THE SUPREME COURT'S decision upholding the power of government to search a newspaper office for documentary evidence of someone else's crime is a staggering blow to freedom of the press. What is more, the impact of the decision will be almost as heavy on the rights of all citizens. What the court has said is that if the police can convince a judge there is probable cause to believe evidence of a crime is contained in your private files - a crime not committed by you but by anyone, anytime, anywhere - they can rummage through your papers and premises until they find it, or choose to abandon the search. Previously, it could be argued that the police could only ask for that evidence or get a court order requiring you to produce it.

This assault stands on its head the history of both the First and Fourth Amendments. Both additions to the Constitution grew out of long battles between the press and the British Crown, which had used search warrants to seek evidence of seditious libel. Until a decade ago, the Fourth Amendment was thought to guarantee that the private papers of anyone - diaries, for example - were always immune from the prying eyes of government. And until yesterday's court decision it had been far from clear that the government could invade those private papers to obtain evidence of a crime committed by someone other than their owner.

The effect of this decision on the operation of the news media in this country could be dramatic. In a situation like Watergate, for example, a newspaper (or its reporters) would be foolish to retain documentary evidence that might reveal the sources of its information. Had this decision been in place before Watergate, it is not hard to imagine the conditions under which some judge could have been convinced this newspaper had evidence of some crime - perhaps totally unrelated to Watergate - in its files. Once a warrant to search for that evidence had been issued, the way would have been open for investigators to look at everything in the files until they found it. If, in the process, they happened to come across by accident the names of the Watergate sources, the government would then have acquired information that no court would have authorized it to obtain.

We do not yet know what effect the decision will have on the ability of news media to obtain confidential information of any kind. But we share the disquiet of Justice Potter Stewart, who wrote in his dissent, "It requires no blind leap of faith to understand that a person who gives information to a journalist only on condition that his identity will not be revealed will be less likely to give that information if he knows that, despite the journalist's assurance, his identity may in fact be disclosed." The majority of the court, while grudgingly granting a reporter's right to keep confidential information in some situations, has now made it unsafe for him to keep that information any place but in his head.

While the problems of confidentiality may not be as great for most private citizens as they are for the news media, they do arise. There is little reason to think the court will be any more generous in its handling of the privacy of a doctor's confidential files - or those of any other citizen - than it has been with the press's. If the police can convince a judge that the picture you took of the Washington Monument has in it evidence of, say, a purse snatching (even if you aren't aware of it), they can obtain a warrant to enter your home and dig it out of your desk - looking at all the other pictures that are there, in the process.

The court, while refusing to find any distinction between the problems its decision causes for newspapers and for other citizens, did note that judges should be especially careful in issuing warrants to search news-media offices. "Properly administered, the preconditions for a warrant . . . should not affort sufficient protections against the harms that are assertedly threatened . . .," Justice Byron White wrote. But even if all the judges in the nation were as sensitive to issues of personal privacy and freedom of the press as are some members of the court (which they aren't), this decision would represent a vast grant of discretionary power for government to invade the privacy of American citizens, and assuch is a terrible setback to individual and press freedom.