delivered the Opinion of the Court.
We granted certiorari to review the decision of the court of appeals in
People v. Hill,
I.
In 1988, the prosecution charged Edwin D. Hill with six counts of aggravated robbery in violation of section 18-4-302, 8B C.R.S. (1986), after Hill allegedly robbed several individuals at gunpoint on separate occasions. Hill pled not guilty by reason of insanity and a trial was held to determine if Hill was legally insane at the time of the offenses, pursuant to section 16-8-105(2), 8A C.R.S. (1986). Hill presented an expert witness, Dr. Dean Plazak, who testified at Hill’s first sanity trial that in his opinion, at the time of the armed robberies, Hill suffered from a condition known as cocaine delusional disorder. According to Dr. Plazak, this disorder is characterized by a psychosis resulting from brain damage caused by long-term cocaine use. Dr. Plazak also testified that this psychosis can remain with a drug user for months after the user has ceased to ingest cocaine, and that its symptoms, which resemble those of paranoid schizophrenia, include delusions, visual hallucinations, and “formication.” 1
The jury returned a verdict finding Hill legally sane at the time of the commission of the armed robberies. Another jury found Hill guilty of five of the six counts of aggravated robbery at the conclusion of a separate trial on the merits. Hill appealed the decision to the court of appeals, asserting that the trial court committed reversible errors during both trials.
The court of appeals affirmed Hill’s conviction on the merits, but reversed the judg *823 ment of sanity and remanded for a new trial on the sanity issue. People v. Hill, No. 90CA0466, slip op. at 1, 13 (Colo.App. July 23, 1992) (not selected for publication). The court of appeals held that the trial court abused its discretion in denying Hill’s challenge for cause as to one of the prospective jurors and that this error was not harmless. Id. at 5.
Before commencement of the defendant’s second sanity trial, we issued our decision in
Bieber v. People,
Hill conceded that, after Bieber, Dr. Pla-zak’s testimony was no longer relevant to the issue of Hill’s legal insanity at the time of the robberies because it only supported a settled insanity defense. Hill presented no other expert witnesses to testify to his mental condition at his second sanity trial. Instead, Hill relied solely on his own testimony as evidence of insanity and contended that this evidence was sufficient to overcome the statutory presumption of sanity.
Hill testified that his long history of drug use resulted in his becoming involved in various criminal activities and suffering from extensive loss of memory. He also testified that he suffered from a mental disease or defect in that “[his] mental process was that of a drug addict.” He stated that “during those years” he was not thinking at all, was “just going through the motions,” and that “only sometimes” did he perceive the difference between right and wrong. He explained that “something may be right in the eyes of the social public, but, to me, it wasn’t or that it didn’t make that much of a difference.” Hill further testified that during the three-month period in which he committed the charged offenses he suffered from feelings of isolation and depression.
During extensive and detailed questioning, Hill repeatedly denied having any recollection of the armed robberies for which he was charged. He denied any memory of the people or places involved. As a result of his claim that he suffered nearly total lack of recall as to any of the details of his daily life when the crimes were committed, he did not testify as to his mental condition at the specific times of the commission of the offenses, but only generally during a three month period surrounding the crimes and in the years preceding the crimes.
The trial court found that no evidence had been introduced about Hill’s state of mind at the time of the commission of the offenses, and that Hill’s state of mind at unspecified times was a result of voluntary intoxication. The trial court concluded that the statutory presumption of sanity had not been overcome and issued the following instruction to the jury:
Instruction No. 4
The law presumes everyone to be sane. The People have rested on the presumption of sanity. No evidence of insanity has been presented by the defense.
The jury returned a verdict finding the defendant sane at the time of the commission of the armed robberies.
On appeal, Hill argued that the trial court erred in submitting this instruction because it effectively directed a verdict against Hill in violation of his Sixth Amendment right to a jury trial. Hill did not challenge the trial court’s finding that the defense did not present any evidence of insanity, although Hill’s trial attorney had disagreed with the trial court’s conclusion. Nor did Hill argue on appeal that evidence of the state of mind of the defendant, while probative of settled insanity, was also admissible to show that the
*824
defendant was insane at the time of the commission of the offenses pursuant to the statutory definition of insanity.
