During the Moscow Trials in the 1930s, a story circulated that Stalin had never been a Bolshevik, but was an old Tsarist spy who had escaped detection after the revolution and remained in the party.
Discussing the story, Trotsky rejected it, and said: even were it shown to be true, it would add nothing to our political and social understanding.
It would only confuse the issues posed by Stalinism, seen in Marxist terms as a social and political phenomenon that had arisen as a counter-revolution against a working-class regime in an isolated and backward country whose development towards socialism was impossible.
Suppose it turned out that Tony Blair, or some of those who were very close to him, had been agents of the American CIA at the time of the invasion of Iraq in 2003? Would that clarify or confuse the picture of the British/US relation then?
Would it add to our understanding of the how and why of it?
Surely it wouldn't. It would lead us to view as a spy story what is perfectly understandable as a story of political delusion, of political corruption, of political and military subordination of one country to another, and of the hollowness of our pluto-democratic system which allowed Blair to do what he did.
Blair went with Bush because he saw advantage in playing Robin to the neo-conservative Batman in Washington, and big disadvantages in refusing to do so.
He thought that the USA's military world supremacy would be both irresistible and sufficient in reshaping the Middle East. He thought it would be easy for the USA to use its military might as a tool of social engineering and to remodel Iraq.
He saw the issues through the eyes of Bush and the naively arrogant and bungling neo-conservatives in Washington.
In Britain, Blair had largely destroyed the old democratic structures of the Labour Party and substituted his own secretive, manipulative, and autocratic "presidential" rule for collective rule by the Cabinet. Parliament had long previously lost the power to control the Government, short of overthrowing it.
Robert Harris is a former friend of Tony Blair who fell out with him over the invasion of Iraq.
Harris's 2007 thriller The Ghost, and now Roman Polanski's film based on it, link the "Blair" character to the CIA, and so inevitably reduce the politics of the invasion of Iraq to a story of subterranean "secret agent" manipulations.
That is perfectly legitimate. That is the sort of thing a thriller, book or film, is supposed to do; and certainly aspects of the now public story - the deception of Parliament about the "weapons of mass destruction" in Iraq, for example - lend themselves to such treatment.
Is it well done? In the story, a ghost writer (Ewan McGregor) is hired to help write the memoirs of a former British prime minister (played by a convincingly powerful Pierce Brosnan), now holed up on a bleak island somewhere off the US coast.
A previous "ghost" has been found dead on the beach, seemingly victim of an accident. The new "ghost" treads in the footsteps of his predecessor, picking his way towards the ex-PM's secret.
The secret, once it is uncovered, adds nothing to our understanding of Blair's role in the war. Politically, the story is anti-climactic.
Even so, as a thriller I found it gripping and, on its chosen level, almost convincing.