A U.S. premiere for MacMillan

Music can be so fundamentally inexplicable that composer James MacMillan doesn't even try to account for moments in his new

Violin Concerto

that might leave the most seasoned Philadelphia Orchestra patrons saying "What?" - though more out of curiosity than confusion.

The MacMillan concerto, to be given its U.S. premiere at the Kimmel Center on Thursday, has such event status that it's also part of the orchestra's appearance at Carnegie Hall next week. At age 51, MacMillan is among a handful of composers not only able to write large works and have them performed, but also to be encouraged to write more. Still, one had to ask, upon hearing the Violin Concerto's 2010 world premiere on radio last spring, why does the third movement have the orchestra counting aloud - emphatically - in German?

He laughed. "I don't know where that's come from," he said Tuesday, having just arrived from his home in Glasgow. "It's a mystery thing that came in a dream or two - a couple of daydreams as well as night dreams. I'm not trying to be obtuse. I would honestly tell something were there something to tell."

The mystique is shared with vocalizing in other works. Five years ago at the London Proms, his Scotch Bestiary asked the orchestra members to sing. "At the places I've done it, including Toronto," he said, "the players are fine with it. There's an 'Amen' chorus and a couple chords where they get to pick notes out of the air."

A memorial for his deceased mother, the Violin Concerto is typical MacMillan for embracing all manner of musical events. Inclusion of the "Dies Irae" chant for the dead makes sense, as do folk songs his mother sang to him as a child - but try to separate them from the quasi-folk music that he invented for the concerto. In the final movement, not only do the men in the orchestra count in German, but the women have more enigmatic texts - something like symbolist poetry - spoken amid the music's textures. (At least he gives orchestras the option of using microphones. At May's spring's world premiere in London, he says, "the women weren't directed very well. They sound kind of lost.")

Tellingly, MacMillan doesn't question, despite some mixed reviews at the premiere, his inclusion of that effect. He's a hearty, almost blue-collar presence, with confidence born of the slow but sure gestation of his creative personality. Though composers such as George Benjamin and Thomas Adès were discovered young, hailed as geniuses, and then suffered creative blocks, MacMillan wasn't noticed until age 30 and then for a 1990 instrumental tone poem whose title had limited marquee value - The Confession of Isobel Gowdie - based on a 17th-century witchcraft trial.

The piece set the tone for a harmonic language that never flinches from dissonance, juxtaposes all sorts of musical events in opposing key signatures, and is inspired by ancient legends. And despite his outsider status in the United Kingdom's music community, Isobel Gowdie has been recorded five times - just one indication of his popularity. He lives in Glasgow with his wife, three children and one grandchild, not in a great music capital like London, but he enjoys plenty of performances and being a local celebrity. He's also prolific: 2010 was a four-concerto year for him.

As a devout (he would say radical) Catholic, he's naturally attracted to sacred texts, and thus has often composed for the lower-profile choral world. But for all his religiosity, there's none of the soothing simplicity of Arvo Part or John Tavener. For MacMillan, his religious life is about struggle, expressed in ways that aren't easy on the ears and sometimes with hard-to-parse progressions of musical thought. Also, he doesn't project great certainty, a specific doctrine, or any attempt to evangelize. "I think I give witness to where I've come from - a culture that's been largely shaped by a Judeo-Christian experience that belongs to all of us," he says.

Rather than identifying with Part and Tavener, MacMillan sees himself as part of creative tradition of composers embracing their religion of origin later in their lives, as Arnold Schoenberg did with Judaism, and Francis Poulenc with Catholicism.

He's also aware that artists haven't fared well with some of the more narrow religious communities. And "narrow" is one word that can't be applied to MacMillan. The "Amen" chorus in his Scotch Bestiary, for example, coexists with a mad succession of popular-music styles, trumpets playing with sardonic wah-wah mutes, and circus marches.

Yet his devotion is unquestionable. One of his latest successes is a full-blown choral/orchestral St. John Passion, championed by the dean of British conductors, Colin Davis, both on CD and in concert performances both in London and Boston. MacMillan plans to go further with the St. John Gospel in ways that deal with Christ's Resurrection. Also looming in his mind is the Gospel of St. Luke, which he'll study with a panel of religious scholars at Duke University. "I'm trying to extricate myself from John and go over to Luke," he says without a hint of irony. "They're very different."

However, the spiritual starting point of many composers - expressions of natural phenomenon - is a frontier that MacMillan is only starting to explore. "It may be a hangover from my early modernist period," he says, "when I shunned influences by something so old-fashioned and so 19th-century. But as I've gotten older, I care less about that and embrace the 19th century even more."

Given the turbulence of his current music, the possibility of musical hurricanes could be cataclysmic.