It might be said that Andy Warhol's most important art work was himself, or at least himself and the circle that he created around him - The Factory.
This was a strange world of white-faced ingenues, addicts and porn stars that floated in and out of his pool of light - some to tragic ends, like Edie Sedgwick, others like Viva into an endless retelling of the same story, others still, like Lou Reed, reliving the experience of the Velvet Underground for all the decades to come.
The strangest part of it was Warhol's studied neutrality, the blandness of his face and appearance - there, but somehow inactive. And that may be the key to Andy Warhol, as artist, as (non-) performer, as entrepreneur. Because that is what he was - a businessman in art. He made no bones about it. "Making money is art and working in art is good business," he said in his autobiography.
As an illustrator, Warhol's early works were ink drawings of some of the adverts he came across in his activity as a commercial artist. Later in the 1950s he moved into painting - recognisably close to the pop art of Robert Rauschenberg and Roy Lichtenstein, though they were uneasy about his open sexuality (both later emerged as gay). But whatever their differences, theirs was an art celebrating - monumentalising you might say - US popular culture, particularly the comic and Hollywood cinema.
This was the language of the modern post-war world: full of colour and promise, and of endless diversity. Warhol's early work was distinguished by a kind of mark, drips of paint that were signs of the painter's hand, reminders that this was an object lived and interpreted through feeling.
But the marks of the artist's presence disappeared quickly in Warhol's journey. His representations of objects, events and car crashes show little feeling. They are reports from a relentlessly eventful world where very little lasts beyond its moment. That surely was what was meant by the "democracy of consumption" celebrated in a key show in 1964, The American Supermarket. The shelves of this hyperstore, however, were filled with representations of consumer goods, the beginning of the fusion of art and the market. Out of this were born the Coca Cola cans, the Campbell's Soup tins and the dollar bills that made Warhol famous.
Of course some critics have celebrated these images as symbols of the "Zeitgeist", ironic responses to the consumer culture which parody it by exaggeration. It seems to me that this is a brave attempt to give Warhol an artistic status that he seemed less and less interested in as time went on.
He is quite clearly celebrating that culture, providing a secular society with altars at which to worship (he himself was a practising Byzantine Catholic and turned to religious themes in his last painting on the Last Supper).
His most famous works - the portraits of Mao and Marilyn Monroe - make a similar statement. They echo the marketplace and merge with it. Mao, of course, was not the icon that Marilyn was, but Warhol did much to make him one just as the silk screen versions of the murdered Che Guevara created one of our most emblematic faces. And it was not only the subject matter that worked in this way - it was the method too. These were silk screened, produced in multiple runs by machines. Once again Warhol had no need to be there.
The Factory produced films, ranging from the six-hour film Sleep to the 24-hour epic Empire State. The camera simply gazed, left by its absent operator until the timer turned it off.
If the artist has disappeared, and there is no feeling or response, then the subject and the object merge. The image becomes the object.
I was reminded of something John Berger wrote in his wonderful Ways of Seeing:
"The publicity image belongs to the moment... yet it never speaks of the moment." Berger argued that all that the commercial arts could offer was a promise of future individual transformation - a false promise, of course, because that future was unattainable. Andy Warhol, it seems to me, locks us in a present place - a glamorous, sexy place perhaps, but nevertheless a place from which there is no escaping, just an empty hall of mirrors.
That is the opposite of what we have a right to expect from art.
Andy Warhol: Other Voices, Other Rooms is at The Hayward Gallery, London, from 8 October to 18 January 2009