The trial of Lindsay Clancy forces us to confront a tragedy almost beyond human comprehension: A mother killed her three young children — 5-year-old Cora, 3-year-old Dawson and 8-month-old Callan — inside their Massachusetts home.

We must begin with those children. They were innocent, vulnerable and entitled to the protection of the person they trusted most. Their names and stolen futures must not disappear beneath arguments about psychiatric diagnoses, prescription drugs or criminal law. Whatever verdict the jury reaches, three beautiful young lives were taken, a father's world was destroyed, and an entire family was shattered.

Clancy does not deny killing her children. The central question is whether she was legally responsible for her actions. Her defense contends that postpartum psychosis had overtaken her mind so completely that she could no longer understand the nature or wrongfulness of what she was doing. Prosecutors argue that although she was mentally ill, she remained capable of planning, making decisions and appreciating that her conduct was wrong.

That distinction is difficult but essential. Mental illness and legal insanity are not synonymous. A person can be profoundly depressed, anxious, suicidal or emotionally unstable and still remain criminally responsible. For an insanity defense to prevail, the illness generally must have so severely impaired the defendant's mind that she could not comprehend the wrongfulness of her actions.

The prosecution points to evidence of deliberation. Clancy sent her........

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