That's a famous line from the 19th century philosopher Ralph Waldo Emerson, but it could also apply to a modern American hero: the Rev. Martin Luther King Jr.

As the nation celebrates King's national holiday Monday, it's easy to freeze-frame him as the benevolent dreamer carved in stone on the Washington Mall. Yet the platitudes that frame many King holiday events often fail to mention the most radical aspects of his legacy, says Jeanne Theoharis, a political science professor at Brooklyn College and author of several books on the civil rights movement.

"We turn him into a Thanksgiving parade float, he's jolly, larger than life and he makes us feel good," Theoharis says. "We've turned him into a mascot."

Many people vaguely know that King opposed the Vietnam War and talked more about poverty in his later years. But King also had a lot to say about issues not normally associated with civil rights that still resonate today, historians and activists say.

If you're concerned about inequality, health care, climate change or even the nastiness of our political disagreements, then King has plenty to say to you. To see that version of King, though, we have to dust off the cliches and look at him anew.

If you're more familiar with your smartphone than your history, try this: Think of King not just as a civil rights hero, but also as an app -- his legacy has to be updated to remain relevant.

Here are three ways we can update our MLK app to see how he spoke not only to his time, but to our time as well:

No. 1: He's an environmental hero

When former Attorney General Eric Holder spoke at a King commemorative event in 2011, he described King in an unusual way:

King, he said, helped "plant the seeds for what would become our nation's now-thriving environmental justice movement."

There's a connection between what King was saying in the 1950s and '60s and this explosion of ecological thinking that emerged around 1970, where everyone was saying it's all about interconnectedness. Drew Dellinger, author and activist, on King's environmental legacy

People don't think of King as an ecological activist. He didn't live long enough to see the environmental movement take off. He died just a few months before the Apollo 8 astronauts took the iconic "Earthrise" photo over the moon, which is often credited with sparking widespread environmental awareness..

But he still inspires environmental activists because they say he was so eloquent in articulating a core belief of their movement: the interconnected nature of life.

One of the most difficult challenges climate change activists face today is convincing political and business leaders to act on behalf of future generations and people in other parts of the world.

King, though, offers a great example of how to address this challenge, some say. He constantly talked about the interconnectedness of life.

In his "A Christmas Sermon on Peace" just five months before his assassination, King delivered one of his most famous quotes:

"All life is interrelated. We are all caught in an inescapable network of mutuality. Whatever affects one directly affects all indirectly. We are made to live together because of the interrelated structure of reality."

Some even describe King as an ecological hero.

Activist and author Drew Dellinger says King's insight is profound.

"That's the essence of ecology," says Dellinger, who wrote an essay for Common Ground magazine entitled, "Martin Luther King Jr.: Ecological Thinker."

"The first law of ecology is that everything is connected."

Other environmental activists have noticed King as well. In his essay, Dellinger cites Larry L. Rasmussen, author of "Earth-honoring Faith" and a professor emeritus of social ethics at Union Theological Seminary, describing King as "one of the great ecological thinkers of the 20th century."

King wrote more about nature and the fragility of life on Earth than people realize, says Dellinger. King once warned that "cities are gasping in polluted air and enduring contaminated water."

Dellinger also cites this quote from King: "It would be foolhardy for me to work for integrated schools or integrated lunch counters and not be concerned about the survival of the world in which to be integrated."

When he was pouring through King's sermons and books, Dellinger says, he discovered constant references to science and nature. He came across notecards written in King's hand rhapsodizing about "all this galaxy of wonder" and "stars that guide sailors in storms."

Some activists concerned by melting glaciers caused by climate change have found an unlikely source of inspiration in King's words.

"There's a connection between what King was saying in the 1950s and '60s and this explosion of ecological thinking that emerged around 1970, where everyone was saying it's all about interconnectedness," Dellinger says.

Gwendolyn Keyes Fleming, a former EPA regional administrator who wrote about King's environmental legacy, says King also inspired a generation of activists in the 1970s and '80s who fought to remove landfills that had been placed near communities of color.

