Justice Andrew Mead is expected to make a decision soon in the case of John Havlin, a Hampden man accused of stabbing his 5-year-old daughter last year. Mr. Havlin’s attorney argues that his client is not guilty because of mental illness. Though this is a difficult standard to prove, if ever there were an example of insanity as a proper defense, this case is it.

The public often is suspicious of an insanity defense — sometimes correctly, sometimes not. But its value and the reason that it should be available to defendants are amply demonstrated in this case. Mr. Havlin was by all accounts a fine father when he began suffering from an undiagnosed mental illness that caused him eventually to seek help. Unable to wait for care through emergency-room services on one occasion, he left before getting treatment.

Deepening delusions apparently followed that were so severe he stabbed his daughter in the belief that he was protecting her from a greater evil. He was later diagnosed with paranoid psychosis, has had months of treatment and is on medication. His daughter has fully recovered from the stab wounds. Since being released, Mr. Havlin has done volunteer work, spent time with his children and tried to reassemble his life.

The case is noteworthy not only because it exemplifies the value of a defense that shields the accused who has neither the capacity to intend to commit harm nor the grasp that he was doing so but also because of the support that defense received from the state prosecutor. In his closing arguments, Deputy District Attorney Michael Roberts said, “I seldom find myself arguing for the same verdict as the defense, but it’s my opinion that would be the most appropriate verdict.” It is a credit to Mr. Roberts that even in the adversarial atmosphere of a courtroom, he could argue the state’s case while seeking a verdict that would help Mr. Havlin and his family.

If Justice Mead agrees with both the defense and prosecution, Mr. Havlin would be recommitted to a mental-health facility for at least six months. Given the fact that he already has received treatment and been deemed fit for release and that his reconnecting with his family probably is most important for all concerned, the six-month requirement is unfortunate. But it is better than a long stretch in prison for a crime he was mentally incapable of understanding.