The poignant and heartfelt documentary “Cow,” directed by Andrea Arnold, is a displaced movie. For all its virtues, its most interesting action isn’t onscreen but only hinted at. This lacuna reflects a widespread documentary practice that’s also a conventional lapse in aesthetic judgment. Arnold and her crew followed a single dairy cow named Luma, at a cattle farm in Park Farm, in Kent, England, for an intermittent four-year period. The film (which opens today in theatres and is streaming on many services) observes her through the cycle of life, from calving and giving milk through the travails of old age, amid the structured workdays on the farm. Employees guide Luma and the many other cows to the stalls where they’re connected to electrical milking machines, into fields where they romp, into pens where they feed, into other pens where Luma is mated with a bull and becomes pregnant again. But the filmmakers are absent.

“Cow” filters out the basic personal element. It doesn’t show the crew’s interactions with the farm workers, who seemingly pretend that the crew and equipment aren’t there; the efforts of the crew to stay close to Luma; or the conspicuous intrusion of Arnold and her colleagues setting up shop, with their equipment, at the farm and in the nearby fields where the animals graze. For that matter, the film also excludes the backstory on which it inevitably depends: the negotiations by Arnold and the producer, Kat Mansoor, with the farm’s management that set the terms for filming there, the implicitly transactional basis for the film’s very existence. These omissions are not a new issue with documentary filmmaking; the self-erasure comes up often (as in the Oscar-nominated “Honeyland,” in 2019) and seems baked into other agrarian films such as “Gunda” and “The Four Times,” as if both implying and concealing the incongruity of filmmakers’ presence in natural and rustic settings.

In “Cow,” the drama is thin and the ideas are distant. Closeups of Luma are the film’s emotional engine. (The cinematographer is Magda Kowalczyk.) These images have a soulfulness that’s then, at times, amplified into majesty by compositions in which sky, terrain, and other animals fill the frame with a sense of united, grand-scale, wide-ranging power. They’re the exception—and they merely punctuate the film’s sense of observational, reportorial information delivery. It’s all the more reason why the movie, despite the ambling pace, resembles the narrow train of a Hollywood fictional narrative.

The paradox of “Cow” is the paradox of modern documentary filmmaking, an aesthetic revolution that was born—in the United States and in France—on the basis of technological innovation, the ability to film while recording synchronized sound with lightweight and portable equipment. In 1960, Robert Drew produced “Primary,” in which he was able to dispatch a crew to Wisconsin to embed with the campaigns of Senators John F. Kennedy and Hubert Humphrey as they competed for the Democratic Presidential nomination. Around the same time in France, the filmmaker Jean Rouch and the sociologist Edgar Morin collaborated on “Chronicle of a Summer,” in which they and their associates stopped people in the street to ask about their lives, and then probed further to explore the connections of work, private life, and politics—and to examine their own filmmaking process.

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Morin coined the term “cinéma vérité” for their project; Drew eventually borrowed the term for his work (he also used “candid drama” and “reality filmmaking”). Unlike Rouch and Morin, Drew (whose movies included “The Chair,” “The Children Were Watching,” and “Crisis”) didn’t feature the filmmakers discussing their work on camera or overtly interacting with their subjects. Nonetheless, Drew centered on the filmmakers’ presence, not their absence; for him, filmmaking was a unified field in which he and his crew were inevitably participants in the events that they recorded. That participation is revealed most often in the behavior of the subjects, who, far from going about their business as normal, speak to the filmmakers, address the camera, perform for it, and acknowledge its presence and its power. Drew’s candid dramas are based in the subjects’ realities—starting with the very fact of being filmed—and the emotional and intellectual authority of his work is rooted in these self-conscious performances.

All great ideas are subject to corruption; all original artistic practices risk being reduced to formulas. Much documentary filmmaking practice has devolved to the so-called observational or fly-on-the-wall method, where filmmakers get permission to embed themselves in a setting that interests them and collect recordings with the authenticity of the real—while excluding the reality of the filming. They shape their material as the fiction of what it would have been had the filmmakers not been present. At its best, this very idea resounds with a kind of directorial chutzpah—it’s what Frederick Wiseman has been doing since the beginning of his career, and the audacity comes from what he does with his disappearing act, creating scenes of immense length and complexity, in which his participants seem to be performing not for his sake but for their own, as an essential aspect of their identity and social lives. His narratives have vast arcs allowing for vast digressions because his principal subject is the abstract frameworks of institutions. He does the analytical work that seemingly builds out under the action as it advances.

Few documentary filmmakers have followed Wiseman into the radical potential of self-elision from onscreen events. Far more often, the arm’s-length observation is a sedimented convention, an unconsidered assumption about the very definition of documentary filmmaking—a choice, and a bad one, that is taken for essential and natural. The past decade-plus has been a time of grand renewal in documentary filmmaking; the very notion of creative nonfiction has been developed and expanded, as in the work of such filmmakers as Robert Greene, Penny Lane, Khalik Allah, Nanfu Wang, Theo Anthony, Courtney Stephens, and Pacho Velez, among many others. At the same time, new technology has made its power felt, often negatively. Digital video cameras, which dispense with the high cost of shooting film and make it possible to record enormous amounts of material, invite a snippety editing process that squeezes as much material as possible into a film’s standard duration.

It would be absurd to even suggest a moratorium on the observational method, but it’s no less absurd to consider it uncritically—and the most vital criticism that the practice needs is that of filmmakers themselves, in action. “Cow” reflects an admirable and fascinating motive: to enter into the daily experience and the inner life of a farm animal. Arnold, a major artist of cinematic fiction, has made characters’ self-presentation, their sense of performance in daily life, a crucial part of her most original drama, “American Honey.” In “Cow,” Arnold hasn’t considered her subjects or her place in their world as stringently or as originally. The strongest moments of “Cow” evoke her fervent and empathetic dramatic sensibility but not a strong awareness of the possibilities of documentary form. In this, she’s far from alone; more than any one filmmaker’s omissions, “Cow” reflects the unquestioned conventions of the time.