HERE comes Joseph Kosuth, the inventor, or maybe go on, risk it - the father of the hardcore Conceptualism. Joseph K, the commando whose incendiary ideas might have levelled 20th century art. And he dresses like a commando too, all in black from his black shoes to his black waistcoat and baggy black cords, to his round, black spectacles. Like some of his work, there is also a touch of grey, here courtesy of a short clipped beard. The major break from this conceptual rigour is a pair of small, blue eyes, peeping out from behind heavy lenses.

For a long time in the 1950s and 1960s, it seems, artists were thinking in terms of the end of art. As Reinhardt said of his all black paintings: "I am painting the last painting anyone can paint", which, seems to have been a popular activity at the time. Artwork had to have that kind of ringing finality about it. The New York centred cart world, in the full heat of minimalism, was heading towards a silent meltdown. And then, and then, like Superman from the mid West, came Joseph Kosuth, a boy with miraculous superpowers, ready to lead the way out.

Born in Toledo, Ohio, in 1945, Kosuth had studied academic drawing and painting from the age of 10 he continued painting until he was 20. He recalls that his mother would go to collect his early art prizes, as the competitions he won were frequently for, adults. "I didn't tell anyone my age until I was 28, otherwise nobody would have taken me seriously. Otherwise there is no way I would have had something purchased by the MoMA when I was 20," he says.

His ideas about art changed in the mid 1960s. Although he had been a painter, he began to develop serious misgivings about the medium. "You began to realise these were no longer windows to another world, to realise the paint on the painting and the paint on the wall were the same paint. It meant then that the painting was a painted canvas object loaded with so much prior meaning that it would eclipse anything that I as an individual artist would have to say. Art needed to be invented to question the nature of art." And they were off.

Standing on the shoulders of artists like Sol LeWitt, Kosuth and some others began to experiment with reducing the importance of an art object although, as Kosuth clarifies: "It was a shift away from the importance of materials and objects, not the elimination of them". Lawrence Weiner, for example, suggested his "statements" - short descriptions of ideas for possible works - were sufficient in themselves. Joseph Kosuth, however, took another approach.

His best known work, the one that gets into the text books when somebody wants to explain Conceptualism, is One And Three Chairs. Something of an heir to Magritte's famous Ce n'Est Pas Un Pipe, the work has three parts - a wooden chair, a life size photo of that chair and an enlarged dictionary definition of the word "chair".

The artist said later that "art is the whole and not part. And the whole only exists conceptually". Each of the work's three parts could only offer a part of the concept of a chair, and the three could only ever come together in the mind of a viewer. So although these three parts were the carriers portions of its information, they were not parts of the artwork in a traditional sense: they were simply devices used by the artist for making meaning. Phew!

Kosuth first made One And Three Chairs in 1965. (It is remade every time it is exhibited.) By the time 1968 rolled around, he was a significant enough figure to command the cover of Newsweek - although he never did. In that busy news year, a prepared cover feature on "the new art" was bumped by events in Czechoslovakia.

"The article scared the living crap out of me because of the way I was being treated in it. I was dealt with as a kind of eccentric artist and it made me realise the whole body of ideas I thought I was fighting for was being subsumed under the usual art historical model of the expressionist artist, all the usual cliched crap that they like to talk about instead of the art, right?" So Kosuth struck on a new plan: ... I went out and put together a movement".

By 1970, Kosuth had co curated a major show at MoMA with the uncannily prophetic title of Information. The battle against formalist traditions was far from over (as well as making art, Kosuth spent much of the decade firing off essays championing the cause in journals such as Art Language) but the Conceptualists had certainly moved from the lunatic fringe to a place much closer to the heart of the art world.

Now, at the age of 52, and with something of the status of an old soldier, Kosuth has arrived at the Irish Museum of Modern Art, singing the praises of its director like a Best Actor on Oscar night and generally appearing like someone very comfortable within the institutional framework of inter national contemporary art.

THE years have not served to give much cosy aura to Kosuth's hardcore, mostly monochrome work. A walk through the rooms that make up IMMA's current retrospective is a voyage though some austere text based work, which may reward engagement but certainly offers no cheap thrills.

Given his suggestion that "all art is linguistically based" it is no surprise that the writings of James Joyce and Ludwig Wittgenstein have featured heavily in Kosuths more recent work, including Guests and Foreigners, the new, immense black and white text installation that snakes through the corridors at IMMA. But if the ingredients in the work are obvious - it is "dangerously predictable" to do work on Joyce in Dublin, Kosuth says - its meanings are far less so.

"In a way I don't like to say too much about the new piece. When people come to the museum, I want them to complete the work. Too much of contemporary culture is about sitting passively, consuming the creative production of other people. What is important for me is that people come there and have to deal with the work... I want people to discover it for themselves."