My favorite private-eye trick is the one I learned about from Errol Morris.

You probably know Morris as an Oscar-winning documentary filmmaker. Roger Ebert called his first film, Gates of Heaven, one of “the ten greatest films ever made.” With The Thin Blue Line, Morris dramatically freed an innocent man imprisoned on a murder rap. In The Fog of War he extracted a confession from Robert McNamara, getting the tightly buttoned-up technocrat to admit “[we] were behaving as war criminals” for planning the 1945 firebombing of Tokyo, which burned to death 100,000 civilians in a single night.

You may also know that Morris is the author of the recent massive, fascinating book called Believing Is Seeing: Observations on the Mysteries of Photography, which won rave reviews for the way it looks not just into the frame of a photo but behind, beneath it—the way truth is “framed” in every sense of the word.

You may even think, as I do, that Morris has become one of America’s most idiosyncratic, prolific and provocative public intellectuals.

But what’s less well known about Morris is that he brings to his work the invaluable experience he picked up working as a private eye. And he hasn’t given up the private-eye impulse: He’s back on the case, two cases actually—two of the most electrifying and controversial cases in the past half century.

Born in suburban Long Island, Morris graduated from the University of Wisconsin. After a stint of cello study in France, he talked his way into the Princeton graduate philosophy seminar of Thomas Kuhn, an icon of postmodernism, the man who coined the term “paradigm shift.” It wasn’t exactly a meeting of the minds. In fact, it almost cracked Morris’ skull, which is what Kuhn seemed to be aiming to do at the climax of an argument when the esteemed philosopher threw an ashtray at Morris’ head.

“The Ashtray,” Morris’ five-part, 20,000-word account of that episode and their philosophical clash over the nature of truth, is a good introduction to the unique kind of writing he’s doing now. (Don’t miss the section on the obscure Greek philosopher of irrationalism, Hippasus of Metapontum, a digression worthy of Jorge Luis Borges.)

After the ashtray incident, Morris eventually did two stints as a private eye. If there is one subtext to all of Morris’ subsequent films and writings, it is the private eye’s creed, the anti-postmodernist belief that “the truth is out there.” Truth may be elusive, it may even be unknowable, but that doesn’t mean, as postmodernists aver, that reality is just a matter of subjective perspectives, that one way of seeing things is just as good as another.

“I’m amazed,” Morris said when we spoke recently, “that you still see this nonsense all over the place, that truth is relative, that truth is subjective. People still cling to it.” He calls these ideas “repulsive, repugnant. And what’s the other word? False.”

But I digress (something impossible to avoid in writing about Errol Morris). I wanted to tell you about his private-eye trick, which he learned from a hard-bitten partner.

It wasn’t a blackjack-, brass knuckles-type thing. “It went like this,” Morris explained. “He’d knock on a door, sometimes of someone not even connected to the case they were investigating. He’d flip open his wallet, show his badge and say, ‘I guess we don’t have to tell you why we’re here.’

“And more often than not the guy starts bawling like an infant, ‘How did you find out?’” And then disgorges some shameful criminal secret no one would ever have known about otherwise.

I have a feeling about why Morris likes this. There’s the obvious lesson—everybody’s got something to hide—and then there’s the subtle finesse of the question: “I guess we don’t have to tell you...” No water-boarding needed, just an opening for the primal force of conscience, the telltale heart’s internal monologue. It’s one of those mysteries of human nature that private eyes know and Morris has made his métier.

For three decades Morris has painstakingly produced brilliant documentaries on subjects ranging from pet cemeteries (Gates of Heaven) to jailed innocents (The Thin Blue Line) to lion tamers (Fast, Cheap and Out of Control) to cosmologist Stephen Hawking (A Brief History of Time) to Holocaust deniers (Mr. Death), Vietnam War architects (Fog of War) and Abu Ghraib’s “bad apples” (Standard Operating Procedure). And more recently, in 2010, a long-forgotten, insane tabloid war over “the manacled Mormon” sex scandal in Britain. This film, Tabloid, is a strange, delicious documentary that uncannily anticipated the current tabloid scandal there. And (like Gates of Heaven) Tabloid is really an investigation into the nature of perhaps the ultimate mystery: love.

He hasn’t stopped making films; indeed, he’s making one now with Ira Glass of “This American Life” dealing with cryogenics, of all things. But films take time, so in the past five years, Morris has turned to writing, developing a unique new genre that combines philosophical investigation with documentary transcripts and inventive graphics.

