Yoko Ono told me: "It will be like a homecoming without being there." She will not be at the first exhibition of her Sixties art, Yoko Ono and Fluxus, that opened at London's Royal Festival Hall on Friday. "Something else cropped up." She is 63, a Beatle's widow for 17 years, and still an artist of the invisible.
Among the exhibits are her enigmatic "instruction paintings". No canvas, no paint. Just printed instructions for "paintings to be constructed in your head". One instruction reads: "Imagine dividing the canvas into 20 different shapes". Art, for her, was all in the mind. Still is.
The most poignant artwork is a sheet of paper bearing the inky footprints of her murdered husband, John Lennon.
Ono, daughter of a Japanese banker, was the inspiration for Fluxus, an anarchic, anti-art group of New York artists formed in 1961 five years before she met Lennon. The group caused outrage by offering bits of rubbish as art and staging absurd happenings. Yoko's "bed-in" (in which the couple held court in bed) was pure Fluxus, whatever else it looked like. So were her films and photographs of naked bottoms, her own and Lennon's included.
At the time, it looked like a one-woman plot to change the world through art. She was ridiculed. Today, it is worth asking whether she succeeded. After all, the spirit of Fluxus is now pandemic: you see it in pickled sharks and art college degree shows filled with nothing but conceptual oddities - balloons in string vests, spanners suspended on springs from the ceiling.
Well, did it work? "I was just groping at the time," she says, "but things are resolving themselves in a very interesting way. Nowadays, people are having fun with art. When I was doing that, people thought I was crazy. I felt lonely, like an outsider.
"I wish John could have known all this would happen. He would have loved it."
She revealed how Lennon had suffered by championing her art. It had been her art that had first attracted him to her. He had visited her show at the Indica gallery in London in 1966, where he was confronted by her Painting to Hammer a Nail, a block of wood with hammer and nails supplied (a version of it appears in the exhibition).
"Can I hammer a nail?" Lennon asked. "It will cost you a shilling," Ono told him. "Then can I hammer an imaginary nail?" It was the correct reply.
"That's how we met," she said. "John is one of the few people who understood my art. But by understanding it, by taking it seriously, he became lonely, too.
"John took part in the 'bed-in' and stood up for me, proclaiming that he understood. I deserved all the abuse, not him, because it was me who was sticking my neck out.
"Now that everybody is doing conceptual art, they've forgotten the abuse. But they should give John credit for his foresight."
Ono also suffered by becoming a Beatle's wife. "The minute I got together with John, people decided I didn't exist. But I was still working as an artist."
The year before they met, her career had reached a peak with a performance by the Fluxorchestra at the Carnegie Recital Hall, New York, of her Sky Piece to Jesus Christ, in which performers bound the orchestra with tape, carrying them off one by one as they became immobile.
The year she met Lennon she performed her Cut Piece in which the audience was invited to cut off her clothes. It exposed some unexpected pathology in the cutters, particularly males.
"Fluxus is a wind going round and round that hasn't stopped for 30 years," she said. "It has a unique voice, one of the few art movements that has a sense of humour."
Even the naked bottoms refuse to go away. Ono organised what might be called a Bottomfest in a small German town three years ago. The whole population took part. There were blown-up photographs of bottoms on hoardings, bottoms on walls, bottoms on carrier bags, bottoms on umbrellas and posters of bottoms to greet arrivals at the airport. "People who landed were very surprised," she said, "and so were the townspeople. Surprised and happy."
It was the artist George Maciunas who founded Fluxus, inspired by Ono's concerts in her loft in New York. He chose the name partly because of its dictionary definition: "A fluid discharge, especially an excessive discharge from the bowels ... a continuous moving on ... a flowing stream." His Fluxus manifesto of 1963 proposed purging the world of dead art.
When Ono published the texts of her instruction paintings last year, she wrote of an exhibition in a Berlin museum in 1992 in which she had displayed not only the instructions but the canvases (such as Smoke Painting, a canvas that the public is invited to burn with cigarettes until it disintegrates).
She said: "A guy asked, if I'd wanted people to construct the paintings in their heads, why did I display canvases instead of just exhibiting the instructions by themselves? I started to laugh. The world had changed so much. 'I did that in 1962.' 'Why don't you do it now?' he asked. I just said, 'You're right.'"
Yoko Ono and Fluxus: Ballroom, Main Foyer, Royal Festival Hall, South Bank, London, until 23 March (10am-10.30pm), admission free.
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