'I'm the king, may I come in?" said Edward VIII when he knocked on a door in Glasgow. He had inherited a talent for slum-visiting, even if he lacked the stamina for kingship. Ten months after his accession, he toured the coalfields and furnaces of South Wales, rendered idle by the Depression. The visit brought on the second most famous remark of his reign. Standing, bowler hat in hand, at the abandoned steelworks at Dowlais on a damp November morning, he told hundreds of unemployed, "Something must be done." The crowds cheered.
But what did the comment signify? The king was obviously upset by what he had seen - the majority of men had lost their jobs when the factory closed and families were going hungry. The comment endeared him to the unemployed; to many, he became a hero. But it also pointed up the impotence of the king's position: he had no executive power. "Something must be done", by somebody else. Within a few weeks, he had quit his post anyway.
But this symbolic engagement in the lives of their people had given the monarchy a new role. In Britain, the only occasion on which most people see the king or queen engaged in politics is the state opening of parliament. This little piece of theatre, played out in its colourful pomp - gilded coaches, cavalrymen in glittering breastplates, the trumpet fanfares and artillery salutes - is a modern reinvention, begun by Edward VII, who was said to have wanted an opportunity to outshine his cousin, the Kaiser. The ceremony usually draws the protest of some leftwing politician at the indignity of democratically elected MPs being summoned to listen to an unelected monarch reading a speech in the unelected House of Lords.
But you might as easily see the humiliation the other way round. George V apparently told his son that he "knew of few worse ordeals than being obliged to deliver somebody else's speech, at the same time balancing on his head a 2lb gold crown". If so, he had a remarkably limited imagination.
Many of the other functions performed by the occupant of Buckingham Palace are similarly contrived to add lustre to the machinery of state. Consultation, encouragement and warning have become accepted as the only three legitimate political activities of a constitutional king or queen. The most celebrated instance is the regular meeting when the prime minister calls on the Queen. It is almost impossible to find out what is discussed, for no agenda is circulated and no record is kept. They are just private conversations.
James Callaghan used to relish them. John Major's office privately described them as a weekly session "on the psychiatrist's couch". Major himself felt that the sessions were the oral equivalent of his being able to kick the cat, and he often returned exclaiming, "She knows so much!" In the early years of his prime ministership, Tony Blair attended much more frequently than John Major, although later he sometimes preferred to talk by telephone.
"I think it's rather nice to feel that one's a sort of sponge and everybody can come and tell one things," the Queen has said of these encounters. Former prime ministers say much the same thing. "I don't want to be melodramatic," one told me, "but being prime minister is a very lonely job, and you really can't trust anybody, so to have someone to talk to who isn't after your job, who won't leak what you've said to the press and won't use what you've said against you - that's a real relief."
Even prime ministers can be awestruck, though. Eddie Mirzoeff, filming for a BBC documentary, was astonished by how nervous John Major was. "You could almost see him trembling," he recalled. A portion of the soundtrack (which was never broadcast) has the Queen describing the garden to Major, as they strolled the grounds together. He is heard to interject, "Oh yes. And it's all the work of Calamity Brown." The Queen, with a studied tact not always displayed by her eldest son, does not correct him.
The problem with opinions is that there is always someone else with a different one. Since the function of a modern monarchy is to act as a unifying symbol, it is therefore usually better not to voice them. Yet this vow of silence may be tolerable only if you are content to spend your apprenticeship like Edward VII, eating, shooting and having sex. Prince Charles could not settle for pleasure and his staff have learned to become accustomed to his return from periods spent staying with the Orthodox monks of Mount Athos in Greece or communing with nature in his Gloucestershire garden, when he would fire off instructions about how he was going to save the world.
Each instance has tended to be seen as merely another "it really is appalling" outburst. But, discounting their often tetchy tone, there has been an underlying unity to his apparently scattered pronouncements. Anyone who paid close attention to his speeches, or talked to his friends, would have known what he thinks about all manner of subjects, from global warming (deeply troubled) and globalisation (unsustainable) to the decision to go to war in Iraq (an utter mystery). He detests much modern architecture and distrusts much modern medicine. We know that he despaired of George W Bush and was baffled by Tony Blair's chameleon-like ability to be all things to all people.
None of these opinions was either surprising or unique. In many of them, the prince was doing no more than reflecting the anxieties of his people. Despite the synthetic outrage of newspaper columnists at the Prince of Wales's "political" interventions, does his opinionising amount to anything more than a modern spin on "Something must be done"? It hardly seems likely to provoke revolution.
Even devoted campaigners against monarchy admit, in fact, that their movement has no fizz and has failed to find any real place in modern Britain. The republican cause may be less marginalised than the Flat Earth Society, but it has much less presence than the battle for animal rights. It has few advocates among the country's politicians, and it features in none of the manifestos of the major parties. As it is, faith in pure democracy, with the king or queen replaced by an elected head of state, remains a minority pursuit. Even during the 1990s, when the British monarchy seemed so embattled, the number of people telling opinion pollsters that they would vote in a referendum in favour of replacing it with a republic was no more than one in five.
"Republicans are edged into such an absurd cul-de-sac because there is no effective space for them in the public arena," complained the marxist Tom Nairn 20 years ago. "There is no serious republican campaign or movement, no republican press, and no recognised or avowable anti-monarchic stance in everyday argument and debate. It is this climatic fact that defines republicanism from its first syllable as posture and wilful eccentricity." It was, in short, a hobby for cranks.
The climate of public opinion has changed over the postwar period only to the extent that politicians have been willing to trim the monarchy of some of the trappings of imperial excess. It was fitting, perhaps, that the royal yacht Britannia's last major international appearance was in the harbour at Hong Kong as the base from which Prince Charles could watch the British flag hauled down and the colony returned to Chinese rule. John Major's government then dithered and delayed over a scheme to replace Britannia; inevitably, when Labour won power in 1997, it was cancelled. Major now privately regards his failure to act as a mistake, one that made Elizabeth the first British monarch since Charles II not to have an official yacht. Arguably, though, in an age when owning a yacht has more than ever become the preserve of plutocrats, retiring the royal yacht was an appropriate, even astute, piece of public relations. But it does not follow that the British people has lost its taste for pomp and circumstance or its enthusiasm for the monarchy itself.
The very perceptive - and mildly republican - Spanish princess Eulalia drew a comparison in 1915, when she asked what liberty the British people would acquire by casting off royalty. "They would gain as little as if, by a popular uprising, the citizens of London killed the lions in their zoo. There may have been a time when lions were dangerous in England, but the sight of them in their cages now can only give a pleasurable holiday-shudder of awe - of which, I think, the nation will not willingly deprive itself."
Certainly, if we were devising a system of government for the 21st century, we should not come up with what we have now. The arrangements are antique, undemocratic and illogical. But monarchies do not function by logic. If they work, they do so by appealing to other instincts, of history, emotion, imagination and mythology, and we have to acknowledge that many of the most stable societies in Europe are monarchies, while some of the most unstable and corrupt have presidents.
Of course, we could easily pack all of them off to live out their lives in harmless eccentricity on some organically managed rural estate. But why bother?
© Jeremy Paxman 2006.
· This is an abridged extract from On Royalty, to be published by Viking on October 6, price £20. To order a copy for £18 with free UK p&p, go to theguardian.com/bookshop or call 0870 836 0875. Jeremy Paxman will be in conversation with Robert McCrum, the Observer's literary editor, at 7pm next Wednesday at Imperial College London, SW7. Tickets: 0845 456 9876.