It began with a three-part, 25,000-word New York Times series on the question of the arrangement of some rocks in the road in two 150-year-old photographs taken during the Crimean War. (The “rocks” were actually cannonballs; they just looked like rocks in the photos.) I know: You’re running for the exits. Twenty-five thousand words on some rocks on a road?! But believe me, it becomes an absorbing intellectual adventure story.

I suppose I should disclose that I make a brief appearance in what became the first paragraph of the first chapter of the book, Believing Is Seeing. Wherein I ask Morris incredulously, “You mean to tell me that you went all the way to the Crimea because of one sentence written by Susan Sontag?”

To which he replied: “No, it was actually two sentences.”

Sontag had implied that the rocks in one of the photographs had been “posed,” and this lit a fire under Morris, who believes that everything in photography is “posed” in one way or another, not merely by what’s put in the frame, but by what’s left out.

To illustrate the near-impossibility of establishing veracity in photography he engaged in what might seem like a mad, hopeless enterprise: to see whether the cannonballs were initially on the road or placed there—posed for ideological impact. An investigation that involved him going halfway around the world to the Crimea to find the road and subsequently interviewing “shadow experts” on the time of day each photograph might have been shot.

As one commenter wrote:

“Don’t miss the excursus on the use of albatross eggs to provide the albumen for photo emulsions in early film developing. Or the meditation on Descartes’ Meditations. Or the succinct and devastating deconstruction of deconstructionists’ dim witted view of truth (just because we can’t necessarily know it, they rashly conclude it doesn’t exist). This leads to his critique of the correlative misreading of the film Rashomon [it’s not an ‘all points of view are equally valid’ manifesto] and his desire, expressed in a footnote, for a Rashomon about Rashomon.”

OK, that was me, writing back in 2007 when the series first appeared.

One of Morris’ advantages in his investigations is his disarming personal style. He’s a friendly, genial-looking, unpretentious guy, who reminds me of the old “Tinker, Tailor, Soldier, Spy” and Alec Guinness’ amazing, offhandedly profound portrait of the disarmingly unassuming, apparently empathetic George Smiley. And it occurred to me that in his own way, Morris is our Smiley. Robert McNamara, for instance, thought Morris understood him. And he did—just not the way McNamara understood himself.

But as wily as Morris is, I was worried when he told me about his latest obsession: the Jeffrey MacDonald murder case. “Oh my God, no,” was my measured reaction, “Not that!”

For the past four decades the MacDonald affair has been a toxic swamp that has drawn in some of journalism’s best and brightest writers.

“Yes, that,” Morris replied, telling me that MacDonald is the subject of his next book, titled A Wilderness of Error. In fact, he said, the book is the culmination of 20 years of fascination with the case, going back to a time in the early ’90s when Morris and his wife visited wig shops in Fayetteville, North Carolina, to investigate the wig-fiber evidence at the MacDonald crime scene. He is not a MacDonald partisan in that he doesn’t necessarily believe prosecutorial errors are proof of innocence, rather evidence of uncertainty.

If Errol Morris is that excited about the MacDonald case, it’s a sign we can’t say “Case closed.”

It is, you’ll remember, one of the past half century’s most controversial murder mysteries. The central question remains in dispute: Is MacDonald an innocent man wrongly convicted of murder or is he the ultimate con man?

It began in 1970 and soon became a national scandal widely known as the “Green Beret murder case.” MacDonald, then a Green Beret doctor with an unblemished record, was accused of murdering his wife and two young daughters in his home at Fort Bragg, a key Green Beret base. MacDonald blamed the crime instead on a band of hippies—including a woman in a floppy hat and blond wig—whom he claimed he unsuccessfully fought off as they invaded his home chanting, “Kill the pigs!...Acid is groovy!”

From the beginning the case was fraught with cultural implications. Who was guilty: a Green Beret or Manson-like hippies? After being exonerated at an Army hearing, MacDonald was convicted by civilian prosecutors and given a life sentence that he’s still serving, while spending every waking moment proclaiming his innocence.

You’ve probably heard of how two big-name journalists got involved in tormented relationships with MacDonald, then in fractious relationships with each other. First Joe McGinniss (of recent Sarah Palin biography fame), who seemed to intimate to MacDonald that he believed in his innocence but then came out with a book (Fatal Vision) that sought to nail him. MacDonald sued McGinniss for breach of trust.

