¶1(dissenting). The judge stated that “lack of criminal responsibility is something that must be addressed by an expert witness and after examination,” and that “it certainly cannot be raised by the individual himself or by laymen.” The defendant made no suggestion to the judge that he was prepared to produce expert testimony. The judge then excluded the de*378fendant’s testimony that he “was being treated for mental illness during the month that the crime occurred.” Ante at 373-374. “The defendant wished to testify,” according to the court, “that he ‘used to hear noises,’ that very shortly before the robbery he visited a mental health clinic, and that he felt better ‘now because [he was] under a psychiatrist’s care.’” Ante at 376. The court identifies no other evidence available to the defendant bearing on his criminal responsibility at the time of the alleged crime.
¶2While it is true that “[i]n an appropriate case the very facts of a crime themselves might be some evidence of the existence of legal insanity,” Commonwealth v. Mattson, 377 Mass. 638, 644 (1979), thus raising the criminal responsibility issue on which the Commonwealth carries the burden of proof, this is not such a case. In my view, Commonwealth v. Mills, 400 Mass. 626 (1987), notwithstanding, the meager facts the defendant sought to place in evidence in this case, in the absence of expert testimony, fall far short of being sufficient to place the defendant’s criminal responsibility in issue. See Commonwealth v. Mills, supra at 636 (O’Connor, J., with whom Nolan and Lynch, JJ., joined, dissenting). Accepting the defendant’s proffered testimony as true, one can only speculate whether, at the time of the alleged crime, “as a result of mental disease or defect he lack[ed] substantial capacity either to appreciate the criminality [wrongfulness] of his conduct or to conform his conduct to the requirements of law.” See Commonwealth v. McHoul, 352 Mass. 544, 546-547 (1967), quoting § 4.01 of the American Law Institute’s Model Penal Code, Proposed Official Draft (1962).
¶3I continue to adhere to the view I expressed in Millsthat the criminal responsibility issue is not raised by evidence that demonstrates no more than a possibility that the defendant was legally insane at the time of the alleged offense. The judge’s observations about the necessity for expert testimony, understood not as an academic pronouncement but rather as a ruling in this case that without expert testimony the proffered testimony was insufficient to raise the criminal responsibility issue, were, in my judgment, correct. Furthermore, even if *379the judge incorrectly thought that expert testimony is always required, her misconception was harmless. Since the evidence that had been admitted, together with the proffered evidence, was insufficient to raise the criminal responsibility issue, and the proffered evidence had no bearing on any other issue in the case, the judge correctly excluded it. I would affirm the conviction.