested in delusions which render the individual incapable either of knowing the nature and character of his act, or of understanding that it is wrong, [that] he is legally insane under the California formulation of the M’Naghten test.” (Skinner, supra, 39 Cal.3d at p. 782.)
Furthermore, our holding does not prevent the defense from presenting evidence of mental disease, defect, or disorder to support a claim of unreasonable self-defense based on a mistake of fact. A defendant who misjudges the external circumstances may show that mental disturbance contributed to the mistaken perception of a threat, without presenting the jury with the same question it would confront at a sanity trial. The jury must find there was an actual, unreasonable belief in the necessity of self-defense based on the circumstances, and it should be so instructed. (See People v. Tewksbury (1976) 15 Cal.3d 953, 963 [127 Cal.Rptr. 135, 544 P.2d 1335].) An insanity defense, on the other hand, requires no showing that the defendant responded to objective circumstances. At a sanity trial, and only at a sanity trial, the defense can maintain that purely delusional perceptions caused the defendant to believe in the necessity of self-defense.
In sum, defendants who mistakenly believed that actual circumstances required their defensive act may argue they are guilty only of voluntary manslaughter, even if their reaction was distorted by mental illness. But defendants who contend they killed in self-defense because of a purely delusional perception of threat must make that claim at a sanity trial. Unreasonable self-defense and legal insanity are distinct theories, and must be adjudicated separately.
Here, defendant’s claim of unreasonable self-defense was based entirely on a delusional mental state that amounted to legal insanity. The trial court properly denied his request for an instruction on unreasonable self-defense.
III. DISPOSITION
We affirm the Court of Appeal’s judgment.
Cantil-Sakauye, C. J., Baxter, J., and Chin, J., concurred.