The composer Howard Skempton tells the following story about the late American composer Morton Feldman. In 1979 he was editing the score of Feldman's Spring Of Chosroes for Feldman's London publisher, Universal Edition. Skempton had been an admirer of Feldman's music since the mid-60s, but here the score seemed uninspired, empty. “I couldn't see those characteristic meaty chords,” he said. So he rang Universal's director, Bill Colloran, and expressed his misgiving.

“Morty's in London now. Why don't you ring him and talk to him about it yourself?” Colloran said. It was Feldman's sabbatical year from his post at the University of New York at Buffalo.

From the mid-60s Skempton had been one of Feldman's original group of musical advocates in the UK, a group which included the pianist John Tilbury and the composer Cornelius Cardew (both, at different times, members of AMM). Now he was put into the awkward position of having to approach the great man and tell him that he didn't think he was writing up to his usual standard. Skempton rang the number and told Feldman what was on his mind. “I expected to be blasted,” he said. There was a long pause. (This seems characteristic; in his music Feldman is a master of long pauses.) “You may have a point,” Feldman said quietly, then gave an astonishing explanation: “I'm into anonymity at the moment.” Rather than blast him, he invited Skempton to visit him at the house he was staying in, off Finchley Road in North London.

They spent the best part of a day discussing the piece Feldman was then composing, Violin And Orchestra. “It was his first really extended piece,” Skempton recalls, “and it was already over an hour long. At one point he showed me some of the samplers [nothing to do with music technology, but pieces of embroidery stretched on small wooden frames] and rugs he was collecting. He said one of the things he loved about them – and this had to do with the particular virtuosity of the children and women who made them: he was fascinated by what was largely a female craft – was the little irregularities and asymmetries. He was obviously charmed by that.”

At one point, Feldman noticed that one of the samplers’ wooden frames was missing a screw, so he decided to replace it. “It took half a day to do this,” Skempton said. They walked all the way down Finchley Road, so the composer could find a shop that sold the correct type of screw. Then they came back to the house and Feldman carefully put the screw into the sampler frame. “That was the tempo of the day,” says Skempton. “I thought, ‘No wonder his pieces take four hours to perform.’”

This episode contains a lot of clues about Morton Feldman's music and about Feldman himself. For instance, he wasn't just showing off his collection of rugs, he was explaining to Skempton a principle of his music: the idea that an acute, patient attention to minute variations in a pattern (like the small irregularities and asymmetries in an Anatolian village carpet) was as good as a vast amount of grand compositional drama.

Another clue was the long time scale he needed to get this very personal message across. Since his death in 1987 he has become best known for the very long pieces written in his later years, like String Quartet II which lasted five hours when performed in London in 1984 by The Kronos Quartet, or the four and a half hour For Philip Guston (1984) that covers four CDs. These pieces require considerable stamina on the part of audiences, particularly in venues with uncomfortable seats.

Feldman explained the idea behind writing such long compositions (a kind of professional suicide for any composer who depends for his reputation on the public performances of his music) in a lecture in Toronto in 1983: he wanted to “get rid of the audience”; to write in a way that depended on a more intimate relation between performer and listener. The two needed time to get to know each other, he felt. In the same lecture, he related the origin of the idea with a boisterous, New Yorkish anecdote that would not be out of place in a Woody Allen movie.

“I got the cue from a big time publisher I would see once in a blue moon. He turned to me one night after a concert at the Russian Tea Room [a famous showbiz watering hole in Manhattan] and said, ‘Feldman, do you mind if I tell you something?’ I said, ‘Go ahead.’ He said, ‘You're not going to make it – unless…’ I said, ‘Unless what?’ He said, ‘You're a fabulous composer, really wonderful. Unless...’ ‘Unless what?!’"

Here Feldman paused in his narration. “‘You need a little drama.’” He paused again. “‘Not much. But you need a little drama. Just a little bit.’

“So after living for 25 years with absolutely no drama in my music – I had plenty in my domestic life – I started to think about that.”

