Almost a century has passed since an eccentric New York millionaire, John Armstrong Chaloner, published his remarkable autobiography. Committed by his family to a mental hospital, he cleverly talked his way to freedom in Virginia. From his refuge, he asked, "Who's looney now?"

The Supreme Court tackled the question in June with its gummy opinion in the case of Eric Michael Clark. A Texas court pondered the question last week in the retrial of Andrea Yates for drowning her five children. Once again the millwheels of the law are grinding away at the insanity defense. How does a defendant prove he's legally deranged? How does the state prove he's not?

To say that the high court "tackled" the question in the Clark case is to tie a vigorous verb to a limp reality. Justice David Souter, writing for a 5-4 court, barely got a hand on a question of law that has baffled English-speaking jurists for at least 400 years. Justice Anthony Kennedy was no more lucid in dissent. They agree on this much: There is such a thing as lunacy, and lunatics should not be punished as others are punished. But how to prove? How to punish?

Justice Souter spelled out the facts. In the early hours of June 21, Officer Jeffrey Moritz of the Flagstaff, Ariz., police was alerted to a pickup truck that had been circling a residential block with its radio blaring. Moritz was in uniform. He turned on the emergency lights and siren of his marked patrol car. Eric Clark, the truck's driver (then 17), pulled over. Moritz got out of the patrol car and told Clark not to move. Less than a minute later, Clark shot the officer, who died soon thereafter. Clark ran away but was arrested later that day with gunpowder residue on his hands. The gun that killed the officer was found nearby stuffed into a knit cap.

Police charged the youth with first-degree murder. He pleaded insanity and was first committed to a state hospital. After two years he was found competent to stand trial. In court he relied again on a defense of paranoid schizophrenia, i.e., that "at the time of the commission of the criminal act he was afflicted with a mental disease or defect of such severity that he did not know the criminal act was wrong."

The prosecution, for its part, argued that Clark knew perfectly well that Moritz was an officer. He was in police uniform and was driving a marked police car, with its siren and emergency lights in full operation. Moreover, Clark had fled the scene. A few weeks before the homicide he had told some people he wanted to shoot police officers.

The trial court found the defendant guilty and sentenced him to life imprisonment. The appellate courts affirmed and the case moved up to the U.S. Supreme Court. After barely two months' deliberation the high court also affirmed. Nothing in the concept of "due process," said Justice Souter, prevents Arizona from defining its insanity test narrowly. The state must demonstrate only that Clark was not afflicted with a mental disease "of such severity that he did not know his criminal act was wrong."

After 16 years on the high court, Souter remains a stylistic disappointment. Now and then -- rarely -- he gets off a sentence of remarkable clarity. Mostly he writes in paragraphs of impenetrable prose, festooned with a fungal clutter of unnecessary footnotes. He says:

"Though Clark is correct that the application of the moral incapacity test (telling right from wrong) does not necessarily require evaluation of a defendant's cognitive capacity to appreciate the nature and quality of the act charged against him, see Brief for Petitioner 46-47, his argument fails to recognize that cognitive incapacity is itself enough to demonstrate moral incapacity. Cognitive incapacity, in other words, is a sufficient condition for establishing a defense of insanity, albeit not a necessary one."

In the final paragraphs of his 39-page opinion, Souter abruptly lapses into lucidity. He quotes approvingly from authorities who doubt that psychiatrists and psychologists are "more qualified than any other person" to give an opinion on whether a defendant's mental condition adds up to insanity.

Justice Anthony Kennedy, joined by Justices Stevens and Ginsburg, filed a salty dissent. (Justice Breyer dissented separately.) He fumes that Souter has invented an unworkable theory to justify a verdict of insanity. Then the majority applied the theory unfairly by holding Clark's counsel responsible for not having divined Souter's novel approach.

The long and short of it is that Eric Clark has been found sane enough to serve 20 years in the slammer. If he ever was crazy, nobody proved it.