Historians are apt to be wary of fictive questions. What if the South had won the Civil War? How would the American West have developed without the railroads? What if Dewey really had defeated Truman?

The trouble with such questions is that every ounce of data available to mull over has already been shaped, minutely and irreversibly, by occurrences opposite to the ones dreamed up. Yet history is often more art than science. Even the most hidebound empirical analyst –try as he might to skirt moral reflection and reduce history to a social science — is likely to pose a fictive question or two in a day’s work. Was Thomas Jefferson’s presidency indispensable to the Louisiana Purchase?

Or try this one. What if Michael Paine had been more alert to the safety of his family in the fall of 1963?

Paine was the estranged husband of Ruth Hyde Paine, a nice Quaker housewife who invited Marina Oswald and her two small children to stay at the Paines’ home in a Dallas suburb while Marina’s husband, Lee Harvey Oswald, looked for work. Among the Oswald possessions stored in Mrs. Paine’s garage was a $20 mail-ordered bolt-action Mannlicher-Carcano rifle tied securely in a green-and-brown blanket roll.

Michael Paine recalls thinking vaguely that the blanket roll was camping gear. His separation from Ruth the previous September had been enlightened and amicable; no hard feelings. He would come home frequently to have dinner with Ruth and their two kids, and putter with his drill press and table saw in the garage.

But in April of 1963, seven months after Ruth and Michael had split and five months before Marina Oswald was to move in, Michael saw the now familiar photograph of Lee Oswald brandishing a rifle.

Lonely after her separation from Michael, Ruth Paine had met the Oswalds at a party. Lee seemed a little odd, what with his weird insistence that his Russian wife not learn English. But Marina was nice, and Ruth wanted to improve the Russian she had been dabbling with for some years.

At Ruth’s request before one of his family visits, Michael Paine had stopped at the Oswalds’ apartment to meet the couple and to drive them home for dinner. The photograph was conspicuous on a table.

“I had expected,” Paine recalls, “to find a theoretician, you know, somebody who was interested in philosophy or politics or something like that. But he was obviously, clearly, proud of this picture, and I came to think it was a true icon he had of himself.”

In Mrs. Paine’s Garage and the Murder of John F. Kennedy (Pantheon, 209 pages, $22; click here to order), Thomas Mallon wonders at many what-ifs, including the horrific consequences of Mr. and Mrs. Paine’s latitudinarian do-goodism. Now in his early 70s and living alone in Boston, Michael Paine is still resolutely non-judgmental.

His family roots go back to the Revolution, and he is a great-great grandson of Ralph Waldo Emerson. His father was George Lyman Paine, who combined family wealth and progressivist politics.

“I grew up,” he tells Mallon in an interview, “feeling that dedicating one’s life to trying to make a better world was a very good and valuable thing to do. And raising babies shouldn’t interfere too much with it. So I had that feeling with regard to Lee…I didn’t find fault with the way he spent his life.”

So Michael Paine — separated in 1962 from his wife of four years (approximately: he could never quite remember the exact year of his marriage to Ruth) and not hugely attentive to his own two young kids — could sympathize with a 23-year-old loser already saddled by two children.

On weekends, during the time Marina was staying with Ruth Paine, Lee Oswald would visit. Ruth gave him driving lessons and made the connection that landed him a job at the Texas Book Depository. A month after the murder of John F. Kennedy, Ruth remarked to a journalist that she would “forever have to live with my regrets that I did not perceive this incompetent yet striving man as a dangerous person.”

Yes — and sometimes, as Mallon reflects near the end of his brilliantly coalescing what-ifs, “a refusal to think the worst of people is precisely what brings it out.”

Mallon glances over “the paranoid style in America’s political character” as displayed by conspiracy buffs who stretch out the Kennedy assassination’s agony on the Internet. But his real interest is “the nation’s transcendent and optimistic strain, whose evasions have sometimes led it down garden paths when night was falling.”

What if a capacious love of humanity were less often tied up in a colossal indifference to the human condition?