¶1ON REHEARING
¶2INTRODUCTION
¶3The Attorney General petitioned this Court to grant a rehearing and reconsider our 1990 Opinion No. 9, filed January 24, 1990. In the second sentence of the argument part of its brief, the State agrees that the opinion testimony of the state fire investigator would probably have been excluded if there had been an objection to it. The Attorney General contends that defense counsel’s failure to object did not constitute fundamental error because, so the argument goes, it was the intention of the defense from the outset to concede that defendant started the fire, but had done so accidentally. State’s brief, filed June 30, 1988, at 12. If we accepted the State’s contention that defense counsel had such an ill-advised intention, it would cause us to doubt that it was a decision which a reasonably competent criminal defense counsel would make.
¶4As a second purported “issue,” the State’s brief in support of the petition for rehearing suggested, by innuendo, that a rehearing should be granted because the court’s decision was rendered by only three justices, one of whom concurred only in the result. We, of course, were well aware of that circumstance. There were only three sitting justices. However, three votes constitutes a majority of the Court. In this instance there were three votes for the result and the opinion issued. The petition for rehearing also garnered sufficient votes, and the Court has had the benefit of additional briefing and further oral argument.
¶5The consensus of the Court as presently constituted, i.e., a full complement, is to agree with the State’s argument that our earlier conclusion basing a reversal on the doctrine of fundamental error was inappropriate. Our earlier opinion, as published above, concluded with the statement that: “Walter’s claim that he received ineffective assistance of counsel at trial is, of course, intertwined with defense counsel’s failure to attempt to prevent the jury from hearing and considering the arson investigator’s indictment against Walters.” Accordingly, we now conclude that reversal of the judgment is more aptly based on the lack of effective assistance of counsel.
¶6*55INEFFECTIVE ASSISTANCE OF COUNSEL
¶7On rehearing, the State contends that an opinion as to an ultimate issue is admissible under the rules of evidence and Idaho case law. Idaho Rule of Evidence 704 states that:
Rule 704. Opinion on ultimate issue.— Testimony in the form of an opinion or inference otherwise admissible is not objectionable because it embraces an ultimate issue to be decided by the trier of fact.
¶8I.R.E. 704 (emphasis added). Rule 704 has not opened the door to all opinions on every subject, particularly in a criminal trial. State v. Pinero, 778 P.2d 704, 711 (Hawaii, 1989). Rule 704 must be read in the light of Rule 702. Expert testimony is only admissible when the expert’s specialized knowledge will assist the trier of fact to understand the evidence and determine a fact in issue. I.R.E. 702. Opinions which directly pass on the credibility of witnesses are generally not allowed. State v. Lindsey, 149 Ariz. 472, 720 P.2d 73 (1986), State v. Myers, 382 N.W.2d 91 (Iowa 1986), State v. Rimmasck, 775 P.2d 388 (Utah 1989), State v. Pinero, 778 P.2d 704 (Hawaii 1989)., The Arizona Supreme Court in State v. Lindsey, a child sexual abuse case, explained that the basis for precluding expert testimony on the credibility of a witness was the danger of usurpation of the jury function and the lack of need for expert testimony on the truthfulness of witnesses. It said that:
Thus, even where expert testimony on behavioral characteristics that affect credibility or accuracy of observation is allowed, experts should not be allowed to give their opinion of the accuracy, reliability or credibility of a particular witness in the case being tried. Nor should such experts be allowed to give opinions with respect to the accuracy, reliability or truthfulness of witnesses of the type under consideration. Nor should experts be allowed to give similar opinion testimony, such as their belief of guilt or innocence. The law does not permit expert testimony on how the jury should decide the case ... [T]he expert’s function is to provide testimony on subjects that are beyond the common sense, experience and education of the average juror
¶9State v. Lindsey, 149 Ariz. 472, 720 P.2d at 76. Generally, expert testimony that purports to determine whether a particular witness is truthful on a particular occasion is not permitted because there is no reason to believe that experts are any more qualified to render such opinions than are jurors. State v. Rimmasck, 775 P.2d 388 (Utah 1989). In a criminal trial where the expert opinion, as in this case, involves the weighing of the credibility of witnesses based upon their out-of-court statements, special caution must be exercised by the trial court to make certain that the expert’s opinion is based upon his or her expertise and that it will assist the trier of fact in determining a fact in issue. Historically, the evaluation of the credibility of witnesses has been committed solely to the jury and they alone have the responsibility to determine the guilt or innocence of the accused.
