Whether a criminal trial seeks to determine the truth or merely to do justice , the process cannot work without evidence, and evidence cannot materialize without witnesses to recount it.
Anyone directly concerned with a trial's outcome (a victim or a defendant) usually wants to introduce any evidence favoring his side. ''The accused,'' says the Sixth Amendment, ''shall enjoy the right to have compulsory process for obtaining witnesses in his favor.''
Against this policy of full disclosure, the law has placed some specific limited exceptions. A physician may not, without permission, disclose what a patient told him concerning an ailment. A lawyer may not reveal what a client said; or a priest a penitent's words in the confessional.
Conversely, no one can compel the patient, the client, or the penitent to tell the jury the contents of the conversation. A refusal to testify, ordinarily the prelude to contempt of court, is in these instances blameless.
We call these exceptions to the principle of full testimony a ''privilege'' allowed because society is prepared to forgo vital evidence in order to permit efficient functioning of particular confidential relationships.
However worthy the social purpose behind any testimonial privilege, the fact is that application of the privilege cuts off the flow of information. Every sealing of a witness's lips requires the jury to search for the truth with that much less light.
Against this background, courts, legislatures, and journalists are struggling to fashion appropriate treatment for the situation that arises when a newsperson - having received information under promise of confidentiality - faces an order to testify about a source.
The most popular remedy, a ''shield law,'' spares a journalist from punishment should he decline to furnish a jury with the source's name (or testimony that would lead to unmasking).
Underlying the shield law is the premise that without ironclad guarantees of anonymity, individuals with vital information will decline to contact reporters, thus leading to constriction of newsgathering and the unhampered triumph of evil. The correctness of this largely untested assertion is, however, beside the point. The real question is whether the proposed remedy makes sense.
Who does the shield protect, the reporter or the source? Who holds the shield? In the lawyer-client paradigm, the privilege belongs entirely to the client. The attorney cannot betray the secret even if he wishes to, even if he believes his silence will engender a grave injustice. Indeed, an attorney who discloses what a client has said is subject to severe discipline, perhaps disbarment.
In the journalist-shield context, the source is entirely at the reporter's mercy. By definition, he cannot come forward to insist on confidentiality. The kind of serious professional sanctions that would befall a blabbing lawyer, priest, or doctor are not available to restrict a reporter.
Suppose the fact of disclosure by a particular individual is itself news? The Pulitzer Prize-winning investigative reporter is famous for asking a reluctant interviewee, ''Which would you rather be, the source or the story?'' Given the competitive pressures of journalism, the shield may easily turn into a sword, or at the least a journalistic decision to disclose only certain information.
Remember, we are not talking about a reporter receiving information ''off the record.'' The shield law impedes the production of evidence. What if the reporter, by shielding the source, deprives an innocent defendant of vital evidence, thus letting the real villain escape?
Some shield laws meet this problem by allowing disclosure ''to avoid a miscarriage of justice.'' Fair enough. But who is to decide? As any judge who presides over criminal trials will agree, it is difficult for someone in the business of spotting such miscarriages to identify them.
At least judges make their decisions in a context of principle and the glare of public attention. Judges have to satisfy the public and the higher courts. Does the reporter's shield face an equal test?