Bartlett Sher was skeptically incredulous when it was suggested that “Hamilton,” the epoch-shaking masterwork, could strike someone as being so substantial as to be an opera.

“Based on what?” asked Sher, the highly lauded director of work on Broadway and in opera.

The defense rose and made its case: “Hamilton” is “through-composed” (no spoken dialogue) and has a weighty libretto, serious narrative discipline, a steady musical motif that undergirds the recitative and many beautiful songs that erupt like arias.

Sher, whose production of “The Bridges of Madison County” comes to the Orpheum in Minneapolis June 21-26, was unmoved. He would concede only that “Hamilton” has “a continuous and complete rhythm that is controlled by the music.” It’s historic and a great piece of work, but opera is more than simply a form.

“The cultures are totally different,” he said. “Completely different in history and sensibility, traditions, the training, the hierarchy of where the power is.”

Well, that settles that, right?

“In the broadest sense, if a piece is sung through, then it is an opera of some kind,” said Conrad Osborne, a New York-based opera critic, singer and actor.

However, he emphatically added, “compared to opera as we know it,” musicals such as “Hamilton” or “Caroline, or Change” or “Next to Normal” are very different in aesthetic, style and vocal usages that “do not allow for the exploration with such depth, emotional range and richness as classical music.”

So in a way, Osborne and Sher are saying the same thing — even though Osborne is willing to entertain a slightly broader interpretation of the question. Opera and musical theater are not merely formats defined by their construction. A pamphlet and a phone book are bound sheets of paper with information, for example. Would that make a particularly lengthy pamphlet and a small phone book reasonably the same thing? No. Each has a different function and intention.

“It is a historical definition, not a technical distinction,” said Kevin Smith, president of the Minnesota Orchestra and for 20 years head of Minnesota Opera.

Thus there are subsets — operetta, light opera, rock opera (or “hip-hopera,” as “Hamilton” is sometimes described), the archaic singspiel, French court opera, Beijing opera — meant to define boundaries that might or might not mean anything to theatergoers.

“It’s like bel canto, which is a style of opera but also a philosophy of composition,” Smith said. “Opera has a distinct lineage. I view musical theater as a generic term, and opera exists within that.”

Is one better than the other?

Definitions aside, is there an intrinsic benefit for a musical to be discussed as an opera? Is there a cachet to the term — a sense that opera is a higher art form, more refined and prestigious, somehow richer? (Which is exactly what an opera aficionado would say.)

Opera comes out of the European classical music tradition. It is, indeed, the grandest of the performing arts — a combination of orchestral music, singing, acting, dancing, design and costume.

The style of composition, too, is more complex than pop music. “If someone spends their day listening to classical music and comes to ‘The Shining,’ they will recognize that,” said Eric Simonson, who directed the Minnesota Opera’s recent world premiere. “The music in opera is a piece of art.”

Singers, too, are vastly different. “When you hear an opera singer sing a particular moment, it’s an event,” said composer Kevin Puts, who has written two operas for Minnesota Opera.

The requirements of training and sound are far more rigorous, and the stress on the unamplified voice to fill a 2,000-seat hall could be compared to the taxing race a thoroughbred horse runs. You can’t do this stuff every day. “The Shining” was performed only four times over eight days.

Musical theater performers, on the other hand, belt out their stuff eight times a week, and if their voices (given a lift by microphones) aren’t tuned to the highest standards, they can sell a song with charisma and razzle dazzle. (Rex Harrison, anyone?)

While the high art of opera may indeed be something to which a composer and librettist might aspire, the label of opera does them no favors. We are now 60 years into the rock ’n’ roll era, and while musicals have been dragged into the modern era, opera and the rest of the classical scene have been squeezed out of the popular consciousness. Gone are the days when Beverly Sills or Luciano Pavarotti would visit a TV variety show. When Renee Fleming sang the national anthem at the Super Bowl, how many football fans had a clue who this was?

Osborne is not sanguine about the current state of new opera, with a few notable exceptions such as Benjamin George’s “Written on Skin,” William Bolcom’s “A View From the Bridge” and David Little’s “Dog Days” (which Puts also recommended).

“I haven’t encountered anything that is remotely ready to add to the canon,” Osborne said. “John Adams, Philip Glass — there is something there, but I don’t find it compelling.”

Meanwhile, “Hamilton” grosses nearly $2 million a week and “The Lion King” is chugging past $1 billion, 19 years after it tested in Minneapolis. To be fair, though, the very best musical theater production might struggle to beat out the very worst Adam Sandler movie if box office were the only arbiter. Still, in the world of live performance, theater has swamped opera.

It’s an old and familiar song

This question of blending or blurring is not new. The Gershwins’ “Porgy and Bess” is the original poster child for the debate.

“It can come off seeming like a musical and like an opera,” said Osborne of that 1935 work. “I saw Trevor Nunn’s [2006] production at the Savoy in London — and that was for a commercial theater audience — and it was miserably sung, even though he had done a thoroughly operatic production years earlier” for television.

Kurt Weill considered “Street Scene,” his collaboration with Elmer Rice, to be a synthesis of European opera tradition and the American musical. Frank Loesser’s “The Most Happy Fella,” from 1956, is often mentioned as being nearly an opera. “Les Misérables” clearly aspired to be taken more seriously than a mere musical.

Purists who shriek when you bring up Andrew Lloyd Webber’s name will allow Stephen Sondheim’s “Sweeney Todd” (occasionally performed by opera companies) to enter the conversation. Sondheim popularly said the difference between musical theater and opera is this: One is done in an opera house for an audience expecting an opera; the other is done in a theater for an audience expecting a musical.

Osborne remembered Leonard Bernstein saying that in the late 1950s “we all felt that, any day now, someone was going to come along and do for the musical what Mozart did for the singspiel.” Whether that has come to pass depends on your understanding of terms and your willingness to bend the rules.

Simonson, who works in both forms, erects a pretty strong wall.

Even for “Hamilton”?

“There was not a bit of me that said it’s an opera,” Simonson said. “He is drawing from Beyoncé, ‘Jesus Christ Superstar,’ ‘Sweeney Todd,’ hip-hop, Paul Simon, Puerto Rican music, jazz, gospel. If you talk about the musical and jazz being two of the original American art forms, he’s taken that to new heights. ‘Hamilton’ is something that’s been waiting to happen for 20 years, and no one had the guts to do it.’ ”

And whether we want to consider it opera or musical theater?

“I think the lines are a little bit arbitrary and pointless,” said Puts.

Yeah, but isn’t it fun to argue about?