¶10While experts may render a wide range of opinions based upon their special skills or knowledge, we are not prepared to go so far as to hold that an expert may tender his opinion as to the ultimate fact in a criminal trial — the defendant’s guilt or innocence of the crime charged. The expert’s opinion in this case is a far cry from that given in State v. Crawford, 110 Idaho 577, 716 P.2d 1349 (Ct.App.1986), where the opinion which was permitted was a medical doctor’s evaluation that a victim had suffered great bodily injury. See, Lampkins v. United States, 401 A.2d 966, 970 (D.C.App. 1979); State v. Carlin, 40 Wash.App. 698, 700 P.2d 323, 325 (1985). Although the opinion in State v. Crawford involved an ultimate issue, it was clearly based on the doctor’s area of expertise and would have been helpful to the jury in resolving one of the issues before it. The opinion did not involve a statement of belief about the defendant’s guilt or innocence, and his credibility, which is clearly the situation before the Court in this case. There is no reason to believe that the arson expert who *56testified in this case was any better qualified to evaluate truthfulness than the jury. There was no need for the testimony and there was clearly the danger that a jury might give the arson expert’s opinion undue weight.
¶11As to the admissibility of Dillard’s opinion, the State candidly acknowledges that “a timely objection to this testimony probably could have been sustained.” Brief in Support of Petition for Rehearing, 5. Because there was not a timely objection, we have concluded that although the fundamental error doctrine has been recognized by this Court, in this instance, where the main thrust of the appeal was based on the constitutional right to effective assistance of counsel, we had no reason to apply it. More properly, we should have and could have decided that the reversal should have been based on the ineffective assistance ground.
¶12As stated in Carter v. State, 108 Idaho 788, 702 P.2d 826 (1985):
As was indicated above, trial counsel failed to move to suppress these statements. Carter asserts that this amounted to a failure to provide reasonably competent assistance of counsel. The right to counsel under both the sixth amendment to the United States Constitution and art. 1, § 13 of the Idaho Constitution encompasses the right to effective counsel. Strickland v. Washington, 466 U.S. 668, 104 S.Ct. 2052, 80 L.Ed.2d 674 (1984); State v. Tucker, 97 Idaho 4, 539 P.2d 556 (1975).
The Supreme Court recently set out the test for analyzing an ineffective assistance of counsel claim under the sixth amendment.
The benchmark for judging any claim of ineffectiveness must be whether counsel’s conduct so undermined the proper function of the adversarial process that the trial cannot be relied on as having produced a just result. Strickland, supra,104 S.Ct. at 2064.
The Court outlined a two-pronged test that the defendant must meet in order to prevail on an ineffective assistance of counsel claim. First, the defendant must show that counsel’s, performance was deficient, that counsel made errors ‘so serious that counsel was not functioning as the “counsel” guaranteed by the sixth amendment.’ 104 S.Ct. at 2064. Second, the defendant must show that counsel’s errors prejudiced the defense. ‘The defendant must show that there is a reasonable probability that, but for counsel’s unprofessional errors, the result of the proceeding would have been different.’ 104 S.Ct. at 2068.
¶13Carter, 108 Idaho at 794-95, 702 P.2d at 832-33. In Carterthe ineffective assistance of counsel issue was raised in post-conviction proceedings because the defendant in the prosecution action had been denied the assistance of counsel at the time he was being subjected to custodial interrogation. Trial counsel failed to move for suppression of any testimony of the deputy sheriff regarding a statement the defendant made during custodial interrogation, and this Court ruled that counsel’s failure amounted to ineffective assistance of counsel under the Stricklandtest. Carter, 108 Idaho at 795, 702 P.2d at 833.
¶14The first prong of the Stickland test is met in this case. Trial counsel’s inexplicable failure to object to Dillard’s extremely damaging opinion that the defendant was guilty of setting the fire was an error so serious that we rule that counsel was not functioning as the “counsel” guaranteed by the sixth amendment. Trial counsel’s failure to object to Dillard’s damaging opinion is on par with the failure by counsel in Carterto move to suppress the damaging testimony of the deputy sheriff. See Carter, 108 Idaho at 795, 702 P.2d at 833.
¶15The second prong of the Stricklandtest is also met in this case. To satisfy the second prong, the defendant “must show that there is a reasonable probability that, but for counsel’s unprofessional errors, the result of the proceeding would have been different. A reasonable probability is a probability sufficient to undermine confidence in the outcome.” Strickland, 466 U.S. at 694, 104 S.Ct. at 2068. There is little question that Dillard’s opinion was prejudicial to Walters’ defense. When an *57arson expert declares that it was the defendant who set the fire in the house, there can be little doubt that the jury was impressed and influenced by the authoritative statement. Had Dillard been prevented from declaring his damaging opinion, there is at the least a reasonable probability that the outcome of the proceeding would have been different. Most certainly, that probability is “sufficient to undermine confidence in the outcome.” Since all of the evidence which led to the defendant’s conviction was circumstantial (there were no eyewitnesses or confessions), it is impossible to say that the accusation by the arson expert did not have a substantial influence on the verdict. Absent Dillard’s testimony, there is no overwhelming evidence of the defendant’s guilt. The second prong of the Strickland test is therefore met, and the defendant has met the burden of showing that he was denied effective assistance of counsel at trial.
¶16The judgment of conviction is reversed, and the cause is remanded for a new trial.