What we know is that the British Empire has been one of the dominant forces of the modern world, changing more lives than fascism, Soviet Communism or the revival of Islam. Inventors from Lancashire and Scotland, and the thinking of Adam Smith, Wesley, and the aristocratic revolutionaries of 1688, have had a bigger influence on today's world than Marx.
One cannot begin to speculate about what sort of places India, Africa, Australia, North America or many other stretches of the world would be had there been no Imperial Britain. As for the "mother country", with its global language, its many races, its awkward Euro-scepticism and its sprawling, now-declining imperial cities, Britain itself seems still half- stunned by the experience of so quickly gaining and then losing the empire.
This has, in short, been one heck of a happening - what Ian Jack has described recently as, "a grand, endlessly interesting thing to have happened to a small and unlikely country".
And what about us, the children and grandchildren of the unlikely ones? Where are we left now the whole thing has finished? Is it a scattering of boarded-up Mafeking Streets, and a useful base-camp for Microsoft, and that's it? Will the Scots and the English still rub along under one national flag?
The answers are no, and yes, respectively, though some think, and have always thought, that the end of Empire means the end of Great Britain - that the two were the same. The archetypal Imperialist politician, Joseph Chamberlain, spoke for millions when he said, early on in the century, that the identity of England (sic) had become identical to that of the Empire: "England without an empire! Can you conceive it? England in that case would not be the England we love ... It would no longer be a power, if not supreme, at all events of the greatest influence, generally well- exercised, on the civilisation and the peace of the world. It would be a fifth-rate nation ..."
We have been getting used to losing that role and slipping down the ratings for half a century. Though we have not broken up, loss and nostalgia have become central to state-occasion Britain. It was entirely natural, of course, that they dominated yesterday's emotion-sodden farewell to Hong Kong. But loss and nostalgia can be found in endless ways, large and small, in the Beaten Retreats, tattoos, commemorative services, honours lists, Queen's broadcasts and so on that surround national politics.
I was of the last generation to learn Imperial history of the old school - Clive, Nelson, Churchill - and grew up with a hazy false memory, learned from family photos, of Another Britain, a lost place of pith helmets, firm jawlines, crisp naval uniforms. And there it was again on that Hong Kong parade ground, the Britain whose most characteristic tune isn't "Rule Britannia" but a trembling Last Post.
Enough! We have, as a country, come close to drowning ourselves in all that. It was right that Tony Blair went to Hong Kong (looking grim and uneasy; he hasn't yet got the trick of looking solemn but self-assured).
But seeing him as the bands marched past was depressing too. For 50 years political leaders have been trying to reshape a national purpose, and for 50 years they have been failing. Much of the rest of the country has got on with it, in culture and business, but politics has lagged far behind, so that Britain today seems a disjointed place, polarised between Post- Modernism and post-imperialism, with too little in between.
So enough Last Posts and folded Union Flags. Enough "Britannia" and enough weary self-deprecation from the Prince of Wales. We should not leave Hong Kong with too much regret. The critics of Chris Patten's Hong Kong policy are mostly fatuous. We had to give the colony up. However belatedly, it was also right to assert the kind of politics we believe works best everywhere.
That is not a hopeless cause. The symbols of our departure include the backward-looking Imperial insignia, kilts and Gurkhas. The incoming People's Liberation Army may seem, by contrast, the wave of the future, representatives of a growing superpower. But things are not so simple. It is the domestic values of the departing ex-colonial power - liberalism, free speech, the rule of law - which are in the global ascendant, and the authoritarian, pithless post-Marxism of the ageing Peking elite which is in retreat.
Whatever happens in the next few months and years, Hong Kong will bury them, not the other way about. Peking, as the Chinese capital, is applauding the return of a part of China. But Peking, as a citadel of anti-democratic and illiberal politics, is applauding a future which will quite soon destroy it.
So we need less Imperial nostalgia and a little more optimism back home. Let us have more, please, of the inclusive, liberal group of islands that we have it in us to become, and which it is Blair's job to help shape through the millennium. More, please, of a modern, reformed democracy, with a Bill of Rights and an adult relationship to its European partners. More of the unapologetically ethical and moderately "green" voice of Robin Cook's Foreign Office. More, in short, of a plausible national future, which means liberalism, democracy and multi-cultural openness. We have to remember that the Empire was finally smashed not by the Hong Kong treaty, but in the global war against fascism half a century before. And though that was a great cause to die for (has any empire been destroyed as usefully?), we still haven't fully caught up with the victory we helped to achieve.
Because of the Empire, the UK is now an ethnically diverse union of countries with a global language - more like a small America, in some ways, than like France. Yet because of the Empire, we are also still a hierarchical, cod-medieval country with a terrible capacity for lacrymose nostalgia. Now we must choose.
Of course, with others, we should do our duty in helping to keep an eye on Hong Kong. But in a broader sense, we must stop looking back, either guiltily or sentimentally. A great break of this kind has happened before, at least to England. For decades, late-medieval London was in trauma at the loss of France - remember Calais, engraved on Mary Tudor's heart? But what followed was a great outpouring of national energy, a liberation, and the shift to early-modern Britain. Perhaps Hong Kong is the Calais of our times.
It is time to move on. Let this be the last Last Post.