Philip Glass is hard to pin down. Not that he has airs; on the contrary, he’s one of the most straightforward composers you’re likely to meet. “I’m a very bad celebrity,” he says. “I’m not interested in it at all.” And he doesn’t act like one. In conversation, he asks your opinion about things, and when he chuckles at one of his own punchlines, he looks at you hopefully, waiting for a laugh. Yet he’s still hard to pin down, because there have been many different Philip Glasses over the composer’s 81 years. If you think you know who Philip Glass is, you probably don’t.

You’ll know the outlines, of course. Glass is one of the most popular and prolific composers alive. His output is veritably Bach-like in its range and quantity. To some people, Glass is “Koyaanisqatsi,” the pulsing, mind-bending film in which Glass’s music and Godfrey Reggio’s images play equal parts. To some, he’s “Einstein on the Beach,” the breakout avant-garde opera he created with director Robert Wilson in 1976. Some would name the symphonies he wrote based on the albums David Bowie recorded in Berlin in the late 1970s, “Heroes” and “Low.” (He’s currently working on a symphony — his 12th — based on Bowie’s third Berlin album, “Lodger.”)

In the new-music world, some hail his masterpiece as “Music in 12 Parts,” the four-plus-hour work he wrote in the early 1970s as a summation of his distinctive musical language. Operagoers might name the 2015 revision of “Appomattox,” covering a century of American history, from the Civil War to the Rev. Martin Luther King Jr. And “a lot of people like ‘The Hours,’” Glass says, referring to the score of the 2002 film with Meryl Streep, Nicole Kidman, and Julianne Moore, one of three of his film scores nominated for an Oscar. “Why not? Beautiful women, and a nice story, and all that.”

All of these works are Glass, and none of them are. Glass is mercurial: constantly changing, shifting, reinventing himself. A practicing Buddhist, he seems to embrace the idea that nothing is permanent. “You listen to what he’s been doing, and it’s radically different,” says the performance artist Laurie Anderson, who has been a friend and collaborator of Glass’s since they were part of the alternative performance scene in downtown Manhattan, 40 years ago. “Many people get a style, do the style, repeat the style, are known for the style. Phil never did that. That’s another thing I find so wild about his work. It changes in ways I don’t think other people’s does.”

What do you mean, people might say? The conventional wisdom on Glass, propagated by fans and detractors alike, is that he always sounds the same. His music, people say, is about repeating the same things over and over: He’s a guy with a gimmick. It’s New Age-y because it incorporates non-Western approaches, such as Indian classical music and the music of indigenous peoples of Mexico. It isn’t really classical music. People take these things as a reason to like his work, or to reject it. But none of it is really accurate.

Philip Glass, considered one of the most influential composers of he 20th century, talks about his first job breaking records in his father's store. (Zoeann Murphy/The Washington Post)

The classical music world has long viewed Glass as an outsider. Glass isn’t a composer who is offered teaching gigs, although he’s been a mentor to scores of young musicians. And this year’s Kennedy Center Honors, which he will receive Sunday, is one of only a handful of major awards he’s received, all late in his career. “My first big award was the Praemium Imperiale,” the Japanese international arts award founded in 1988, he says. “I was 75. I had long ago given up any thought of getting any.” In 2015, this was followed by Canada’s Glenn Gould Prize and the National Medal of the Arts, bestowed by President Barack Obama. “My feeling is, you have to be about 75,” he says of the accolades. “At that point, when it happens, it’s, ‘Oh! That’s nice!’ ” He mimes the kind of surprised pleasure one might evince at a thoughtful gift from an office mate. “I mean,” he adds, “if it happens in your 40s, it’s maybe nicer, but I think if I got an award earlier, I would have been very suspicious — of myself. I didn’t have to worry about that.”

Indeed not. When Glass was in his 40s, he had only recently stopped supporting himself and his music by driving a New York taxicab.

The octogenarian Glass is interviewed on a cold sunny day in October, in the brownstone in Manhattan’s East Village that he’s lived in since 1984. He’s as easy to recognize as his music: a once-tall man, now slightly stooped, with a duck-toed gait and a clipped version of what the painter Chuck Close has described as his “dendritic, Medusa-like curls.” (Close has put those curls through a range of permutations since he first photographed Glass in the 1960s.) His house, like him, is comfortable but not ornate: sun from a small back patio streaming in over low built-in shelving holding pieces of Asian art and other objects. Glass is affable and almost avuncular, sharing favorite stories in the manner, common to family members and older celebrities, of one who has done it many times before. Having carved out time in his schedule to talk, he is doing all he can to make the time count.

Usually, he’d be working — something he does 10 to 12 hours a day. When he was a student at the Juilliard School, he says, “I put a clock on the piano. And I said, I’m going to sit here from 8 o’clock to 11 o’clock. I just sat there. And after a while, I just wrote, because I had nothing to do.” Ever since, he’s put a premium on showing up every day. “It’s not about writing fast,” he says, “it’s about being able to spend hours. If you can solve the stamina problem, it helps a lot.”

The result of this steady output is evident in more than 30,000 manuscript pages of music: 27 operas, 11 symphonies, 8 string quartets, 20 piano études and 50-odd films, among many other works. In the 2007 film “Glass: A Portrait of Philip in 12 Parts,” by the director Scott Hicks, the composer Nico Muhly, who was Glass’s assistant for several years, says, “He’s written himself into a situation where he doesn’t really get to have a day off.” There are always people waiting for things.

