All leakdom could be said to be divided into two parts: the top-level leak serving some administration purpose, and the unauthorized subterranean, whistle-blowing leak that tends to defeat the administration's purpose.
President Reagan once complained of being, "up to my keister in leaks." He deeply resented leaks because they suggested some loyalty other than to him.
The current prime example of a damaging leak is The New York Times story of Dec. 15, 2005, about government eavesdropping without a warrant, which plunged the administration into something of a crisis. The Times' only attribution in the story was, "According to government officials." The president has ordered an investigation to discover the leaker. As far as I know, no whistle-blower leak investigation has ever led to successful prosecution.
Perhaps the unauthorized leaker of the century was the FBI's Mark Felt, aka Deep Throat. President Nixon, in time, came to suspect Mr. Felt but took no action against him. Nixon told his subordinates he feared Felt might make revelations about him that would prove even more damaging.
The other famous leaker of the 20th century was the RAND Corporation's Daniel Ellsberg, who delivered to The New York Times and The Washington Post the 7,000 page history of American involvement in the Vietnam War that came to be known as the "Pentagon Papers."
Deeply resentful of unauthorized and especially anonymous leaks, the White House nevertheless likes leaks that it can control. One such leak that spun out of control was the unveiling of the covert CIA identity of Valerie Plame, whose husband had cast doubt on the existence of an Iraqi nuclear program. If a leak can be said to have boomeranged, this one did. It is now the subject of a two-year special investigation. Vice President Cheney's chief of staff, I. Lewis Libby, has been indicted for perjury.
Mr. Libby has testified that his superiors also instructed him to leak information from a secret intelligence report about supposed Iraqi weapons of mass destruction. The investigation is still in progress.
Leaks can be a dangerous game, but danger is not likely to discourage them - either from the top, or from deep inside the government. As I have written before in these pages, the ship of State is the only kind of ship that leaks mainly from the top.
• Daniel Schorr is a senior news analyst at National Public Radio.