JOHN HINCKLEY JR. tells us, by means of an unsolicited letter to Newsweek Magazine, that he is opposed to any change in the rules governing the insanity defense in federal court. Aren't you amazed? Less surprising is the president's decision to ignore this particular sage of St. E's and to go forward with his proposals for a dramatic change in this rule of law. On Monday, the White House sent to the Hill legislation that would limit the insanity defense to those cases where the defendant did not have the state of mind required by the statute -- the intent to do the act with the knowledge that it is wrong.

Thus, if a killer thought he had a daisy in his hand and not a gun, or if he thought he was shooting a pumpkin and not a policeman, he would not have the necessary intent to commit the crime and would be acquitted by reason of insanity. On the other hand, if he knew what he was doing was wrong and intended to do it nevertheless, he could not then raise as a defense the history of his childhood, his disappointments in love or any mental disease or defect that might explain why he intentionally committed the crime. Those factors would only be considered at the time of sentencing.

The president's proposal is dramatic in that it severely limits the availability of the insanity defense, but it is not unexpected. As we have learned more about mental illness, courts have tried to reflect that learning in judicial decisions that greatly broadened the defense. More recently, though, lawyers and criminal justice experts have questioned the direction of this trend. The federal courts in the District of Columbia, for example, now apply a standard much more narrow than the Durham rule of 25 years ago. But many think the defense should be limited further and the question of mitigating mental illness saved for the sentencing.

The Hinckley case was surely the straw that broke the camel's back. Even Mr. Reagan, who maintained a presidential objectivity after the verdict, gives us a hint, in his message to Congress, of what his true feelings must have been when a Washington jury acquitted his assailant. He characterizes the present rule as one that "permits the introduction at trial of massive amounts of conflicting and irrelevant testimony by psychiatric experts, thereby complicating the trial process and deflecting the attention of the jury from the critical issues." This kind of testimony is described elsewhere in the message as "a confusing swearing contest between opposing psychiatrists." The president believes that the insanity defense, as it is currently applied "is seen not only as time-consuming, confusing and expensive, but also as a defense that is not available to less affluent defendants and favors those persons able to hire an impressive array of psychiatrists." These sentiments reflect more than the justified reactions of Mr. Hinckley's victim; they express the frustration and sense of injustice that is widespread in the general public.

Congress may not want to go as far in limiting the defense as the president recommends. But surely it has an obligation to look carefully at various formulations of the defense that have been suggested by scholars and that have been adopted by state courts. A compassionate, informed and hardheaded review is in order.