On November 22, 1968, which marked the five-year anniversary of the assassination of John F. Kennedy, in Dallas, and was seven months after the assassination of Martin Luther King, Jr., in Memphis, a daguerreotype of Frederick Douglass appeared on the cover of Life magazine, for a special issue devoted to “The Search for a Black Past.” I don’t know how to explain why I thought of that photograph while watching Diamond Reynolds’s Facebook video of the police shooting of her boyfriend, Philando Castile. But I did. I’d been sitting at the breakfast table, crying, like so many people, while reading the news about the sniper in Dallas who shot twelve police officers, killing five of them, and had decided not to watch any of the footage of what happened that night for the same reason I’d decided not to watch any of the videos earlier in the week: watching had, oh, three or four murders ago, begun to feel like a kind of complicity, as if we’re all prisoners marched out of our cells and into the prison yard to serve as spectators for the next execution: the gun fires; we flinch; we return, helplessly, to our cells. So I’d skipped the footage of two policemen, in Baton Rouge, shooting Alton Sterling, and I’d swiped past Reynolds’s video, from Minnesota. But then, after reading about the sniper, I thought: maybe watching people shoot one another has become an obligation of American citizenship. So I forced myself to watch. And, as I did, the screen went black—the police had thrown down Reynolds’s phone, and put her in handcuffs—and you could only hear voices, the muted, distant sound of Reynolds crying and praying, and, closer, the urgent voice of her four-year-old daughter, and right then I remembered that photograph of Douglass.

Douglass believed photography would set his people free by telling the truth about their humanity. He escaped from slavery, in 1838, just about when the daguerreotype came to the United States. He’d been advertised as a runaway, in the newspaper, with a little woodcut: a caricature of a black man. In 1841, he sat for his first photograph, in a dark suit, with a stiff, white collar, staring straight into the camera. “Have we been listening to a thing, a piece of property, or to a man?” the abolitionist William Lloyd Garrison asked, when he took the stage after Douglass spoke in public for the first time. “A man! A man!,” came the cry from the crowd. His voice, his face, his photograph, proof: I am a man.

Douglass went on to become the most celebrated orator of his day and also—a fact established in a terrific new book, “Picturing Frederick Douglass”—the most photographed man in America. Douglass was photographed a lot because he was famous, but also because he was fascinated with photography. “Negroes can never have impartial portraits at the hands of white artists,” he said. “It seems to us next to impossible for white men to take likenesses of black men, without most grossly exaggerating their distinctive features.” But photographs, he thought, would tell the truth. He had this hope, too: he believed photography would help to realize the promise of democracy. “What was once the special and exclusive luxury of the rich and great is now the privilege of all,” Douglass said. “The humblest servant girl may now possess a picture of herself such as the wealth of kings could not purchase fifty years ago.” Reynolds works as a housekeeper in a hotel. She owns a camera that can take moving pictures and stream them, live, to the whole world, instantly. It was just that kind of technology that, Douglass predicted, would usher in an age of equality, justice, and peace:

The growing inter-communication of distant nations, the rapid transmission of intelligence over the globe—the worldwide ramifications of commerce—bringing together the knowledge, the skill, and the mental power of the world, cannot but dispel prejudice, dissolve the granite barriers of arbitrary power, bring the world into peace and unity, and at last crown the world with just[ice,] liberty, and brotherly kindness.

Against the overwhelming evidence of history, many people have this same faith in technology in the twenty-first century. “The whole soul of man is a sort of picture gallery, a grand panorama, in which all the great facts of the universe, in tracing things of time and things of eternity, are painted,” Douglass said in a lecture, in 1861, called “Pictures and Progress.” These days, it’s data companies that say this sort of thing. “My iPhone 5 can see every point of view, every panorama, the entire gallery of humanity,” a Sprint ad promised, not long ago. Can your phone really see every point of view? Facebook has a say in what footage to stream on Facebook Live. No doubt the people there are doing their best, in deciding what to allow and what not, or in writing algorithms to make those decisions, but their chief consideration isn’t the public good, which is how a newspaper would make that decision; their chief consideration is consumer satisfaction. (And, lately, newspapers are following Facebook, instead of leading.) There are reasons to stream a murder live. I’m just not sure Facebook has reckoned the cost.

There is no technological fix for atrocity. The posting on the Internet of footage from iPhones and dash cams and body cams, capturing the bodies of so many black men, bleeding, crying, dying, is meant as both evidence and a call to action. And it serves those ends, by exposing the truth. But the posting and reposting have a lot in common with the replication and circulation of photographs taken by spectators at lynchings in Jim Crow-era America. Black newspapers sometimes published those photographs, so that “the world may see and know what semi-barbarous America is doing,” as an African-American editor, in Kansas, put it. More often, though, far more often, those photographs were sold as postcards.

A daguerreotype of Douglass appeared on the cover of Life in November, 1968, after a series of unbearable events. In March, President Lyndon B. Johnson’s National Advisory Commission on Civil Disorders, the Kerner Commission, reported, “Our nation is moving toward two societies, one black, one white—separate and unequal.” Between February and April of that year, in Memphis, African-American garbage workers went on strike and marched, wearing signs and carrying placards that read “I AM A MAN.” At one march, which was meant to be peaceful, a group of young men tore the sticks from the placards and began using them to smash windows. The police then began attacking the protesters. King, who was there, was devastated by the violence. Days later, he was assassinated. Race riots then broke out in more than a hundred cities, where protesters faced police in riot gear. That summer, violence broke out around the Democratic National Convention. Douglass had been largely forgotten until one of his autobiographies, out of print since the eighteen-fifties, was reprinted, in 1960. His daguerreotype, on the cover of the “The Search for a Black Past,” was Life’s answer to the riots, the proof of the photograph: I am a man.

On Saturday, at a press conference in Warsaw, President Obama warned against making comparisons between 1968 and 2016. He said, “When we start suggesting that somehow there’s this enormous polarization, and we’re back to the situation in the sixties, that’s just not true.” But that’s not to say there’s no reason to look backward. Johnson signed the Voting Rights Act and the Civil Rights Act, and, in response to riots in the cities, he also signed the Law Enforcement Assistance Act, marking the beginning of the modern criminal-justice system, as my colleague Elizabeth Hinton, a historian, points out in an extraordinary and important new book, “From the War on Poverty to the War on Crime.” It may be that there aren’t answers to be found in the search for a black past. No tape can be rewound. But a frame has been frozen in time, in black and white. This face, this proof: here was a man, here are men, white and black, to be mourned.