Then the New Yorker’s Janet Malcolm produced a book, The Journalist and the Murderer, which accused McGinniss of treachery and became a huge media-ethics kerfuffle because of Malcolm’s dramatic opening sentence, which still echoes in the dusty classrooms of J-schools throughout America: “Every journalist who is not too stupid or too full of himself to notice what is going on knows that what he does is morally indefensible.”

I had thought the case was finally dead.

“It’s not dead!” Morris exclaimed, “He’s got another appeal coming up” (most likely in April).

“On what?” I asked, unable to believe there could possibly be a scintilla of evidence or testimony that hasn’t been combed over in the past 40 years.

“Two pieces of new evidence,” Morris replied. “One involves this federal marshal, James Britt, who was with Stoeckley [Helena Stoeckley, supposedly the woman in a floppy hat and blond wig] and who says that he heard the prosecutors threaten Stoeckley when Stoeckley said that she was going to insist that she had been present in the house that night.” (Stoeckley herself is now dead.)

“The other piece is the DNA evidence of an unsourced hair [untraceable to MacDonald or anyone else in the family] under the fingernail of one of the murdered children.”

Which means...the possible presence of another person at the scene of the crime.

Morris claims he has uncovered more Helena Stoeckley evidence on his own.

“There are too many coincidences,” Morris says. “For instance, it just so happens that the first officer, the officer who heard [MacDonald’s] statement [about the woman in the floppy hat], noticed on his way to the crime scene a woman who answered to that description standing in the rain and fog at 3 in the morning. He couldn’t stop because he was answering an emergency call, but the minute he heard the description, he made the connection.”

“Are you saying that MacDonald could be as innocent as Randall Adams in The Thin Blue Line?

“I think so much of the evidence has been lost,” Morris said wistfully. Lost too, perhaps, is any hope of certainty.

This is one of Morris’ greatest strengths, what Keats called “negative capability”: the ability to hold conflicting perspectives in the mind without “irritable” reaching after certainty. (So many conspiracy theorists just can’t bear the irritation of living with uncertainty.)

Any entanglement with the Jeffrey MacDonald case is risky, if you ask me, but Morris is not afraid of risk. As if to prove it, Morris tells me he’s considering plunging into the most dangerous labyrinth of them all—the Kennedy assassination. Abandon all hope ye who enter there.

Last November 22, the New York Times posted a six-minute mini-documentary Morris carved out of a six-hour interview with Josiah “Tink” Thompson, the author of Six Seconds in Dallas.

Another remarkable coincidence: Thompson was my philosophy professor at Yale, a specialist in the works of Soren Kierkegaard, the gloomy Danish proto-existentialist best known for the “leap of faith” notion—the idea that to believe in God one must abandon the scaffolding of reason for the realm of the irrational, even the absurd. The Lonely Labyrinth, Thompson’s book on Kierke­gaard, is still widely admired.

At the same time he was leading students through the labyrinth of Kant’s Critique of Pure Reason, Thompson worked as a consultant for Life magazine on the JFK case and wrote his influential book on the ballistics evidence in Kennedy’s assassination—an attempt to prove through pure reason (and science) that the Warren Commission was wrong. That Oswald could not have fired the number of shots attributed to him in six seconds from his antiquated Mannlicher-Carcano rifle. Which meant there had to have been at least one more gun­man. (Others have since claimed to have disproved Thomp­son’s contention.)

More coincidences: Thompson eventually quit his promising academic career to become—yes—a private detective working with David Fechheimer, a legendary investigator who had also employed...Errol Morris.

After reading a story I’d written that discussed Thompson’s arguments, Morris called him and arranged an interview. “He drove from Northern California to Florida, where I filmed him,” recalls Morris. “I wondered why [he drove] because we offered to fly him in. So I’m interviewing him. He gets up. He walks off. He comes back. And he has a Mannlicher-Carcano, just like the one Oswald used.”

“That’s why he didn’t fly?”

“Exactly. He wanted to demonstrate for me the enormous difficulty of firing those shots in rapid succession.”

My feeling is that the real JFK mystery is what was going on inside Oswald’s head, not inside the chambers of the Mannlicher-Carcano. Why was he doing it? What was his motive? Were others involved, even if they didn’t fire a shot?

But if anyone can solve it...

I have a fantasy that someday Errol Morris is going to show up at the door of some old guy no one has connected to the Kennedy assassination before and say, “I guess we don’t have to tell you why we’re here.”