By Richard Reeves
Presenter, BBC Radio 4's Political Roots
Remember the old bragging song, Lloyd George Knew My Father?
Richard Reeves' interview with Nick Clegg was an animated affair
Well, Nick Clegg's got a better one, for the philosophically minded anyway: "Isaiah Berlin knew my granny".
The Liberal Democrat leader has Russian emigre blood in him, and on one side of his family has connections to the great liberal philosopher. And Clegg's mother spent her early years in a Japanese prisoner-of-war camp. It is these familial roots that animate his liberalism, more than any grand intellectual inspiration. He doesn't follow in the footstep of a previous Liberal leader, Jo Grimond, by re-reading Mill's famous essay On Liberty every New Years' Day.
LISTEN TO THE PROGRAMME
Political Roots: Liberals is on BBC Radio 4 on Sunday 29 November at 2245 GMT
The repeat is on Wednesday 2 December at 2045GMT
Or listen after the Wednesday edition
"There was something floating around in my family," Clegg told me for the Radio 4 programme Political Roots. "I don't want to make it sound as earnest as it does - we were a perfectly normal family in that sense - but there was something very much in my upbringing which had an almost romanticised view of Britain as a home of liberty."
Not that he doesn't like to talk: Our interview over-ran because he enjoyed knocking ideas around.
At one point he became so animated he knocked the tie-microphone off his suit!
If you were feeling unkind, you might suggest that neither his ideas or his enthusiasm count for vey much.
After all, it has been a long time - 80 years - since the Liberals, had a sniff of power.
But if recent polls are anything to go by, he may hold the Parliamentary balance of power within a matter of months if the next election produces a hung parliament. So how deep are Clegg's liberal roots? What really makes him and his party tick?
The answers seems to fall into two main categories: Obstinacy and optimism.
Liberals have a republican dread of people and institutions wielding power over others.
"I've probably as a character got a bolshiness in me as well, which abhors arbitrary, illegitimate power," says Clegg, "whether it's the kind of pantomime that passes for our democracy in Westminster through to people arrogating to themselves authoritarian powers over other people. I think there's a bolshiness to Liberalism, which says that if anyone has power, that power's got to constantly be challenged and justified and held accountable."
As well as this bolshiness, bloody-mindedness almost against the wielding of arbitrary power there is also in liberal thought - and evidence in Clegg's liberal instinct - an innate optimism.
Liberalism as a political philosophy has always been founded on a conviction that free people will lead good lives; that when people are left to their own devices, those devices will be mostly good.
"I see Liberalism funnily enough not really even as a political label," says Clegg.
"I see it as a description of a state of mind - a generosity of spirit, generosity of heart, great optimism - because it starts with a very intuitive belief that individuals can do great things if they're, if they're empowered to do so. It's very, very optimistic."
Clegg is trying valiantly to maintain "equidistance" between the two main parties - although it is of course very difficult to remain equidistant between two constantly moving poles.
In recent days, he has stressed that he will not keep a defeated Labour party in power, and suggested, somewhat opaquely, that he will support the party with the "strongest mandate" from the electorate.
But in our interview it seemed that his heart was with what sees as the more acceptable elements of the Labour Party rather than with the Conservatives.
"A progressive believes that there's nothing inevitable, there's nothing fatalistic, there's nothing static about the human condition," he told me.
"A conservative is pessimistic. A conservative thinks, well basically, 'Yeah we can tick around the edges, but life is kind of as it is'.
"Now I so happen to believe that the Conservative Party represents a world view that is modest in its ambitions of what it thinks we're capable of doing together."
His critique of Labour, though, is different: Equally tough, but on the basis of what Labour has done in practice rather than with what it believes in theory.
"Labour I think has progressive instincts as far as social progress is concerned," he said.
"I think the great tragedy of Labour - and it's a real tragedy, it's an absolute tragedy - is that despite those progressive instincts, it has deployed deeply regressive and authoritarian means."
What all this mean by this time next year is not clear. Clegg, by instinct rather than design, has made his party more liberal. Whether this makes them a more important political contender is another question.
Richard Reeves is the director of think tank Demos.
Political Roots: Liberals is broadcast on
BBC Radio 4
on Sunday 29 November 2009 at 2245 GMT and repeated on Wednesday 2 December at 2045 GMT