The prime minister will use his Guildhall speech tonight to take "head on" the argument that Britain is embarked on an inevitable path of decline. It's an interesting decision - perhaps one Sir Humphrey would have described as "brave" - since it invites people to examine an argument which, so far, has not had a wide airing.
Later, I'll outline the Cameron case for the defence; first, here's the case for the prosecution. This, in other words, is not my view but the combined views of those who say that Britain is in decline:
The economy, stupid
Britain has the biggest deficit in the G20 and now faces a decade of retrenchment.
China's rise cannot be halted. The world is moving East.
The City of London's in decline
For more than 20 years, London has been one of the principal financial centres on the planet - but now it's in decline. Higher taxes, public and political hostility to banking and financial services and the rise of the East are leading people to leave the City of London or, just as seriously, never to choose to come here in the first place.
A coalition government committed to "re-balancing the British economy" will lack the will to maintain the City's pre-eminence.
A declining military
The British army was humiliated in southern Iraq, some claim. They had to rely on the Americans coming to their aid in the "Charge of the Knights".
Despite showing extraordinary bravery British forces in Helmand have been no more successful than they were in Basra and have, once again, needed the Americans to come to the rescue.
British defence cuts are so serious that soon we will have aircraft carriers without aircraft and need to share the one we do have with the French.
After the Iraq war, Britain has lost the will to project its power around the world.
In Europe but not running it
Britain is prepared neither to join the EU fully nor to pull out of it.
The first Eurosceptic prime minister in two decades has, in effect, put Britain's relationship with the EU on hold - able neither to go forward nor go back (see my earlier post on Europe's push-me-pull-you).
David Cameron has felt forced to present a 2.9% rise in the EU's budget as a victory just days after pledging a freeze.
For the next 18 months the PM's energies will be taken up by defending Britain's rebate.
Marginal on the world stage
The era of the dominance of the post-war powers is coming to an end, illustrated by the G20's replacement of the G8. Inevitably the pressure to re-structure the UN threatening our role as one of the Permanent Five on the Security Council will eventually succeed.
America is led by a president who, unlike most of his predecessors, has no affection nor historical attachment for Britain.
Margaret Thatcher was a lover of the military and a cold warrior. Tony Blair was a lover of the military and a neo-con. David Cameron is neither. He's a sceptic about the military and about projecting British power abroad.
Gordon Brown briefly counted on the world stage thanks to Britain being in the chair of the G20 during the banking crisis and his mastery of international financial architecture.
The prime minister's foreign policy focus on the "British national interest" means that Britain is forced to charm the Chinese despite its human rights abuses and to re-set its relationship with Russia despite the murder of a Russian on British soil almost certainly by Russian agents.
In contrast David Cameron was a minor player at the G20 Seoul summit. It is hard to see him being a major player in the EU or the UN.
That, to repeat, was the case for the prosecution. Later, I'll post David Cameron's argument against Britain being in decline.
It's a debate that is, of course, far from new. Ever since the end of World War II, ever since an American secretary of state said the Britain had "lost an empire and had yet to find a role", Britain has worried about its status in the world.
Some politicians - Nick Clegg perhaps - would say that Britain should stop pretending to be what it is not and should accept that our power and status depend on being one of the biggest and richest countries in the EU. That, though, is not the view of most Conservatives. So, even though David Cameron is temperamentally relaxed about his status, he will be feeling the pressure all British prime ministers feel to show that Britain matters.
One intriguing little story emerges from my time in Seoul. President Obama chose not to have a bilateral meeting with David Cameron at the G20 summit. I think we can safely assume that this was not because Downing Street did not ask for one. If the PM had cared as much as Gordon Brown - whose orders to officials to do everything they could to secure a meeting for him with the president at the Pittsburgh G20 summit leaked - we could easily have seen a repeat of the "Obama snubs PM" story.
Update, 10:21: This debate is not only happening here. Look at the words of Robert Kagan, former foreign affairs adviser to Senator John McCain and former department staffer on ABC yesterday:
"Britain has taken itself out as a major player in the international system, at least for a while, with the kind of cuts that they've made in their national security budget.
"The problem is, the United States doesn't have the luxury of doing that. I mean, Britain can become a free-rider in the international system, but that is the price that they've paid."
PS: An earlier version of this post read "Mansion House" instead of "Guildhall"; apologies.