Pierre Boulez, who will be 79 next month, is one of the great elder statesmen of the musical world. As a young, radical composer in the 1940s, he embraced Schoenberg’s 12-tone technique, and became a leading light of serial composition in the avant-garde of the 1950s. He took up a second career as a conductor, which led him to posts with the Cleveland Orchestra, and later the BBC Symphony and the New York Philharmonic.

For many people, his magnum opus of the 1970s lay directly neither in composition nor conducting, but in the creation of IRCAM (Institut de recherche et coordination acoustique/musique), situated next to the Pompidou Centre in Paris. IRCAM, synonymous with the marriage of computers and music, has become as controversial as the avant-garde endeavours of Boulez's youth. But, along with IRCAM, the ever-pragmatic Boulez created the 31-member Ensemble Intercontemporain, a performing group which can put flesh, as it were, on the compositional explorations at IRCAM, and which has established itself as one of the world's finest groups propagating new music.

It was in his offices at IRCAM that I met Boulez, who showed, along with the expected sharpness of word and mind, a graciousness and charm which, though less well celebrated, are essential parts of the man and his make up.

Boulez's musical taste remains mostly focused on the 20th century - although he has conducted Rameau, Handel, Haydn, Beethoven, Berlioz, Wagner, and, more recently, Bruckner. That favoured century saw its leading figures grapple with two major revolutions, each separated by roughly 50 years: the breakdown of tonality - the music of which Boulez the conductor reads with uncanny perception - and the challenge of serialism, which Boulez, the composer, directly influenced.

Is there now, 50 years on again, any similar shared concern for the composers of today? "Yeah, there is a kind of sharing of reconstitution, more or less. Not reconstitution of the values of the past - some are doing that but I don't think that's very productive - but certainly reconstitution of continuity, of narrating something, more than just formal preoccupations."

Although the sounds of the 1950s seem inextricably linked to his public image as a composer, the pure formalism of the 1950s, he says, was something "we were very quickly out of".

He draws a parallel with architecture. "Architecture could not have changed without the change of material. Concrete, glass, steel have changed the possibilities. At the same time the architects had to invent and to have the imagination with this material. The material was provoking their imagination and their imagination was provoking the use of the material. I think in music that's the same. You try to find new material, it provokes the imagination. You know, at the beginning of the 1950s, at the end of the 1940s, we were very negative. We did not want this or that, we wanted something else. There was a lot of no, no, no."

Computers and technology have changed all that. "Now, with the new material, you don't need to be negative. On the contrary, you have all the difficulty of being positive, because you have so many possibilities in front of you."

Boulez once said he found "the obligation to use all 12 notes to be unbearable because the results are so predictable". How has this attitude manifested itself in his music over the years? "Because I create a kind of hierarchies, let's say, or organisation, for certain moments in a work. You know, for me, this kind of Schoenberg theocratic view that you invent one series of 12 tones, and then the whole work is there - that's like God said: 'fiat lux', and light was there - for me this kind of theocratic approach is really something which is not for human beings at least, because you need to absorb every accident of your life.

"If you are working and suddenly something unforeseen happens, you have to absorb it, because that makes the richness of the work. Take even very organised works like, for instance, Die Kunst der Fuge. I suppose when Bach worked his first fugue, he could not really imagine all the transformations he would write later. Because, maybe one day he was in 6/8, the next day he was with dotted rhythm, and so on. There is something which depends on the mood of the day."

He muses on organisation and its opposite: "Even the most improvised things are with something organised. But you must accept a kind of destroying of the organisation in order to get somewhere else".

"My whole effort during my own life was to try to organise, and then disorganise what I had just organised. This kind of dialectic between something solid and something on the contrary you are not sure of, it's the interest of the composition. And also, which is important for me, that you are never sure of what you hear. Because if you are too sure, then you are not interested any more.

"I think that the work of art is based on recognition and lack of recognition. You recognise, you are sure, but are you sure what you have recognised? That's a kind of dilemma. For me there is a strong structure, but you are never sure that the structure is really what you perceive. I think that Proust has written once, toward the end of the À la recherche, he said, a novel is finally made by the man who reads it and not by the man who writes it. I think that's true for the artwork generally, for music also."