Feldman's solution was to write these incredibly long compositions where ‘hardly anything happens’. This was his idea of drama.

Morton Feldman is the embodiment of the enigma of the obscure: the distant, glittering object that is all the more alluring for being remote and indistinct. Of all the members of the New York School, the term now used to corral together the modernist poets, artists and composers who worked in post-war New York, Feldman represents a king-sized lacuna. Although he had many famous friends from among these circles, he seems to exist only as a name in the index of their biographies, while his own biography remains unwritten. Yet he often said that he wanted to be “the first great composer who is Jewish”, and was disgruntled by the lack of fame he enjoyed in his lifetime, that he should be considered by some to be no more than a “footnote in Stockhausen's biography”.

The nearest thing to a decent account of Feldman's life is to be found in the book of essays and lectures published in honour of his 60th birthday in 1985 (Essays, edited by Walter Zimmermann). He was born in Brooklyn, New York, in 1926, and at the age of 12 began to study piano with a sophisticated Russian emigre, Madame Maurina-Press, who inspired in him what he called “a vibrant musicality rather than musicianship.” As a teenager he studied with the composers Wallingford Riegger and Stefan Wolpe.

The turning point came when he met John Cage at a concert in New York in 1949 (in the lobby of Carnegie Hall, where both had fled to escape a piece by Rachmaninov). This was Feldman's introduction to the New York School's loose milieu of painters, musicians and other artistic personalities. In his mid-twenties, Feldman moved into an apartment in Cage's building on Grand Street, in downtown Manhattan, and it was from here that he found friends and acquaintances among the luminaries of this rich period in American cultural history: the painters Philip Guston (a close friend), Franz Kline, Jackson Pollock, Robert Rauschenberg, Mark Rothko, the poet Frank O'Hara. Cage also introduced Feldman to ideas of indeterminacy and graphic notation, a musical aesthetic which liberated his creativity. In music, as the recent Hat Art releases entitled The New York School suggest, he was closely associated with Cage disciples like Earle Brown and Christian Wolff. They were all collegial and competitive. “It was big stakes we were after in those times,” Feldman recalled.

Feldman became Edgard Varèse Professor of Music at New York University at Buffalo in 1973 and held this post for 14 years until his death. His influence as a composer owes much to his example as a teacher.

His artistic obscurity – relative to the stars of the New York School – had nothing to do with his personality, which was gregarious, funny and stimulating, and everything to do with his music, which was concerned with values of reticence, quietness, faintness, stillness; hardly the sort of thing to make headlines in a world where the avant garde was supposed to épater and outrage. As Michael Finnissy, one of his most stalwart British advocates, put it, “I don't think Morton Feldman's music is ever going to be that popular.” There are lots of reasons for this, and they are all good reasons for listening to the music. For one, Feldman was not a musical ideologue or a conceptualist. In this he was very different from Cage, whose music was almost entirely theoretical and based on a cult of the personality of Cage. Much of Cage is unlistenable now as a result. You can listen to Feldman.

Instead, Feldman was interested in musical instruments, individually and in combination with other instruments, and in the sounds they made. It was an aesthetic of radical simplicity, yet with a profoundly esoteric effect. He felt that every note an instrument played was distinct and unique and different from any sound that went before or came after, like stitches in an Anatolian rug.

Listening to pieces like For Christian Wolff for piano, flute and glockenspiel, with its sparse, airy arrangement of widely spaced, pure sounding, unstressed, unaccented notes, you feel that Feldman was so seduced, so intoxicated by the individual sounds the instruments made that he had just thrown his compositional authority to the wind and let the notes proceed autonomously. The personality of the composer seems to have abdicated, or, like the ego of the Sufi mystic, to have been ‘annihilated’ by and in the One of its contemplation. The lights are on but there's nobody home. Another marketing problem. (To underline all this, Feldman chose bone-dry, ultra-modest titles, which either name musical instruments, as in Piano, Three Voices, Four Instruments, or are offered to friends, usually famous: For Samuel Beckett, For John Cage, etc.)

