The novelist Robert Musil, who lived and worked in Vienna, one of the most populously statued cities of Europe, commented on statues and monuments: "[Their] most striking feature is that you don't notice them . . . Why . . . are monuments produced for great men? It looks like fiendish malice . . . one has chosen to hurl them with a memorial stone around their neck into the sea of oblivion."
Who could have named any of the occupants of Trafalgar Square - apart from Nelson - before the Royal Society of Arts launched its campaign to fill the fourth plinth, which has remained empty since Charles Barry laid out the square in 1829? Amid the bandying about of the names of Charles Napier and Henry Havelock, few people have mentioned James II, that unlamented Catholic monarch, troublemaker in Ireland, who overlooks the square from the National Gallery, or the war heroine Edith Cavell, who can be seen further up, outside the National Portrait Gallery.
Statues have embodied collective ideals and memories since the Palladium of Athens and the cult statue of Athena to which the great procession depicted in the Elgin Marbles is making its way with offerings of cattle and woven cloths. Aeneas brought out another precious palladium after Troy fell. It was the thing he most wanted to save from the sack of that city - apart from his own father. He carried the statue off to Rome to found the Roman empire. The statue made it clear that the new city was to be Athens's successor.
So there's a contradiction. On the one hand, statues are invisible and plunge their subjects into oblivion, but on the other, when a revolution takes place, they are often the first objects of a mob's interest. The French Jacobins took the waxworks from Madame Tussaud's uncle's gallery and paraded them in the street to demonstrate their fury against the regime; the Communards, Gustave Courbet famously taking part, pulled down the column with the king's statue on it in the Place Vendome.
In his fine study, The Final Sculpture: public monuments and modern poets (Cornell University Press, 1985), Michael North pointed out, acutely, that: "Sculpture has held a public place in the past partly by virtue of its ambiguity, not because it is the simplest of arts but because it is able to satisfy conflicting desires." The interest stirred by the empty fourth plinth points to wide dissatisfactions, even anxieties - especially around the idea that we, as a society now, don't have a sufficient consensus to agree on a hero or heroine. Can we arrive at more of an idea of what a monument might be, of whether it has a function beyond the naming of national heroes? Can we shape some general principles to help make the decision about the empty plinth, newly made visible after being ignored for so long? How can we outline and satisfy those conflicting desires and ambiguities?
Monuments and memory are intertwined, and sculptures can be, and are, elegies and epitaphs for past horrors as well as triumphs. In Jerusalem, Yad Vashem is called after a prophecy in Isaiah: "I will give you a monument and a name." This is the great memorial museum to the Holocaust. Such public art inaugurates the present in the light of the past, and it frequently establishes identity in sacrifice - on the battlefield, as in all war memorials (including Sargeant Jagger's monumental sculpture at Hyde Park Corner), and in the gas chambers. Mark Wallinger's inspired response to the problem of the plinth, Ecce Homo (a 6ft statue of Christ), also draws on Christianity's symbolism of martyrdom in the true sense of the word, "witness".
Such monuments denounce past evils, and in doing so demonstrate a magical function. Effigies and memorials summon the presence of the past in order to quieten it - the "Never Again" of the war cemetery's prayer. Jochen and Esther Gerz's Counter Monument, unveiled in a suburb of Hamburg in 1986, commemorated the Holocaust through a simple cubic pillar which was increasingly covered in signatures of visitors and passers-by; it fulfilled the magical, ritual practice of memory through records of names, but it was also an exorcism, for it slowly sank into the ground and has now vanished altogether - a symbol, not of obliviousness but of healing, closing the wound.
Memory has taken an ever more dominant place in our preoccupations, caused on the one hand by the turn of the millennium, but on the other hand by political struggles for legitimacy through the use of interpretation of history. We live in a contradiction. We inhabit the fastest-altering social landscape ever, and yet we drench ourselves in records of the past. But securing a consensual view of the past is, if anything, increasingly difficult.
Art now can do something different. It can create tradition and collective identity as it moves. Artists of the postwar generation have shown that they can communicate in new languages and images, and make familiar what was unfamiliar, or disclose their strangeness when they seem quotidian. Artists have been active in one area of social cohesiveness - almost unwittingly. For good or ill, they seem to have put an end to several hard old schisms: between modernity and officialdom, between avant-garde practices (such as temporary interventions or off-limits venues) and public popularity, between general indifference and insider connoisseurship. Contemporary art, especially in its public, sculptural manifestations, is no longer ignored. Contemporary artists have come down from the garret. And the avant-garde, oddly enough, has led the way in using the public forum, the street, the landscape, the unexpected location, for surprising passers-by. And it has been a very successful and popular stratagem. There is growing curiosity among the public about original, contemporary metaphors, about the possibility of some kind of break with the past.
"A state can be judged by the future its sculpture sets out to promise it," John Berger wrote. But this task of facing the future should not necessarily entail solemnity or pomp, and certainly not bombast. One of the great lost monuments of London was Claes Oldenburg's proposal in the 1960s for a pair of knees cut by a miniskirt, which he called London Knees and sketched in place somewhere between Bankside and the London Eye. Outside the Pompidou Centre in Paris, Niki de St Phalle created an exuberant fountain full of her Nanas, multicoloured bouncing figures; and Flower Puppy by Jeff Koons, outside the Guggenheim Museum in Bilbao, has also won public affection.
It isn't the subjects that create the work of art any more than the plot makes the novel. It's the medium of expression, the artist's engagement with thoughts and issues and problems. So perhaps contemporary technologies could be used to great effect in resolving the problem of the empty plinth. This would be in harmony with the tradition of Trafalgar Square, because the forge and the engineering techniques of the Victorian age were demonstrated with pride in the erection of its monuments. There is no reason why we shouldn't use our own contemporary advances in communications: a phantom might even be beamed in, projected by laser. Such technologies could be used to create a contemporary sculpture that speaks to the future, not to the past.
This is an edited version of a recent talk given at the Royal Society of Arts