After a showing of the new documentary The Unknown Known at the Angelika the other day, I shuffled out behind an older couple. He was white-haired and radiating dissatisfaction. And she was saying she'd enjoyed the movie. Former Defense Secretary Donald Rumsfeld had not come off all that bad, she said.

"It wasn't a hatchet job," she said.

This was what was bothering him, plainly. "That guy who was questioning Rumsfeld sometimes," he said, "He should have been harder on him."

"That guy," of course, is Errol Morris, the celebrated documentarian who somehow convinced former Defense Secretary Donald Rumsfeld to talk to him for 33 hours about his career. He has turned those conversations into a documentary that runs just short of two hours.

As in all Morris films, this one had Morris hidden offstage, indeed heard only "sometimes." But also as in all Morris films, pretty much every frame was soaked in point of view, even when he wasn't talking.

This couple's reading, or misreading, of the film was merely the view of two people who happened to have the time at 2:30 on a sunny Wednesday to go watch a documentary in downtown Manhattan. By definition, that makes them totally unrepresentative, but there was a useful allegory in their reaction, one which Morris might appreciate.

Most of Morris' work, the critics say, is about the search for truth. "His films can be dizzyingly discursive," A.O. Scott said in his review at the Times, "but they are also motivated by a doggedly empirical impulse." But the thing is, dogged empiricism demands that you really pay attention. And this movie does, too.

In fact, the inability to see what was really going on is its real subject. People could not see what was really going on when we went to war in Iraq a decade ago. And that is what Morris is, principally, interested in finding out from Donald Rumsfeld. As Morris explained in a recent series of pieces in the Times, the title is drawn from that infamous remark Rumsfeld made in 2003 when questioned on the evidence of weapons of mass destruction in Iraq:

Reports that say that something hasn't happened are always interesting to me, because as we know, there are known knowns; there are things we know we know. We also know there are known unknowns; that is to say we know there are some things we do not know. But there are also unknown unknowns — the ones we don't know we don't know. And if one looks throughout the history of our country and other free countries, it is the latter category that tend to be the difficult ones.

Everyone mocked this at the time. No one, until Morris, seems to have spent much time trying to untangle what Rumsfeld could have possibly meant by it.

Morris does not get a straight answer, but then it's hard to believe he expected one. So his voice intervenes only "sometimes." You have to pay attention. You have to watch carefully.

Does the movie admit that Rumsfeld is human? Yes. For one thing, Morris lets the camera linger on Rumsfeld's grin a great deal. Rumsfeld, it turns out, is a champion grinner. I know this will send some people into fits of apoplexy about Rumsfeld's being smarmy, and he is in places smarmy. But mostly his grins seem sincere and all the more sinister for it.

On Rumsfeld's face, a smile is a curious expression. He has small but rather wide-set teeth and the corners of his mouth draw up in a way that indicates practice and, dare I say it, neediness. There is a vague awareness in these smiles that the strategy has not been working for him. The grin acts as punctuation, and always lingers just a bit too long.

Rumsfeld is eager from the beginning to present himself as affable. "I'm not an obsessive," he tells Morris. He is, instead "cool," which he means in the sense of unflappable rather than fashionable. He is reasonable. He likes the certainty of paper. He writes a lot of memoranda, known colloquially around the Pentagon as snowflakes, many of which are requests for the dictionary definitions of words. Precise definitions do seem to be very important to Rumsfeld, albeit chiefly because the more precise he is, the better it allows him to answer without answering Morris' questions about what was really going on.

He is sometimes up-front in his attempts to obfuscate. He denies that his life in public service was about endless conflicts of personalities. When Morris points out that Shakespeare's histories are all about personal character, Rumsfeld demurs, "That was a different time." A few minutes later he'll get in a dig or two at George H.W. Bush, who was long his rival in high-flown Republican circles. (It sounds like they still aren't that close.)

But most of the time he isn't so obviously craven. He is self-satisfied, which is possibly worse. He criticizes Saddam Hussein as being obsessed with his own image, with being worshipped. He says that as a dictator Hussein was deluding himself. He was "living his image of himself which was pretend." That one could say this about a great many leaders, including secretaries of defense, seems to be lost on him.

It does not, however, seem to be lost on the filmmaker, who is fond of letting remarks like those just hang there. And indeed in that Times series, Morris has been much more literal about his opinion of his subject. "I was left with the frightening suspicion that the grin might not be hiding anything," he writes. "It was a grin of supreme self-satisfaction and behind the grin might be nothing at all." The use of "might," the use of "suspicion": those do still leave the door open a crack.

There might, of course, have been a less unsettling way to make a documentary like this. There might have been a way to do it as a stoning, as revenge for the Bush years, hard on Rumsfeld in the blunt way that satisfies people. There are people who specialize in such things, most of them op-ed columnists.

This movie is, however, something more worth your time. Because truth was such a moving target then, there is a habit, lately, to call the era "postmodern" and dispense with it that way. This has always been too antiseptic a way of putting it, and almost misleadingly so. We weren't living in an era of "radical uncertainty" then. Brilliantly, The Unknown Known points out instead that the chief crime of men like Rumsfeld was the way they weaponized uncertainty. They made it a reasonable sword, unflappable, cool, always right, always assuring you that it knew better than you what was really going on. And that relied, for its power, on all of us forgetting to pay attention.