When Boston tech entrepreneur Paul English recently announced his campaign to erect a monument to Martin Luther King Jr. in Boston, he seemed to be thinking about grandeur. He said of the memorial: “I want it to be epic.’’

“Epic’’ is not a word I’d apply to Dr. King. He embodied the modesty of true greatness. A Boston monument to an African-American Civil War regiment, a park for children now taking shape next to the Children’s Museum, and a wall in Washington, D.C., etched with the names of 58,000 Americans who died in the Vietnam War all remind us that memorials can speak powerfully without shouting.

English is trying to raise $5 million, which would include at least $1 million of his own money, to commission a King monument, a plan Mayor Martin J. Walsh has endorsed. That news got me thinking about memorials. There are lots of them in Greater Boston, and many are wonderful. But some are flops. What makes the difference? What are the qualities of good memorials?

Here’s one thought for starters: A memorial should be durable. It should look as if it’s going to be around for a while, as if the memories and values it represents are going to outlive the temporary fads of any one era. A memorial should be, to quote a famous line by the poet T.S. Eliot, “the still point of the turning world.’’

If there’s a still point in a turning Boston, it’s the Robert Gould Shaw Memorial, on Beacon Street at the edge of Boston Common. The Shaw is the memorial by which others should be judged. It’s been there, unchanged, since 1897. Its marvelous sculpture, by Augustus Saint-Gaudens, depicts black soldiers from the 54th Massachusetts Regiment marching for the Union army in the Civil War, led by a white officer, Colonel Shaw. The Shaw is modest in size and unpretentious in manner, with a New England kind of understatement. (An interesting comparison could be made to another Civil War effort, the Grant Memorial on the National Mall in Washington, where the second-largest equestrian statue in the country rides above a battlefield.) The Shaw’s physical form speaks of the slow skill of human handicraft, and its moral message is the timeless one of human brotherhood. Both keep the Shaw as relevant as ever in today’s world of quick-change imagery and racial tension.

Here’s another rule: A memorial shouldn’t be too realistic. Boston’s Irish Famine Memorial, on the Freedom Trail, is so packed with detail that it lacks dignity. It’s a stage set for lifelike props and actors who seem to have frozen when the curtain went up. It’s cartoonish. Too much realism, too much detail.

Things can go wrong the other way, too. Rather than too realistic, a memorial can be too abstract, so lacking in anything you can recognize that most people won’t have any idea what it’s supposed to represent. An example is the memorial at MIT for Sean Collier, the campus officer who was ambushed and killed by the Boston Marathon bombers in 2013. The Collier memorial was designed by J. Meejin Yoon, who heads MIT’s Department of Architecture. On an outdoor plaza near the site of the shooting, 32 blocks of granite are assembled into an abstract depiction of an open hand. Yoon sees it as a metaphor: “This is strength through many pieces coming together.’’ But without an explanation, you’re unlikely to grasp that idea. The abstract Collier says more about the high-tech brilliance of MIT than about its subject.

A memorial should engage with its surrounding, not stand lonely like a ship at sea. I love the dual statue of Boston’s colorful Mayor James Michael Curley (Lloyd Lillie was the sculptor) near Faneuil Hall. There are two Curleys. One sits on a public bench being your buddy, and the other stands up, ready to roar a political tirade. Their context is simply the city in which you discover them. Maybe you’re stimulated to learn more about the history of immigrant groups in Boston and their rise to power through politics. Unlike the famine memorial, the Curley doesn’t tell you what to think; it just surrounds you with stuff to think about.

The opposite of that kind of civic engagement is another Martin Luther King Jr. memorial, in Washington. Hidden among trees near the Potomac, it fails to respond to anything around it but seems to exist in its own little heaven. It’s huge and bland, words that (probably unfairly) remind me of Paul English’s “epic.’’

A memorial doesn’t have to be grand. The Shaw certainly isn’t. Nor is the promising Martin’s Park at the Children’s Museum, being done by the landscape firm MVVA. It honors Martin Richard, the 8-year-old Dorchester boy killed in the Marathon bombings. I’ve seen the park only in an early model. But I liked the sense of a small, safe space that is complicated enough to give a kid a sense of exploration and discovery, of wandering.

Another rule: A memorial should give the visitor something to do, something that will extend the experience beyond a quick pose for a snapshot. Something must draw you in and make you pause. A world-class example is Maya Ying Lin’s Vietnam Veterans Memorial in Washington. You descend a path into the silent earth, accompanied by the dark wall and its 58,000 names. At some point, you notice your own face reflected back among the names of the dead. When you rise again at the far end, you’ve been taken to a dark place and come out again somehow changed. I suspect Dr. King would have seen it that way.

Finally, a memorial should be unafraid of controversy. One of the functions of public art is to keep difficult issues alive in the public consciousness. Public art is usually political, if only because there always seem to be some people who hate it. Many thought the design of the Vietnam memorial was unheroic. The architectural style of the World War II Memorial, on the Mall in Washington, was cited by some as fascist. The Flight 93 National Memorial, at the site of the Shanksville, Pa., plane crash on 9/11, was accused of looking — of all things — too Muslim.

Conclusion? If Boston does create a King memorial, it probably needn’t be something we all love and admire. Like the man himself, it shouldn’t be shy of confrontation.

Robert Campbell can be reached at camglobe@aol.com.