3
In
Bieber
we rejected the defense of settled insanity and approved a trial court’s denial of an instruction on that defense,
We granted certiorari to determine: ‘Whether a defendant is entitled to have the question of his sanity determined by a jury where he has presented no evidence of insanity sufficient to overcome the presumption of sanity.”
II.
In order to resolve the issue before us, we must first determine whether the statutory procedure in effect in Colorado at the time of Hill’s offenses requires the sanity issue to be submitted to the jury where no evidence of insanity has been introduced. We then must determine whether the court of appeals correctly concluded that the Sixth Amendment right to a trial by jury in criminal cases requires the sanity issue to be submitted to the jury in the absence of any evidence of insanity.
A.
The defense of insanity is raised by a plea of not guilty by reason of insanity made at the time of arraignment.
People v. Low,
A presumption of sanity operates to relieve the prosecution of introducing evidence of sanity until some evidence of insanity is introduced.
See People v. Kernanen,
The requirement that the defendant present only some evidence of insanity to rebut the presumption is wholly proper, since a presumption is not evidence, and the primary purpose of the presumption of sanity is merely procedural convenience in those trials in which sanity is not really an issue.
Kernanen,
178 Colo, at 239-40,
The defendant cites several past decisions of this court in support of his position that the threshold question of whether some evidence of insanity has been introduced is for a jury to answer. It is true that several of our prior decisions have suggested that the question of sanity must be decided by a jury even where the defendant has completely failed to introduce evidence of insanity.
See, e.g., Palmer v. People,
Similarly, in
Henderson v. People,
The holdings of these decisions were driven by plain, unambiguous statutory language. In 1972 the General Assembly chose to alter the procedure by which a plea of insanity is resolved. Although the General Assembly continued to require that a defendant may only raise the issue of insanity through a specific plea, see ch. 44, sec. 1, § 39-8-103(1), 1972 Colo. Sess. Laws 190, 226, it no longer provided that the jury must render a verdict on the sanity issue once the defendant has pleaded “not guilty by reason of insanity.” See ch. 44, sec. 1, § 39-8-105, 1972 Colo. Sess. Laws 190, 226-27. In addition, the legislature codified the presumption of sanity, providing that every person is presumed to be sane until “evidence of insanity is introduced.” Ch. 44, sec. 1, § 39-8-105(2), 1972 Colo. Sess. Laws 190, 226-27.
Unlike the statutory provisions in effect prior to 1972, the current statutory procedure for resolving a plea of insanity fails to include language requiring the court to submit the issue of a defendant’s sanity to a jury once the defendant has entered an insanity plea. Our decisions holding that sanity is always an issue for a jury, which were based *826 on insanity statutes which unambiguously stated such a requirement, are no longer controlling. We conclude that Colorado’s statutory procedure does not require the sanity issue to be submitted to the jury where no evidence of insanity has been introduced.
The defendant argues that while the statutory framework for resolving insanity pleas does not expressly require that the sanity issue be submitted to the jury once an insanity plea is made, neither does it forbid such a procedure. Although it is true that the Colorado Code of Criminal Procedure does not expressly provide whether the court or a jury is to make the initial “any evidence of insanity” determination, we do not agree that the statutory framework for resolving insanity pleas is silent on the issue.
First, if we were to continue to require a trial court to submit the sanity issue to the jury, in the absence of evidence of insanity but after the defendant has entered a specific plea, we would be giving no effect to the legislature’s omission of prior statutory language stating such a requirement. If the legislature intended that an insanity plea should automatically entitle a defendant to a jury determination of the issue, it would not have deleted clear language setting forth such a requirement.
See Colorado Ground Water Comm’n v. Eagle Peak Farms, Ltd.,
Seeond, the legislature has expressly defined insanity as an affirmative defense to a crime.
See
§§ 18-1-802 & 805, 8B C.R.S. (1986);
People v. Serravo,
(1) “Affirmative defense” means that unless the state’s evidence raises the issue involving the alleged defense, the defendant, to raise the issue, shall present some credible evidence on that issue.