It's one of the great musical paradoxes of the last half century that those composers who are most closely associated with serialism have long moved on from the severity and strictness of that technique. Is that an indication of serialism's malleability or a proof of its limitations?

"Well, I think I have seen the limitations, and therefore I want more mobility, more flexibility, more variety. And also to take into consideration acoustical phenomena. You must also take care of the relationship between the lines, that they are governed by a vertical relationship,because, horizontal relationships, you grasp them very rarely, even in the classical music. In Bach, when he makes a canon by augmentation for a long period, you don't remember exactly what you have heard, that's impossible to grasp. But the vertical aspect of music you perceive immediately. And therefore you have to have a strong control of the vertical and then the horizontal can be much more free."

This balancing of elements is a central concern. "For instance, if I have a kind of regular pulse, it will focus the attention, and I don't need to care very much for the organisation of the pitch, because the memory will grasp immediately the permanent aspect of the rhythm. If the rhythm is absolutely inaccessible, because of too complex a relationship, then I use an organisation of intervals which is absolutely fixed, not changing at all. Then my grasp will be of the intervals, and I am reassured with that, so I know that I am disturbed by the rhythm, but I have something very, very solid to grasp. You have always something to organise, a solid core, and then something free around."

Boulez is famous for going back to particular pieces again and again over decades, refining and improving. This is partly because his conducting experience has impinged on his composing.

As in the case of Mahler, he likes to make his scores more practical, more accountable in terms of orchestral players' responses.

Some of the most remarkable transformations have been of a number of the short Notations, originally written for piano in 1945, which he has orchestrated since the late 1970s. The change is like that of a bare branch in winter into a luscious effusion of spring leaf and blossom. Angularity was once a musical badge of honour, but sensuality is now OK.

"That was the time also. It was the time of the war. The time was hard, '45 was stilla hard time, we were hardened. We did not want the kind of pre-war atmosphere at all. There was a rejection of the pre-war. Therefore, somebody like Webern was much more loved than Schoenberg or Berg. Because he was showing us really the hard way. Why did we choose Rite of Spring? Because it was a very primitive way of experiencing music, hard also, the very best side of Stravinsky, I find. The two extremes were some pieces by Webern and some Stravinsky.

"I tried always to have a kind of view, of a synthesis between the German culture and the French/Russian culture. It was for me very important at this time, and it is still, I cannot deny it, the main component of my musical thinking."

He shirks a question about younger composers who stand out for him. Debussy (Prélude à l'après-midi d'un faune), Stravinsky (Rite of Spring) and Boulez himself (Le marteau sans maître) all made their international mark around the age of 30. "I can hardly tell it, it is very difficult for me, because I am too close to my own past, and I have a tendency to judge the other one after my own past. Therefore, I am very careful with myself and I say, maybe I did not recognise exactly what happens, because you can make always mistakes."

He does, however, point out that later generations have had an easier time, with institutions such as the London Sinfonietta, the Ensemble Intercontemporain, Ensemble Modern in Germany, and of course IRCAM, very eager to discover talents. Boulez lived in Germany for 20 years, because the radio stations there were more inviting of new music than any public institution in France.

His supportive endeavours take a new turn this year, with the establishment of a Lucerne Festival Academy, of which he is artistic director. This will deal with the repertoire of the 20th and 21st centuries through a young people's orchestra tutored by members of Ensemble Intercontemporain, along with commissions and workshops for young composers, and master classes in conducting and piano, these latter by Maurizio Pollini.

Pierre Boulez ... on minimalism

"If you are happy with this kind of simple view of things, let's be happy, I am not against happiness . . . " but "Going back to tonality is giving a wrong answer to a right problem."

... on orchestras

"Orchestras are too stiff as an organisation. Nobody's satisfied, but they're all afraid to change the system."

... on opera

"Opera houses are basically busy with repertoire, but the only things which are really new are the views of the directors. Good directors saved the opera."

... on politicians and music

“How many politicians go to a concert? They don’t show the example. The people at the top of the state are really not interested in art, generally speaking. They respect painting or sculpture, because it costs. The art market is an incentive for people to be interested in contemporary art. In music, the more you perform it, the more you spend money.”