"If you go back to the beginnings of the environmental justice movement, a lot of those folks were active civil rights advocates at the time," says Keys, now a partner in the DC law firm of Van Ness Feldman. "Even Congressman John Lewis said that environmental justice is one of the modern civil rights issues of our time."

No. 2: He was a socialist before it was cool

There was a time in American politics when calling someone a socialist was a slur. Not anymore, at least for many younger Americans who are developing a distrust of capitalism.

One of the most popular politicians in recent times is Vermont Sen. Bernie Sanders, a self-described Democratic Socialist who almost captured the Democratic nomination for president. A 2016 Harvard University poll said 51% of young Americans -- 18- to-29-year-olds -- oppose capitalism. And a poll conducted the next year by YouGov and the Victims of Communism Memorial Foundation found that most American millennials preferred to live in a socialist country than a capitalist one.

Young Americans drawn to Sen. Bernie Sanders' message would find that King shared a similar economic philosophy.

The reasons why some millennials prefer socialism have been documented. Lingering scars from the Great Recession; staggering student debt; the greatest economic inequality since before the Great Depression -- all have contributed to an unease about capitalism.

King had similar misgivings.

Many historians describe him as a "democratic socialist," someone who, according to the Democratic Socialists of America, believes the economy should be shaped "to meet public needs" and "not to make profits for a few."

King called for universal health care and education, a guaranteed annual income and the nationalization of some industries.

He was a big supporter of unions, and while there was no Fight for 15 campaign to raise the minimum wage during his time, he once said even menial workers should make enough "so they can live and educate their children and buy a home and have the basic necessities of life."

When King was assassinated in 1968, he was about lead a multiracial army of poor people into Washington to force the nation's political leaders to address poverty.

"He saw that changing the laws was important but that was not in itself sufficient," says Jerald Podair, a history professor at Lawrence University in Wisconsin. "There had to be some sort of redistribution of economic power."

How radical was King's vision? He spelled out it on many occasions:

In 1968, he told a church audience:

"It didn't cost the nation a penny to open lunch counters. It didn't cost the nation a penny to give us the right to vote. But it will cost the nation billions to feed and house all of its citizens. The country needs a radical redistribution of wealth."

In a 1966 speech to his staff, he said:

"There must be a better distribution of wealth, and maybe America must move toward a democratic socialism."

Today capitalism has outlived its usefulness. It has brought about a system that takes necessities from the masses to give luxuries to the classes. The Rev. Martin Luther King Jr.

In 1964, after traveling to Norway to receive the Nobel Peace Prize, King said:

"In both Norway and Sweden, whose economies are literally dwarfed by the size of our affluence and the extent of our technology, they have no unemployment and no slums. There, men, women and children have long enjoyed free medical care and quality education. This contrast to the limited, halting steps taken by our rich nation deeply troubled me."

Even one of King' s most utopian visions -- a guaranteed income -- is getting serious discussion today because of fears that automation will erase many jobs. There's even a new proposal to give every newborn in the US "Baby Bonds," accounts between $500 and $50,000 that parents couldn't touch until their kids turn 18, as a way to combat inequality.

There are those who say King gravitated toward socialism later in his life. But like many Americans today, he saw its possibilities first as a young man.

In a letter to his future wife, Coretta Scott, a 20-something King wrote:

"I imagine you already know that I am much more socialistic in my economic theory than capitalistic ... So today capitalism has outlived its usefulness. It has brought about a system that takes necessities from the masses to give luxuries to the classes."

No. 3: He never let a political disagreement turn nasty

"Reverend Dr. Chickenwing." "A religious Uncle Tom." A "traitor" and a "chump."

That's how one prominent black leader described King when he was alive.

That leader was Malcolm X. While his denunciations of King and other leaders is well known, here's a remarkable fact: There is virtually no record of King making a personal or petty attack against Malcolm X or any other black leaders who criticized him.

This could be an important lesson at a time of bitter political divisions, which are often made worse by social media.

