Robert Caro, the biographer of Lyndon Johnson, thinks the main difference is time. The historian has the time to dig deeper and sift more thoroughly than the journalist can. The historian's relative leisure allows for the correction of mistakes — including errors made by journalists in their haste. Caro was talking about this the other night at the New York Public Library. He spent years prowling around in Lyndon Johnson's early life, he said, only to discover that most of the lore on the subject was all wrong; LBJ had invented it. Caro began getting it right only when Sam Houston Johnson, Lyndon's brother and the often drunken purveyor of family myth, sobered up and started talking straight to Caro.

Sylvia Jukes Morris, now working on the second volume of her superb biography of Clare Boothe Luce, proceeded with the project for years before she discovered that Clare Luce had lied elaborately about her earlier life. Morris virtually had to start over again; her first Luce volume, "Rage for Fame," took 15 years to complete.

One of deadline journalism's inherent defects is that, in order to organize the material and give it punch, a reporter seeks out a narrative line — sometimes the most obvious or sensational, sometimes a merely conventional version, the one that has been arrived at by consensus of the journalistic hive. The narrative line in turn dictates an attitude, the atmosphere out of which the journalist writes. Maybe the story line is that George W. Bush is a little stupid. Or that Al Gore lies about things he did in the past. Everyone loves to hear a story. But is it true?

John Burns, a foreign correspondent for The New York Times, once told me about covering Mao's China. One day in the midst of the Cultural Revolution, Burns was getting his car repaired by a mechanic at the Canadian embassy in Beijing. The mechanic told Burns, "I have been reading your articles." The complacent Burns, expecting a compliment, said, "Oh, really?" The mechanic, not looking up from the engine, said matter-of-factly, "Yeah, they're all complete rubbish, you know. This entire country is a prison, and you don't even know it." Burns was shocked. It was the beginning of wisdom for him as a journalist.

(Check it out: In telling you that story, I made up the detail about the mechanic "not looking up from the engine"; Burns had merely suggested it, by a sort of body English, but I wanted to make my story vivid.)

Historians in a rush may be guilty of both the journalist's errors and the utopian's projections. I have been rereading Arthur Schlesinger Jr.'s "A Thousand Days," which comprises more than a thousand pages about the Kennedy White House, written in the year after JFK's assassination. In his grief, Schlesinger portrayed Kennedy as saint and martyr: "He was a Harvard man, a naval hero, an Irishman, a politician, a bon vivant, a man of unusual intelligence, charm, wit and ambition, 'debonair and brilliant and brave,' but his deeper meaning was still in process of crystallization." In recent decades, a more thorough and honest parsing of Kennedy has edited a lot of the hagiology out of his memory.

Does accurate history matter? Maybe it's necessarily all projection? Maybe there isn't any final draft — only addition and revision, praise and debunking, the endless spin and counterspin of ideology and propaganda? Tolstoy said, "History would be a wonderful thing, if only it were true."

The struggle is to make it truer and truer. Good history matters as much as accurate journalism does. Journalism or history that is corrupted — by laziness, by ideology, by political correctness, by sentimentality, by money — does the work of darkness. Which is to say that, in the absence of truth, bad people prosper.