MacMillan throws light on mysteries of his latest work

First off, what is behind the name? "I have always been intrigued by past practices that have fallen into disuse," says MacMillan. "Practices of basing music on the shape of the rosary, like Biber's Mystery Sonatas. I thought it would be interesting to revisit the form in our own time."

The 17th-century composer Biber based his Mystery Sonatas on the traditional Catholic mysteries: specified points from the life of Christ that are grouped as "sorrowful", "joyous" or "glorious" and used as a basis for reflective prayer. In 2002 Pope John Paul II added a fourth group, the "luminous mysteries", gathering moments in the New Testament that deal with light, both in the physical sense and as a metaphor for enlightenment.

MacMillan says that "whether or not you accept the importance of it, these mysteries were fresh in my mind when I was writing the concerto. They were points of reflection for my music in the way the others would have been for Biber".

So how does the imagery work its way into the score? MacMillan talks of "a kind of fusion" between concerto form and a symphonic poem. Some of his previous concertos have been entirely abstract - the Violin Concerto, for example, and the recent Oboe Concerto - while others, such as the clarinet concerto called Ninian, have had an openly descriptive quality. He says The Mysteries Of Light is more akin to the latter.

"I like the idea of being able to paint pictures in sound in the 21st century. I try to capture the essence of a moment or an image. Sometimes it can be in quite childish ways. That is the joy of composing: it keeps alive a childishness composers should never really lose."

That childishness might play out as the sound of water in the River Jordan or a "quasi-folk dance" running through the concerto. Or it might be less specifically pictorial. "Sometimes purely musical considerations can come to the aid of telling a story. The Catholic idea of Transfiguration is also a musical idea of itself - you can play with music to transform it into something else."

MacMillan describes the style of the new piano concerto as strikingly different from the previous two. Instead of The Berserking's rocky battleground between soloist and orchestra, here the dialogue is softer, using the palette of the orchestra to complement what the piano is doing. "High-speed filigree in the piano can be amplified by instruments to make a kind of meta piano. It's quite a colouristic piece, different from the grind of the first one and the almost slapstick nature of the second."

I ask whether the piano represents a certain voice or character, but he says "it would not be right" for him to lay down a single way of interpreting the piece. "Sometimes the composer is the least able person to explain what his intention is."

Does it matter if the audience does not recognise the Catholic references in the music? "I don't think so. Obviously this is not liturgy; it is not part of a religious ritual, though in many ways it is shaped by both religion and ritual. It has to work in a neutral, secular space where most of the audience will not have any great experience with the rosary. At the premiere in Minnesota some people understood everything, all the little signs and signals. Did they therefore get more out of the music than someone who didn't know? Maybe that question has to be kept open."

MacMillan does acknowledge that it helps if his performers can relate to his sources of inspiration. He says he "had Thibaudet's French repertoire in mind" when he wrote the piece, and describes Olivier Messiaen - also a deeply Catholic composer - as "a presence I can't get rid of whether I want to or not, especially when I write organ music. In fact, I took pains to avoid writing organ music for a while for that very reason." The Mysteries Of Light being overtly colouristic is perhaps a reflection of who MacMillan was writing it for - a reflection of the French soundworld that is deep in Thibaudet's fingers.

But there's another soundworld at play, too. Osmo Vänskä, the Finnish conductor who commissioned the concerto, has long been a champion of MacMillan's music. "Like him, I am a musician of the north," says MacMillan. "When I hear music by other Scots, there are links with Nordic composers in a way that perhaps even English do not have."

It all comes down to Sibelius, he says. "Sibelius is the the gold standard all us northerners reference. There is an attraction to cragginess and an austere beauty, a darkness in the orchestral palette." He laughs as he adds: "There is a northern soul, tempered in my case by something Gaelic and Catholic. Quite the mix!"

The RSNO and Jean-Yves Thibaudet perform James MacMillan's The Mysteries Of Light at the Usher Hall, Edinburgh, on Friday and Glasgow Royal Concert Hall on Saturday.