Feldman was a master of extravagantly faint gestures. In 1976 he collaborated with Samuel Beckett on an ‘opera’ called Neither, the libretto of which Beckett sent the composer on a postcard. In a letter to Beckett, acknowledging how “thrilled” he was to receive the text and reporting on the progress of the composition, Feldman wrote, “It would be as if she [the singer] is singing a tune but it's not there.” On scores, he favoured the indication ppppp, a dynamic level so quiet you can barely hear it. In an essay entitled “The Anxiety Of Art” he wrote, “I was once told about a woman living in Paris – a descendant of Scriabin – who spent her entire life writing music not meant to be heard. What it is, and how she does it, is not very clear; but I have always envied this woman. I envy her insanity, her impracticality.”

Feldman didn't need to envy this impracticality – he already possessed it. Herein lies his uncompromising greatness. Another important marketing problem in Feldman's music is its static quality, a characteristic often seen in tandem with marathon length. He was explicit in borrowing aesthetic principles from painting, particularly the Abstract Expressionists. He wanted his compositions to present a single, flat plane of sound, like an abstract painting, where one concentrates on the variations of texture within a field that could be extended infinitely in any direction. It was an approach that did away with beginning, middle and end, and often with any sort of development.

Any musician who worked with Feldman, though, will tell you this tendency toward the disappearance of the self of the composer was an illusion in practice. A young British conductor told me about rehearsing a piece, a long extravaganza of far apart ‘beeps’ and ‘toots’, under Feldman's direction at a music festival in Banff, Canada. After a gruelling spell of trying to conjure the right balance of austere notes and carefully-sculpted silences from a virtually blank score, a gruff Brooklyn accent boomed from the darkness of the empty auditorium; it was a remonstrance to the conductor: “Your shoulders are too expressive!” None of the players could tell if Feldman was being serious or parodying himself. This is taking understatement to an extreme, of course, but therein lies the beauty of Feldman, or rather Feldman's music.

A central element of the Feldman legend is the stark contrast between the man and his work. Michael Finnissy remembers him as “this huge, fat, ugly guy with this raucous voice, out of whom came this music of exquisite refinement. It was the sort of music you'd expect to come out of some sort of embroidery queen.” Chainsmoking, diabetic and nearly sightless too, one might add. And also enormously charming and attractive to women.

Feldman has had his supporters in Britain ever since Cardew and Tilbury took him up in the late 60s. In 1986, a year before he died, he taught for two weeks at the Dartington Summer School, where the composer Andrew Toovey got the chance to study with him, after years of following the music of the New York School from afar.

“He was there in this little village – Totnes, Devon – devoted to whatever it was we were doing. He was very ill and very overweight. He would stand at the piano for hours and rattle away, telling the same stories again and again.” The encounter spawned Toovey's ensemble Ixion, named after a work by Feldman, which was founded with the original aim of playing Feldman's music, because no one else seemed to be doing so.

A common element in Feldman's British reputation since the late 60s has been the role he played as a musical guru. “We don't have any of those figures over here,” Toovey says. “If we do, they don't live here. [Brian] Ferneyhough lives in California, [Harrison] Birtwistle lives in France. [Michael] Tippett's not really around either, is he?”

Toovey looks with dismay at the posthumous fame Feldman is now acquiring, particularly in Europe, where, he says, “A tradition is growing up of how you perform Feldman – very slowly and with a lot of atmosphere. It's happened very quickly.” Recently, there has been an avalanche of Feldman releases on CD: The Kronos Quartet has released Piano And String Quartet, The Group For Contemporary Music issued a recording of the 1979 String Quartet, and the Swiss Hat Art label has put out half a dozen recordings by Feldman's University of Buffalo ensemble of Eberhard Blum, Nils Vigeland and Jan Williams. Art abhors a vacuum.