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 1982 film in which Glass' 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 as his masterpiece "Music in 12 Parts," the four-plus-hour work he wrote in the early 1970s. Operagoers might name the 2016 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 is. Glass is mercurial: constantly changing, shifting, reinventing himself. A practicing Buddhist, he embraces the idea that nothing is permanent.

"You listen to what he's been doing, and it's radically different," says performance artist Laurie Anderson, a friend and collaborator of Glass' 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. [His work] changes in ways I don't think other people's does."

The conventional wisdom on Glass, propagated by fans and detractors alike, is that he always sounds the same, that he's a guy with a gimmick. His music is New Age-y because it incorporates non-Western approaches; 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.

The classical music world has long viewed Glass as an outsider. He 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 received 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, he says. "I was 75. I had long ago given up any thought of getting any." This was followed in 2015 by Canada's Glenn Gould Prize and the National Medal of the Arts.

"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!' I mean, 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."

In a recent interview, Glass is affable and almost avuncular. 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, as he does 10 to 12 hours a day. As 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: more than 30,000 manuscript pages of music: 27 operas, 11 symphonies, eight 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 Scott Hicks, composer Nico Muhly, who was Glass' 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 2016 memoir, "Words Without Music," which expands on Glass' 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 Nadia Boulanger, the doyenne of composition teachers.

He learned about jazz in Chicago nightclubs, Eastern philosophy from New York friends, and traditional Indian music from Ravi Shankar. 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 German conductor Wilhelm Furtwängler leading Bruckner and Beethoven. Glass adds that 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 gently suggested he might want to take 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 he plays 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."