Do we really have to know that someone wet his pants at the age of 6 and practiced oral sex at 60? Insofar as biography is used to illuminate history, voyeurism has no place in it."

With this pronouncement, Barbara Tuchman, the noted historian and biographer, ended her lecture at the closing session yesterday of a two-day "Symposium of the Art of Biography" at the National Portrait Gallery.

It was not clear that she had anyone in particular in mind, but it was very clear that she felt the private lives of people about whom she writes should be kept private unless they are relevant to the larger story she is trying to tell.

She said white it may be of some clinical interests to learn that Martin Luther was constipated, that fact in no way accounted for the Protestant Reformation.

Tuchman's audience was made up mainly of biographers, and their reactions to her remarks pointed up some of the differences between biographers and historians.

A historian, according to Tuchman, illuminates men and events. A biographer, according to several other speakers, is interested in finding the ultimate truth about is subjects motivations and deeds. Private thoughts and private lives are meat and drink for the biographer.

Justin Kaplan, who won a Pulitzer Prize in 1967 for his "Mr. Clemens and Mark Twain," suggested that Walt Whitman, on whose biography he is now working, was a homosexual.

He said he was interested in solving the riddle of the "publicly excitable but ultimately covert Whitman" who boasted at the age of 71 that he had several children and that he corresponded with a grandchild. Kaplan said no trace of any of these offspring has ever been found.

He noted that Ralph Waldo Emerson, that monument of New England rectitude, had four children and that their existence provided more direct evidence of Emerson's heterosexuality than all the poems and known events of Whitman's life do of his.

Leon Edel, a Pulitzer Prize-winning biographer of Henry James, asked Kaplan why, in the absence of definite evidence one way or another, it was necessary to raise the question about Whitman. Edel himself lectured on Monday about finding "the figure under the carpet" - the secret myths people have about themselves - as one of the biographer's main tasks.

"It is the biographer's obligation," Kaplan replied, "to give Whitman the freedom to understand that love is the root in his life, his poetry, and it doesn't make any difference what kind of love."

On Monday night, Doris Kearas tolf how she gathered the material for her celebrated "Lyndon B. Johnson and the American Dream." The former president would waken for early in the morning when she was staying at his ranch in Texas. She would get up and sit in a small chair and LBJ would get into her bed and pull the covers up to his chin and tell her about his mother and his childhood dreams.

She said Johnson once told her she reminded him of his mother more than any other woman he had ever met. What emerged, she said, was that Johnson felt that his mother loved him only as long as he succeeded.

Edel commented that this would certainly be the "figure under the carpet" in LBJ's life, and Kearns agreed.

Tuchma, who won Pulitzers for "The Guns of August" and "Stilwell and the American Experience in China" and is the author of the current bestselling "A Distant Mirror," a study of the disaster of the 14th century, said that she has used biography, only as a "prism" through which to describe historical events.

"I think of myself as a storyteller and the reader as a listener," she said. In fact, she continued, the reader is essential to the writer, for if it takes two make love or war or play tennis, it takes two to accomplish the purpose of the written word."

Moreover, narration helps both writer and reader see connections between events that otherwise might be obscured. The success of the story depends on how well the historian uses the important facts and omits the trival.

For example, she said, there was the matter of Gen. Stilwell's compenious diaries. He wrote daily about his rages and frustrations, and the angrier he got the more he wrote each day. But he did not write about himself. So the diaries were useful in finding out what bothered Stilwell - rich people, horses Chiang Kai's shek - but not about the inner man.

For Tuchman, the most striking thing about them, she said, was the general's abundant use of bad language.

"I had no idea how common and banal four-letter words were in men's conversaton," she said.

She also had access to seperate accounts Stilwell wrote of several of his dreams. Tuchman said he had thought of his dreams. Tuchman said he had thought of taking them to "analytic friends," but had decided against this because all she would get would be "stock ansers."

Leon Edel rose to his feet. "I'm beginning to see the difference between the historican and the biographer," he said. "Those rages and dreams would be fascinating to the biographer."

"I'm not such a fool, Leon, that I didn't use them to help me understand the man," Tuchman replied. "It's just that he showed his character: he didn't discuss it."