§ 18-1-407(1), 8B C.R.S. (1986). By the plain meaning of this statute, only when some credible evidence supports an affirmative defense is the prosecution required to disprove it as though it were another element of the offense.
See
§ 18-1^107(2), 8B C.R.S. (1986) (“If the issue involved in an affirmative defense is raised, then the guilt of the defendant must be established beyond a reasonable doubt as to that issue as well as all other elements of the offense.”);
Serravo,
In the absence of express statutory language to the contrary, the threshold determination as to whether some credible evidence exists to support an affirmative defense is a matter of law for the court to decide.
See, e.g., Lybarger v. People,
Nothing in the current statute defining the procedure for resolving an insanity defense indicates that the legislature intended to make the procedure for resolving the affirmative defense of insanity different than it is for resolving other affirmative defenses. *827 Under section 18-1-407(2), a defendant has not “raised the issue” of the affirmative defense of insanity, in the sense that it has become an issue for consideration by the factfinder, until the court determines credible evidence on that issue has been presented. See § 18-1-407(2), 8B C.R.S. (1986).
We hold that Colorado’s statutory procedure does not permit the issue of sanity to be submitted to the jury where no evidence of insanity has been presented. Under this statutory procedure, if a trial court properly determines that no evidence of insanity has been introduced, it should not submit the issue to the jury because the presumption of sanity has not been rebutted.
B.
We next address the question of whether the Sixth Amendment right to a trial by jury in criminal cases requires the issue of sanity to be submitted to a jury in the absence of any evidence of insanity. The court of appeals concluded that the trial court effectively directed a verdict against the defendant by instructing the jury of the presumption of sanity and the lack of evidence of insanity.
Hill,
Hill argues that because the Sixth Amendment guarantees a criminal defendant the right to a jury trial, the trial court committed reversible error by instructing the jury that the defendant was presumed sane and that no evidence of insanity had been presented. The court of appeals agreed with this conclusion, and stated that “by removing the determination of defendant’s sanity from the province of the jury, [the trial court] rendered the trial structurally defective and violated defendant’s Sixth Amendment right to a jury trial.” Id. at 830.
In a criminal trial “a judge may not direct a verdict for [the prosecution], no matter how overwhelming the evidence.”
Sullivan v. Louisiana,
We determined in our discussion of Colorado’s statutory procedure for raising the issue of sanity that insanity is an affirmative defense. We must now consider whether sanity should be treated as an element of the charges brought against Hill for Sixth Amendment purposes.
In
Patterson v. New York,
We thus decline to adopt as a constitutional imperative ... that a State must disprove beyond a reasonable doubt every fact constituting any and all affirmative defenses related to the culpability of an accused-[T]he Due Process Clause requires the prosecution to prove beyond a reasonable doubt all of the elements included in the definition of the offense of which the defendant is charged. Proof of the non-existence of all affirmative defenses has never been constitutionally required. ...
Id.
at 210,
Under the Court’s reasoning in
Patterson,
sanity is not treated as an element of an offense in Colorado, for purposes of due process, prior to the introduction of evidence of insanity.
See
In Hendershott v. People, we noted the distinction between the insanity issue and the prosecution’s burden of proving mens rea:
A person who is criminally insane is excused from criminal responsibility for his actions because, due to a mental disease or defect, he lacks the capacity to distinguish right from wrong with respect to the act or to adhere to the right or refrain from the wrong. This is not to say, however, that legal sanity is a proxy for mens rea. The prosecution’s burden of proof on the requisite mens rea for a crime is no less where the defendant has previously been adjudicated legally sane in a sanity trial than in a prosecution where the sanity issue has not been raised at all.
The trial court’s “directed verdict” of sanity in the present case did not impermissibly lessen the prosecution’s burden of proving each and every element of the crime of aggravated robbery as that crime is statutorily defined. Only if the trial court had directed a verdict against the defendant after some evidence of insanity had been introduced would it have impermissibly lessened the prosecution’s burden.
See
§ 16-8-105(2), 8A C.R.S. (1986) (“[Ojnce any evidence of insanity is introduced, the people have the burden of proving sanity beyond a reasonable doubt.”);
Lybarger,
State legislatures generally have considerable flexibility in defining the elements of criminal offenses.
