A column about the importance of ideas in politics ought to start with a quotation from a philosopher: but it is a grey autumn, rain is forecast, and Ed Balls is on the news. This isn't a moment for Hume. We need cheering up.

So let's follow Rodgers and Hammerstein to their musical South Pacific. The sun shines. Waves break on empty beaches. Palm trees rustle. And one character hands out comforting advice. "You've got to have a dream. If you don't have a dream, how you gonna make your dream come true?" she sings.

They should start whistling the tune in Westminster. There's more good sense in the song than a score of thinktank reports. British politics has all but killed itself trying to find out "what works". It has drowned in managerialism and suffocated in analysis. We could do with some South Pacific dreaming.

Watching Balls and Michael Gove battle it out on television today my mind wandered from the facts they threw out to viewers. Twenty per cent of this; a quarter of that; a rise – or was it a fall – in the fifth quintile. Balls in particular was robotically impressive, a man with a mind like an Excel spreadsheet. He knows it all and it amounts to nothing beyond the sterile computations of an Institute for Fiscal Studies assessment.

This is politics by formula: insert X into the equation as the desired numerical outcome and the answer to all the other variables becomes clear. It looks precise, but it is really empty, since all the numbers are baloney. A society reliant on statistical calculation can never be optimistic; all we ever see are the limits and the failures.

Yet this is where much of our politics has ended up. Labour, lacking ideology, surrendered to the cult of measurement. The Conservatives competed in opposition to do the same and at times still are. David Cameron agreed to meet Gordon Brown's arbitrary and theoretical target on child poverty, and even now he is sticking with it. At the final coalition meeting at Chequers before the comprehensive spending review, the Tories decided to bung short-term cash towards the child tax credit even as Liberal Democrats present argued against doing so. The latter wanted an honest reassessment of the sort of society government can create, rather than pretend that something is possible when it isn't.

The coalition understands, hopefully, that it will fail if it continues to sit tests on terms laid out by Labour. Its politics must be recalibrated around a different philosophy. This sounds like – and can be – a way of dodging the consequences of cuts. But there is nothing inalienable about the ideas inherited from Britain's postwar settlement. The welfare state has in some ways led to a better and fairer society but after five decades Britain does not seem notably equal or free or happy. There may be better ways of achieving these ends.

John Maynard Keynes spotted the problem even before it came about. In his new book on the big society, the philosophically inclined Tory MP Jesse Norman quotes an article the economist wrote in 1939: "Why cannot the leaders of the Labour party face the fact that they are not sectaries of an outworn creed, mumbling moss-grown, demi-semi Fabian Marxism, but the heirs of eternal Liberalism?" Heirs, perhaps – but disinherited. There are few liberals in the Labour party these days. The task of thinking liberal thoughts has been left to the coalition.

On Tuesday Nick Clegg will give the Hugo Young Memorial lecture at the Guardian premises, and try to persuade his audience that the government draws its strength from ideology, not opportunism. He will step away from government by measurement and defend the liberal idea of individual human advancement. He has even been reading Karl Popper's The Open Society and Its Enemies. Hayek next, perhaps.

Much of the left will sneer at this: but if I was inside Labour I would worry that Britain's centre-right parties are making a better job of setting out an optimistic philosophy of government than statist conservatives on the left. They have fallen into a negative sulk: everything, Labour predicts, is about to get worse, which only makes sense as a strategy if you have something better to offer.

Labour doesn't. The party has become uninteresting. The coalition is doing the thinking. Yes, the "big society" is waffly, unmarketable and disliked by many Tories. Norman's book won't persuade sceptics. But it is also a serious attempt to replace two misguided philosophies, one on the left and one on the right. Norman attacks Labour's state centralism. More interestingly, he also questions the liberal market economics which not long ago seemed a prerequisite of Tory thinking. He's trying to offer something original and he is not the only one in his party to do so.

Nick Clegg, meanwhile, has got himself in a nasty mess over fees: he's more than unsettled by it. It's hard for him to make a philosophical case while everyone in the street outside is screaming U-turn. But he's right to try. There's an old joke about a speech by a politician who dreamed dreams. "They said it couldn't be done – and it couldn't. They said it would never happen – and it didn't," he tells his audience. But anyone can be cautious. It's those people brave enough to think new ideas who change the world.