Meanwhile, there is the superficially unrelated matter of Andre Durand who paints really bad pictures of the Royal Family. One showed Prince Charles on a rearing horse with William and Harry wearing baseball caps. Another showed an almost naked Diana tip-toeing through the waves on a glass globe. His latest, apparently, shows Diana again, this time eating with her two sons in Pizza Express.
The link between the two is the way the public realm here has become a serious embarrassment. Nobody knows what to do with it, so noble traditions are abandoned to be abused by tinkering fools. Town planning becomes street daubing, royal portraits become glib cartoons. In Hyde Park appalling new gates honouring the Queen Mother assault the eye. The Barbican attempts to add symbolic weight to its mannerist-brutalist arts centre by adorning it with distasteful gold statues. Only in Trafalgar Square where the last plinth remains gloriously empty does a certain tact and honesty prevail - if you've nothing to say, don't say it.
In respectable official circles, vain attempts are made to cling to the pathetic fragments of past grandeur. British embassies around the world are earnestly provided with reproductions of a Gerald Kelly portrait of the Queen, complete with curtain and column, a sad little emanation from the tomb of classicism and the grave of Reynolds. I mean, old boy, it looks about right, doesn't it? No, it does not.
Perhaps it is unfair to blame Britain or modernity for the crisis in the public realm. This week Christie's sold a Trumbull portrait of George Washington at the Battle of Trenton from the Marquis of Bute's collection. It was originally rejected by the city of Charleston as being "too heroic and historical in flavour". What on earth did they want? Well, of course, they wanted a hero-less, history-less democracy. Democracy as a dragging- down rather than a pulling-up goes back much further than the National Union of Teachers.
It was the Americans who perpetuated that odd, corporate desire to put something - anything - called art in front of their office blocks. The phenomenon was memorably described by Tom Wolfe as "The Turd in the Plaza". Art, the executive's education told him, was something to do with the distilled aspirations of the people. But, sadly, by then art had strayed far from any such aspirations and the plazas, even with their art, looked emptier than the Trafalgar Square plinth.
Andy Warhol may have revived the portrait, but only as a crafty, self- regarding denial of individuality. The last chance anybody had to get a portrait painted of themselves in any meaningful, reverent sense was by employing John Singer Sargent in Edwardian London. Similarly, the last time we could build appropriately resonant monuments was just after the First World War when Lutyens built his awesome battlefield memorial at Thiepval - a work in which the classical tradition sombrely contemplates its own Nemesis in the trenches.
Now the problem lies on both sides of the public-art equation. The public realm is only public in the sense that it is a vacancy, somewhere that belongs to nobody and everybody. And art, lacking a coherent audience and a communicable language, drifts into pained self-examination. This can produce great art, but it cannot produce great public art. As Eliot knew, as Picasso knew, the "equals" sign in the equation was severed somewhere around 1920 and the two sides drifted apart.
Subsequent attempts to unite them may have been noble, but they only drew attention to the problem. Henry Moore's altar in Wren's St Stephen Walbrook in the City of London is all very well, but it still looks, depending on your perspective, like a stone Camembert or a gross druidic intrusion into a supremely sophisticated piece of architecture. Moore, I am sure, would see and understand Wren's subtle balancing of the classical dome and the gothic cross. But, in place, the two aesthetics can only result in a contradiction.
Perhaps we should look to Anthony Gormley's huge Angel of the North sculpture to be placed on a hill by the A1. Here is a piece of modern heroism in the public realm - the aircraft-like wings spread, precariously unsupported, across the landscape to astound the passing salesmen in their air-conditioned Mondeos. Plus it is an angel, a clear and traditional symbol of transcendent significance, and it is "of the North", an attempt to evoke the kind of regional identity that was utterly lost in the corporate modernism of the Fifties and Sixties.
Well, I am prepared to give Gormley the benefit of the doubt. But there are problems. The angel seems strained as if it could only be an angel at all in these times by having its angelness stretched to the limits of aerodynamic feasibility. The artist seems to be trying too hard to reach the public realm. Certainly, it is better than the Queen Mother's gates in Hyde Park, but it suffers from the same kind of exaggeration. The gates, in their wild contortions, strain to be meaningful and, in doing so, reveal their meaninglessness. The angel teeters on the brink of the same failure. Possibly, such teetering is what art must now do if it aspires to be public.
Of course, it could be that we simply choose the wrong artists. David Hockney is prone to twee domesticity, but some of his earlier paintings did have a form of contemporary monumentality that at least suggested the possibility of a public role. And there have been other modern artists - Jackson Pollock, Mark Rothko - whose striving for an abstract sense of self or spirituality seemed to show a genuine artistic awareness of the difficulty of reaching out from the canvas or the plinth.
But sadly, in Britain at least, to embrace any such figures as laureates of public art would require a heroic suppression of our national sin of literalness. We always want to be able to say that what we see is what we get. So they paint the street in Harrogate because, in their literalness, the councillors think you make the place more colourful by splashing colour around. Public art cannot be literal, it must be symbolic. If there are no symbols and nothing to symbolise, there's no way to go public, to persuade people to believe that these stones, bronzes or oils are anything more than mute, insignificant minerals.