(concurring):
35 I concur in the majority opinion. I write only to clarify why, in my judgment, Lee was not prejudiced by the erroneous instruction on imperfect self-defense on the facts of this case and under controlling statutory law.
36 The interplay between perfect self-defense and imperfect self-defense is subtle. Perfect self-defense is a complete defense to any crime. See State v. Knoll, 712 P.2d 211, 214 (Utah 1985) ("[Slelf-defense is a justification for killing and a defense to prosecution." (citation and internal quotation marks omitted)); see also State v. Spillers, 2007 UT 13, ¶ 23, 152 P.3d 315 (referring to this first type of self-defense as "perfect self-defense"). It is available to one who reasonably believed that force was necessary to defend against unlawful force:
A person is justified in threatening or using force against another when and to the extent that he or she reasonably believes that force is necessary to defend himself or a third person against such other's imminent use of unlawful force.
Utah Code Ann. § 76-2-402(1) (LexisNexis 2003). But this general rule is subject to a crucial corollary: the use of lethal force is justified only in the reasonable belief that it is "necessary to prevent death or serious bodily injury ... or to prevent the commission of a forcible felony." Id.3
1 37 In contrast, imperfect self-defense is a partial defense, reducing a charge of murder or attempted murder to manslaughter or attempted manslaughter. State v. Low, 2008 UT 58, ¶ 22, 192 P.3d 867. It is available to one who reasonably but incorrectly believed that his use of lethal force was legally justified:
It is an affirmative defense to a charge of murder or attempted murder that the defendant caused the death of another or attempted to cause the death of another . under a reasonable belief that the circumstances provided a legal justification or excuse for his conduct although the conduct was not legally justifiable or excusable under the existing cireumstances.
Utah Code Ann. § 76-5-208(4)(a), (a)@it) (LexisNexis Supp.2006).
138 In State v. Low our supreme court identified the factor distinguishing perfect self-defense from imperfect self-defense: "whether the defendant's conduct was, in fact, 'legally justifiable or excusable under the existing circumstances"" 2008 UT 58, ¶ 32, 192 P.3d 867 (quoting Utah Code Ann. § 76-5-208(4)(a)(ii) (LexisNexis Supp.2007)). In other words, if, under the facts as he reasonably believed them to be, the defendant's conduct was legally justifiable, he then acted in perfect self-defense. If, under the facts as he reasonably believed them to be, he reasonably but incorrectly believed his actions were legally justifiable, he acted in imperfect self-defense.
139 Ordinarily "for both perfect and imperfect self-defense, 'the same basic facts [are] at issue."" Spillers 2007 UT 13, ¶ 23, 152 P.3d 315 (alteration in original) (quoting State v. Howell, 649 P.2d 91, 95 (Utah 1982)). So when would a person ever reasonably but incorrectly believe he was entitled to use force to defend himself? Spillers suggests the answer.
T 40 Spillers shot a man who, Spillers testified, had struck him with a gun on the back of the head and was poised to strike again. Id. 13. The State argued that the evidence permitted the jury to reach one of only two results: either Spillers had committed murder or he had acted in perfect self-defense. Id. 1921-28. But the supreme court concluded that the evidence was amenable to a *1176third interpretation: Spillers was entitled to defend himself against his assailant, but not with lethal foree. Id. 128. In other words, where Spillers's assailant was using his gun as a club, a jury might find that Spillers reasonably but incorrectly believed that lethal force was "necessary to prevent death or serious bodily injury ... or to prevent the commission of a forcible felony." Utah Code Ann. § 76-2-402(1) (LexisNexis 2008). Accordingly, the court held that the trial court erred in denying Spillers an imperfect self-defense instruction. Spillers, 2007 UT 13, ¶ 23, 152 P.3d 315.
[ 41 We learn from Spillers that a defendant is entitled to an instruction on imperfect self-defense if a jury could conclude from the evidence that he reasonably but incorrectly believed he was justified in using lethal force against a non-lethal attack. Stated more generally, imperfect self-defense applies when a defendant makes a reasonable mistake of law-when he acts "under a reasonable belief that the cireumstances provided a legal justification or excuse for his conduct although the conduct was not legally justifiable or excusable under the existing cireum-stances." Utah Code Ann. § 76-5-203(4)(a)(ii) (LexisNexis Supp.2006). On the other hand, perfect self-defense applies when a defendant makes a reasonable mistake of fact-when his conduct was justifiable under the facts as he reasonably believed them to be.4
1 42 We can distill Low and Spillers into a two-part inquiry. To determine whether either version of self-defense is available, we assess both the defendant's understanding of the facts and the defendant's understanding of the law. If the defendant's understanding of the facts is correct (or incorrect but reasonable) and the defendant's understanding of the law is correct, perfect self-defense is available. If the defendant's understanding of the facts is correct (or incorrect but reasonable) and the defendant's understanding of the law is incorrect but reasonable, imperfect self-defense is available. And if either the defendant's understanding of the facts is unreasonable or the defendant's understanding of the law is incorrect and unreasonable, neither perfect self-defense nor imperfect self-defense is available.
143 Here, Lee argues in effect that his understanding of the facts was incorrect but reasonable. He testified that, as the altercation escalated, TH. pointed Lee's own gun at him, Lee grabbed it back, and TH. reached behind him for what Lee believed was "another gun." If this version of events was true, Lee reasonably but incorrectly believed that TH. was about to employ lethal force against him, justifying his own use of lethal force. Lee thus qualified for a perfect self-defense instruction because his understanding of the facts was reasonable and his understanding of the law was correct-if T.H. had a gun and intended to use it, Lee was legally entitled to respond with lethal force.
144 But Lee did not qualify for an imperfect self-defense instruction, because he never claimed that his understanding of the law was reasonable but incorrect, he never claimed that, under the cireumstances as he reasonably believed them to be, he reasonably but incorrectly believed he had a right to respond with lethal force. One can imagine a scenario where imperfect self-defense would have been available. Had Lee testified that he shot T.H. because he believed TH. was pulling, say, brass knuckles out of his back pocket, Lee may have been entitled to an instruction on imperfect self-defense. In that situation, he could argue that he reasonably believed that the cireumstances justified his use of lethal force when in fact they justified only his use of non-lethal force.
4 45 In short, this case presents the very factual dichotomy that Spillers did not: the testimony at Lee's trial allowed only two options-that Lee was "either guilty of murder or [entitled to acquittal] under a [perfect] self-defense theory." See 2007 UT 13, ¶ 23, 152 P.3d 315. Accordingly, I conclude that Lee was not prejudiced by trial counsel's *1177failure to object to the erroneous jury instruction on imperfect self-defense.
. For purposes of this statutory section, a foreible felony includes aggravated assault, most homicides, kidnapping, many sex crimes, and any other felony involving "the use of force or violence against a person so as to create a substantial danger of death or serious bodily injury." Utah Code Ann. § 76-2-402(4) (LexisNexis 2003). An assault is aggravated if the actor uses a dangerous weapon or "other means or force likely to produce death or serious bodily injury." Id. § 76-5-103(1). A dangerous weapon is "any item capable of causing death or serious bodily injury" or, under certain circumstances, a facsimile or representation of the item. Id. § 76-1-601(5).
. Of course, perfect self-defense also applies when a defendant makes neither a mistake of law nor a mistake of fact.