It wasn't what I'd expected. Sitting with composer Tom Johnson in a cafe at the Place de la Bastille in Paris I found myself in a discussion about Irish pronunciation. Johnson had just given me an impromptu performance of a piece he'd made from counting numbers.

And he'd done it in Irish. Munster Irish, he believed, although I don't think it was the particular flavour of Irish that had heads turning at some of the tables around us. But I'd had to tell him that there was a twang in his pronunciation of "cúig" which sounded much more like a Northern accent than anything you'd expect to hear in the south.

Johnson, who was born in Colorado, and spent 11 years as a Village Voice critic charting the new music scene of the 1970s, has been living in Europe since the early 1980s, when the success of his Nine Bells - played by walking in patterns through a grid of suspended bells - was carried on the rising tide of minimalism. He relocated to Europe because that was where the work was. And now it's the US that feels like a foreign country to him.

"I'm so used to European ways, national health insurance, and the idea that you don't just kill people because they were criminals. Some of these European ideas seem so fundamental, it's very hard to go back to the other mentality."

His direct musical roots are American. "I liked the idea of being an experimental composer and following in the lines of Henry Cowell and John Cage." But his musical mentor was Morton Feldman. "He was still young, then. Let's see, when I went back to New York to study with him I was 27 and he was 41. He still was not in Buffalo University. He was a struggling composer, living in an apartment in New York, taking private students, doing what he could. He hadn't even written his best music yet. All of the pieces he's so adored for today were just beginning, Rothko Chapel was in that year. But he had this very open point of view. He wasn't afraid of silence, he wasn't afraid of repetition, he wasn't afraid of anything, and that was what I needed."

JOHNSON SEEMS to have known early on what he was after. "It was always a kind of reduced means. One idea was enough for a piece. The other students were always writing pieces that had three or four ideas, and that never seemed necessary to me. I just wanted to concentrate on one thing. I didn't know why, and of course the word minimalism didn't even exist yet. But it was a desire to simplify and to clarify which seems to be in my genes somewhere."

At Yale he had been "a kind of black sheep," although "one of my colleagues mentioned 30 years later that the only piece he can remember from his years at Yale was the premiere of a four-minute piece that Tom Johnson had written. That pleased me very much. Three singers sang an augmented chord on 'Oooooh' for four minutes, and just in the middle there was a piano which did a one-phrase, 10-second cadenza. It's the kind of thing it doesn't surprise you to think he remembered. It was such a stark idea." Now, like most of his early work, it's gone forever or "in a box somewhere".

Johnson's clarity is extraordinary. His pieces don't hide anything. He wants everything to be obvious. "I always hated this phrase, the art that conceals art. What does that mean? It means you can't let anybody understand anything. I never understood why you have to have the art that conceals art. OK, subtlety is fine, sometimes, but, basically, I want people to know what the piece is about, and to be clear. When I speak, when I wrote criticism, I was very concerned to be as clear as possible. I like human relations which are clear, too, not to pretend you are someone you aren't, and all that."

Music is inextricably linked with mathematics and calculation, in the very physics of sound, in the creation of rhythm. But most composers bury it out of view. "There are a lot of composers who have used mathematics. Maybe Xenakis is the most remarkable. He really got into formulas in his music. It was mostly statistics and probabilities and things, which doesn't have anything to do with the absolute determinism that I prefer.

"Morty Feldman always said, let the music do what it wants to do. And John Cage had another way of saying it, he said we want nonintentional music. Well, both of those ideas mean the same thing. They mean don't try to make the music do what you want, what you feel, what you think. Just let it follow its own path. Something outside of yourself is going to be more interesting than something inside yourself. In this period when we were trying to get away from expressionism and romanticism, this was very important in all the arts. Minimal sculpture was going the same way, trying to do something objective, geometric, not subjective and arbitrary.

"In my case, I didn't want to work with chance, the way Cage did. I wasn't mystical enough to sit down at the piano all day, and listen to notes and say, what is this note wanting to do, the way Morty Feldman did, and, really, I can't tell it what to do, I have to just listen until it tells me what it's going to do. He was really kind of mystical about that. It's wonderful. He could do it. Nobody else seems to be able to do that. My way of letting the music find another path was to find a mathematical formula, a logical sequence, some kind of logic outside of myself that had a deterministic path that the music could follow."

Johnson doesn't really see a connection back to the counterpoint of Bach. "Bach was pretty subjective. He followed the rules of the canon, but within the rules of the canon you can make up a lot of different harmonies, and you can have what they call the tonal answer instead of the real answer, you can cheat in the very entrance of the second voice, already. What I like is forms that don't cheat. I use a lot of canons, but they don't cheat, the second voice does literally what the first voice did."

Of course, Tom Johnson himself does come into it. "Yeah. I'm the one setting up the system of course. And you never can get away from yourself completely, I don't think, in anything you do. You try to get away from that as much as possible. Even Morty Feldman never completely got away from himself and Cage didn't either, certainly. We never get away from ourselves completely, but we can try."

Johnson's most famous piece, much toured by Opera Theatre Company, is The Four Note Opera, probably the funniest opera ever written, and one which, true to its title, uses only four notes. Pirandello and Beckett were important influences, and Mozart, too.

"There's a lot of Mozart in The Four Note Opera, that's very obvious. I had a teacher once who said, nobody wrote better for the voice than Mozart in his operas. It's true. You see how he treats the tenor and how long the tenor can go up to the high notes and which vowels are best for the high notes. You do it the way Mozart did it and it's gonna work. It's amazing. I studied a lot of Mozart scores when I was trying to write The Four Note Opera, that's a strong influence, at least for vocal writing.

"The idea of the four notes came about because I'm always looking for simplicity and I wanted simple melodies, and all the sketches were pentatonic, and one day I thought, sometimes I'm not even using the fifth note, I could do it with four notes. And I thought, hey, if I could pull that off I could call it The Four Note Opera. And I loved the title so much I continued that way."

AS A CRITIC, Johnson was famous for reporting on the cutting-edge developments of the 1970s - the book of his articles is now available as a free pdf download on his website, editions75.com.

I ask him where he thinks the cutting edge is now. He mentions the laptop generation ("It came out of disco and stuff, but these are serious people, this is serious music, innovative, well-constructed and so forth"), sound installations ("machines are so good at doing our music, you can just turn them on and they can be changing all day, and still you have interesting stuff"), and, as an afterthought, silence ("It's like the last frontier in music, the thing that everybody's been afraid of for years and years. There is a whole school of people who are writing long silences. It's called Wandelweiser.").

For Johnson - who has himself written an organ piece with long silences - numbers remain where it's really at.

"Plato said the reality we see is only shadows, the real reality is underneath that. He didn't talk so much about numbers, but I often think that the structure in plants and in chemistry and what's happening with the weather and everything in nature is numbers. There's numbers at the basis of everything. Whole numbers are universal things. Pi and the transcendental numbers - there is something transcendental about these concepts that goes beyond the reality we can see. It's a reality underneath that."

Tom Johnson is at the Project Arts Centre next Wed, playing his pendulum piece, Galileo. Paul Roe (clarinet) performs Bedtime Stories and Malachy Robinson (double bass) Failing. Johnson gives a pre-concert talk at 6pm