The output extends to a memoir, “Words Without Music,” which came out in 2015; Glass wrote the first draft in three months. Essentially a portrait of the artist as a young man, it expands on Glass’s early life: his childhood in Baltimore, playing the flute and learning music in his father’s record shop; his matriculation at the University of Chicago at 15; his degree from Juilliard; and the Fulbright Scholarship that brought him to Paris for two years of rigorous study with the doyenne of composition teachers, Nadia Boulanger. He learned about jazz in Chicago nightclubs and Eastern philosophy from New York friends, and traditional Indian music from Ravi Shankar, whom he assisted on a film project, learning a whole new way of thinking about rhythmic structures. He married the avant-garde theater director JoAnne Akalaitis, the first of his four wives, and had the first two of his four children, Zack and Juliet.

Less expected, though, is the evidence of how deeply Glass is steeped in the traditions of Western music. He devoured recordings: from Bartok and Schoenberg to the great German conductor Wilhelm Furtwängler, whose slow tempos in the symphonies of Bruckner and Beethoven, he writes, “prepared me for Wilson,” his “Einstein” collaborator, another artist who stretches time to the breaking point. Boulanger drilled him in the nuts and bolts of musical technique: species counterpoint, transposition at sight to and from any of the seven clefs. He himself says the sound of Central European art music has been “a solid part of me from an early age,” although it didn’t start to be “audible in my music until almost five decades later.”

This isn’t a side of Glass of which everyone is aware. More common is the view of one gatekeeper he tells of in his book who, perplexed by his scores and unaware of his pedigree, gently suggested he might want to take some composition lessons. In fact, Glass is doing what many composers have done before him: breaking new ground and integrating new perspectives and influences in a framework born of deep knowledge of musical tradition.

Glass also addresses the fallacy that all he does is play the same chords over and over. Certainly the language he developed, unhelpfully labeled “minimalism,” involved subtle variations of similar patterns. But, “It never repeated all the time,” Glass writes in his memoir, “for if it had, it would have been unlistenable.” The chords are constantly shifting and changing; that’s the point.

“Music in Twelve Parts” was a kind of manifesto of Glass’s musical vision; “Einstein on the Beach” was an acme, hitting a world that had never seen anything like its shimmering, chanting choruses and stylized movement.

“But ‘Einstein’ wasn’t the beginning of something, it was the end of something,” Glass says now. “If ‘Einstein’ had been the beginning of something, then I would have written ‘Son of Einstein.’ And it would have been more appreciated, and not as good. People would have liked it, because I sounded like the guy who wrote ‘Einstein.’ But I didn’t want to be the guy who wrote ‘Einstein.’ ” Instead, he wrote “Satyagraha,” his 1980 opera about Gandhi, which met with mixed reactions — some excitement, some dismay — and which has taken almost 30 years to be fully recognized as a major opera.

In the years after “Einstein,” Glass makes it hard for anyone to pin him down. His memoir begins to hop around, losing its linearity and making calculated omissions. He writes of the devastating death of his third wife, the artist Candy Jernigan, of cancer at age 39; he doesn’t mention Holly Critchlow, his fourth and now ex-wife, the mother of his two younger sons, now teenagers, Cameron and Marlowe. Nor does he mention Wendy Sutter, the cellist and ex-girlfriend for whom he wrote the luminous “Songs and Poems for Solo Cello” in 2007. In the case of the personal life, the reason is partly tact. In the case of the music, it’s because, he says, “once the work begins, there’s nothing more to remember.” In the actual act of composing, you lose yourself. “I don’t remember writing ‘Satyagraha.’ But I wrote it.”

In a filmed 1976 interview, Glass is intense and unsmiling, his eyes ringed with black shadows. You can see the drive that underlies the career he subsequently built: the focus it took to create a new music and an infrastructure in which to perform it (his group, the Philip Glass Ensemble, is still going strong). It is a long arc from there to the smiling older man, eyes twinkling, who presents such a warmhearted front to the world. Yet many of his friendships — with Anderson, with Close, with Paul Simon — have traveled that arc with him. “Philip is so full of joy and enthusiasm and ideas and energy,” Anderson says.

The music, too, continues to change and evolve. Like most great composers, Glass has a distinctive thumbprint: You hear a score and know it’s Glass, the same way you hear a Haydn score and know it’s Haydn. But we don’t dismiss Haydn’s music because it all sounds the same. And Glass’s work, while unmistakably Glass, has entered a remarkable late period that verges on the downright Romantic in places: His third piano concerto, recently recorded by Simone Dinnerstein, or his sumptuous 11th symphony.

Though he’s working to create clean corrected editions of all his scores, a project he says will take another 10 years, Glass isn’t invested in his legacy. He has no illusions that you can control what will happen in the future: “The future is not the present projected into the future. The future is what we don’t know.” And as the field of classical music desperately seeks ways to reach a wider, younger audience, and incorporate a wider range of musical traditions, Glass continues to write his music — which is very popular with a wider audience and incorporates diverse traditions — for anyone who wants to notice.

As for his award, “the good thing about this is that we’re going to have art music now at the Kennedy Center” Honors, he says. “The Kennedy Center — I think it was supposed to be the fount of popular culture. But it doesn’t have to be. I mean, why not have other culture, too?”

But surely he knows that for some in the classical world, his music is pop culture?

“Well, that’s . . . ” he says, momentarily at a loss for words. But he’s amused, not angry. He finally finds what he wants to say. “That’s pathetic.”