We turn him into a Thanksgiving parade float, he's jolly, larger than life and he makes us feel good. We've turned him into a mascot. Jeanne Theoharis, author of "A More Beautiful and Terrible History: The Uses and Misuses of Civil Rights History"

Two contemporary leaders in the civil rights movement recently got involved in a Twitter feud that turned personal. The clash between Harvard professor Cornel West and award-winning Ta-Nehisi Coates revolved around political philosophy. But it became so heated that Coates ended up deleting his Twitter account.

King received a cascade of petty insults from Malcolm X and other black leaders during his time, but King's faith tempered his response, says Podair, the historian.

"He was a Christian minister, and he talked about trying to love segregationists, and if you try to have empathy for your enemies, you can also do that with your rivals in civil rights organizations," says Podair , who writes about race in his latest book, "City of Dreams: Dodger Stadium and the Birth of Modern Los Angeles."

There was also a pragmatic side to King's reluctance to get pulled into public feuds. It preserved alliances and kept the door open to new allies -- including Malcolm X.

After Malcolm broke with the Nation of Islam in 1964, he started reaching out to King and other civil rights leaders. He traveled to Selma, Alabama, in 1965, where King was leading the march to Montgomery, to offer support. A friend of Malcolm once said the former Nation of Islam leader believed King "would be the most responsive" of all civil rights leaders to his efforts to reconcile.

"He had come to believe that King believed in what he was doing," A. Peter Bailey told CNN in 2010 "He believed in nonviolence; it just wasn't a show. He developed respect for him. I heard him say you have to give respect to men who put their lives on the line."

The respect was mutual.

When King later learned that Malcolm X had come to Selma to support him, he was quoted as saying, "Hey, that Malcolm is a beautiful brother."

"He always had a deep affection for Malcolm," says Jacob Udo-Udo Jacob, a visiting international scholar in international studies at Dickinson College in Pennsylvania.

"Malcolm obviously challenged and did say many unkind words about King's philosophy of nonviolence, but their disagreements never turned personal or nasty."

The only time King and Malcolm X came face to face was this impromptu meeting in 1964.

King's magnanimous nature didn't just help him deal with other black leaders' insults. It also helped him deal with something people today may find hard to believe: He was persistently unpopular during his lifetime, says Theoharis, the Brooklyn College political science professor and author of "A More Beautiful and Terrible History: The Uses and Misuses of Civil Rights History."

"One of the things that people erase from history is how unpopular and scared of Dr. King most Americans were at the times," she says. "I'm not just talking about in '67 and '68, but I'm talking about in the early '60s."

Theoharis cites King's most transcendent moment: The 1963 March on Washington and his "I have a Dream" speech. Polls showed that most Americans didn't approve of the march, and the following year -- well before the 1965 Voting Rights Act was passed -- a New York Times poll showed that most Americans thought the civil rights movement had gone too far.

The US government thought King was so dangerous they treated him like an enemy of the state.

"We now see his March on Washington speech as the greatest American speech of the 20th century," she says, "but we're uncomfortable grappling with the fact that this is the moment that the FBI decided to have wall-to-wall surveillance of Dr. King."

Today we have wall-to-wall celebrations about King. He's now on par with American titans like Presidents George Washington and Abraham Lincoln. Americans write tributes and sing songs about how much King has changed America.

But here's one more uncomfortable thought that also explains why King remains so relevant:

The country is still divided by many of the same issues that consumed him.

On the last night of his life, King told a shouting congregation of black churchgoers that "we as a people" would get to "the Promised Land." That kind of optimism, though, sounds like it belongs to another era.

What we have now is a leader in the White House who denies widespread reports that he complained about Latino and African immigrants coming to America from "shithole" countries; a white supremacist who murders worshippers in church; a social media landscape that pulsates with anger and accusations.

King's Promised Land doesn't sound boring when compared to today's headlines. And maybe that's what's so sad about reliving his life every January for some people.

Fifty years after he died, King's vision for America still sounds so far away.

Correction: This story has been updated to reflect that Jacob Udo-Udo Jacob is now affiliated with Dickinson College.