See Patterson,
A legislature’s authority to define the elements of criminal offenses is not without limits. Its power to “regulate procedures under which its laws are carried out, including the burden of producing evidence and the burden of persuasion,” does not include the power to “ ‘offend[ ] some principle of justice so rooted in the traditions and conscience of our people as to be ranked as fundamental.’ ”
Patterson,
Contrary to the defendant’s assertion, our prior cases have not held that a defendant has a constitutional right to a jury trial on the issue of insanity in all criminal cases where the defendant seeks to utilize that defense.
See Mundy,
105 Colo, at 551,
In
People ex rel. Juhan v. District Court,
“In a criminal case the defendant can assert as many defenses as can be supported by the evidence. If affirmative defenses such as self-defense or alibi are presented the issues thereon are tried as part of the criminal case, and if any such defense rais *830 es in the mind of a jury a reasonable doubt as to the defendant’s guilt he should be acquitted. The defense of insanity stands upon the same footing.”
Id.
at 259,
The requirement that the defendant initially produce evidence of insanity neither offends any “fundamental principle of justice” nor compromises the prosecutorial burden of proving all statutorily defined elements of a criminal offense beyond a reasonable doubt. We hold that the Sixth Amendment right to a trial by jury in criminal cases does not require the issue of sanity to be submitted to a jury in the absence of any evidence of insanity.
III.
In the present case, the trial court determined as a matter of law that no evidence of insanity had been introduced, and then instructed the jury of its finding and submitted special verdict forms to the jury on the issue of sanity. Since we hold that a trial court may direct a verdict against the defendant on the issue of sanity if no evidence of insanity is introduced, in accordance with Colorado’s statutory procedure and without violating the defendant’s Sixth Amendment right to a jury trial, the trial court was not required to submit the sanity issue to the jury. We do not approve of submitting the issue of sanity to the jury where the court has determined that no evidence of insanity has been presented. However, instructing the jury that no evidence of insanity had been presented was tantamount tó a directed verdict, which is appropriate where no evidence of insanity is introduced. We perceive no error.
Although we do not review the trial court’s finding that there was no evidence of insanity, we emphasize that a trial court may not assess the credibility of witnesses or the weight of evidence in determining if evidence of insanity has been introduced.
See Lybarger,
We hold that Colorado’s statutory procedure and the defendant’s Sixth Amendment right to have a jury determine his guilt or innocence do not require that the question of sanity must be submitted to a jury where no evidence of insanity is introduced. We reverse the judgment of the court of appeals and return to that court with instructions to reinstate the judgment of sanity and remand to the trial court.
Notes
. Dr. Plazak defined "formication" as "a condition that has to do with the individual becoming convinced that there are bugs crawling either on themselves — but in many instances, under their skin.”
. In Colorado, self-induced intoxication is not a valid defense to a criminal charge, but evidence of self-induced intoxication may be offered to negate the existence of a specific intent if such intent is an element of the crime charged. §18-1-804, 8B C.R.S. (1986).
.The definition of “insanity" provides:
"A person who is so diseased or defective in mind at the time of the commission of the act as to be incapable of distinguishing right from wrong with respect to that act is not accountable. But care should be taken not to confuse such mental disease or defect with moral obliquity, mental depravity, or passion growing out of anger, revenge, hatred, or other motives, and kindred evil conditions, for when the act is induced by any of these causes the person is accountable to the law.”
§ 16-8-101(1), 8A C.R.S. (1986).
. Once a defendant is found not guilty by reason of insanity, "the court shall commit the defendant to the custody of the department of human services until such time as he is found eligible for release.” § 16-8-105(4), 8A C.R.S. (1996 Supp.).
. Section 16-8-104.5, 8A C.R.S. (1996 Supp.) provides that the sanity issue and the merits are tried in the same proceeding, before the same trier of fact, for offenses committed after July 1, 1995.
. We note also that a rule permitting the court to determine if there is sufficient evidence of insanity to make it a jury issue is consistent with the practice of a number of federal and state courts.
See United States v. Keen,
