Nobody knew when the mandatory life sentence for murder was removed recently that it was just in time. Rex Law, given 18 months' imprisonment for the mercy killing of his wife, is not only the first person to escape a life sentence for murder in this country, his case is a remarkable testament to justice, mercy and the peculiar, cruel affliction of Alzheimer's disease.

Had the mandatory sentence been still in force, one of two absurdities would have resulted. An elderly man would have spent the rest of his years in prison or, to extend the mercy the case obviously deserved, he would have been found guilty of something less than the murder he plainly committed.

Intention is the quality that distinguishes murder from manslaughter in our law and Law certainly intended to kill his wife, Olga, when he hit her with a mallet and smothered her with a pillow. Two weeks earlier he had tried gentler methods - sleeping pills and gas - which he hoped would take them both while they slept.

The law, of course, does not condone that behaviour, no matter how much legislators might understand it. Euthanasia is not yet permissible in societies such as ours, and will never be if enough people continue to regard human life as sacred and even doctors hesitate to bring on death. Law knew the enormity of what he was doing and expected the judgment he would receive.

This was the crime of a considerate man. Who could not have been moved to read that the 77-year-old had waited until morning to telephone police and report what he had done, saying: "I would have phoned you last night but I thought you would be too busy"? His lawyer, David Bates, challenged the court to use "its merciful artistry to paint in sufficiently bold strokes a portrait of non-custodial justice illustrating sanctity of life".

Judge Tony Randerson decided, wisely, that a nominal prison term was necessary to convey that murder in these circumstances might be understandable, but never excusable. The case has surely set the minimum penalty acceptable under the discretion now given to the courts.

Law accepts the sentence. The loss of his wife, as his lawyer said, was a greater penalty than any the courts could impose.

The tragedy of Alzheimer's disease is that he probably lost his wife long ago. The person he had known for 54 years had turned into someone who probably seldom, if ever, recognised him or remembered much of their life together. That is the nature of this degenerative disease which has come to afflict so many late in life these days.

Many who see friends and acquaintances succumb to the condition are tempted to say they wish to die rather than live on in such a state. More couples than we like to think might have made the vow that Rex and Olga Law reportedly did to release one or the other from the disease. It is easy to say that the partner left to watch the other's decline should seek help before being driven to act on any death pact. This is a crime of love; the killers are not acting in their own interests, they are giving a last selfless service, as they see it, to their loved one.

Perhaps the only way to help people in Rex Law's position is to suggest that whatever a person has said before the disease struck is not necessarily the afflicted person's wish. Alzheimer's literally changes the person's mind and even in that disoriented state, who is to say the person would not choose life?

Nobody, not even the nearest and dearest of lifelong partners, has the right to make that decision for the mentally impaired. That is the statement the court has made as mercifully as humanly possible. It is a landmark for all who deal with this disease of our times, a downside of longevity. Unless medicine finds a way to relieve it, we must live with it.