At the time the Supreme Court decided the Cruzan case last June, only 13 states had laws validating a "living will." The publicity given to Nancy Cruzan's death may prompt the remaining states into action. If so, the young woman's agony will have served a useful purpose.

By this time, almost everyone who reads the newspapers must be familiar with the case. On a January night in 1983, Nancy Cruzan lost control of the car she was driving in southwest Missouri. She suffered horrible injuries that left her in a "persistent vegetative state," a spastic quadriplegic, unable to swallow, with brain damage that was "irreversible, permanent, progressive and ongoing."

Nevertheless, her heart kept beating, and her lungs kept pumping. With a tube inserted into her stomach she remained lifeless but alive. Her parents sought court authority to have the tube removed. A trial court was agreeable. A year before the accident, Nancy had confided to a friend that if she ever became a mere vegetable, she wanted medical treatment stopped.

The Supreme Court of Missouri, in a divided decision, reversed the trial court. Under state law there must be "clear and convincing evidence" of a person's death wish. The friendly conversation did not suffice. On June 25 the U.S. Supreme Court, itself badly divided, affirmed that decision.

The high court noted that the 14th Amendment guarantees that no person may be deprived of "liberty" without due process of law. In circumstances such as those suffered by Nancy Cruzan, a person is at liberty to arrange for life not to be artificially prolonged. But Missouri has power to fix conditions governing the exercise of that right.

As it turned out, following the Supreme Court's decision, the Cruzan family found new evidence of Nancy's expressed wishes. This after-discovered evidence persuaded Missouri authorities to permit withdrawal of the feeding tube. Last week, at 33, Nancy at last found peace in death.

The Cruzan case captured wide attention, but there was nothing novel about it. Between 1976 and 1988, appellate courts acted in at least 54 instances involving a right to refuse medical treatment. Justice William Brennan cited a report that 10,000 comatose persons now are being kept alive through modern medical technology.

What rules should apply? How may the wishes of an unconscious patient be protected? What of the liability of attending physicians? Should the law require evidence that is "trustworthy" or "reliable" or, as in Missouri, "clear and convincing"?

The answers should be left to our state legislatures. Surely that is a reasonable approach to resolving such complex issues of law, morality, ethics, public concern and personal liberty. It is in precisely these areas that old doctrines of federalism come into play.

It should not be up to judges -- especially federal judges -- to write detailed legislation. This was the mistake the court made in the abortion case of 1973. The states are expected to function as laboratories of political experiment. Once a "liberty interest" in a right to die has been defined, the respective states should act.

Justice Antonin Scalia said all this with his usual acerbic eloquence. In a concurring opinion in Cruzan, he expressed a fear that the court is poised to confuse the states as successfully on the right of a comatose patient to die as it has confused the states on the right of a fetus to live.

The federal courts, said Scalia, have no business in this field. It is up to the states to define the point at which life becomes worthless. The states must say when further medical procedures are inappropriate. These limits are not known "to the nine justices of this court any better than they are known to nine people picked at random from the Kansas City telephone directory."

Exactly so. We can empathize with the grief experienced by the Cruzan family. It has been an emotionally draining ordeal, but good may come of it yet. A Lou Harris poll in 1988 found that 56 percent of those polled had orally expressed their desire not to be kept artificially alive. Fifteen percent had executed written "living wills."

I have instructed my own family that if I fall into a persistent vegetative state, let me go gently into that indeterminate dark night. I wrote out my instructions when I first learned of the Cruzan case three years ago. My thanks go today to Nancy in her grave.