A MAJORITY of the Supreme Court seems intent on quietly chewing up an old and honorable rule of the law without quite saying it is doing so. Five justices took another bite out of the 66-year-old exclusionary rule last Tuesday, and sooner or later, if the current trend keeps up, the rule will simply collapse.

The exclusionary rule provides, in its pure form, that the government can never use evidence it has obtained illegally to convict a suspect of a crime. Long the subject of intense controversy, it is, depending upon whose analysis you accept, either a major obstacle to the conviction of criminals or a bulwark of privacy of ordinary citizens. The basic rational behind it is that no better way exists to deter police from conducting illegal searches. A second rational, articulated best by Oliver Wendell Holmes, is that the government should not be permitted to convict people of crime by using the fruits of crimes its own agents commit.

In recent years, however, the rule has been trimmed in highly technical ways that involve the content of legitimate cross-examination of defendants. The result after last Tuesday's decision is that the government still has no chance of putting illegally seized material in evidence if a defendant doesn't testify in his own defense but has a pretty good chance of using it if he does.

To the rule's staunch defenders, such as Justice William J. Brennan, this destroys the rule's effectiveness. It makes a defendant choose between two rights -- his right to testify and his right not to have illegal evidence used against him. To the rule's opponents, the new interpretation simply makes it more difficult for a defendant to lie on the witness stand.

The trouble with having the court tinker in such a manner with this or any other major rule is that the principles involved slip out of the argument. The justices begin to dance on the head of a pin in language whose implications, though often implicit, are real. Tuesday, for instance, they fastidiously refrained from overruling a 55-year-old precedent but gutted it all the same.

A substantial argument can be made that the exclusionary rule has never fulfilled its promises and should be abandoned. An equally substantial argument, which we find more persuasive, is that without the rule the average citizen's chances of being searched illegally would increase sharply. The justices are avoiding that clash of argument, however, and instead are worrying an important part of American law to death.