Mr Greenspan was considered such a guru that last year Senator John McCain suggested putting him in charge of a committee on tax reform, adding: “If he's alive or dead it doesn't matter. If he's dead, just prop him up and put some dark glasses on him.” But Mr Greenspan's reputation is now on the slide and Mr McCain has reinvented himself as a champion of regulation – and is denouncing the “corruption and unbridled greed that has caused a crisis on Wall Street”.

This kind of ideological whiplash is what happens when an intellectual bull market crashes. The current financial crisis can be traced to three of the central ideas of the Reagan- Thatcher era: the promotion of home ownership, financial deregulation and a fervent faith in the market. Each of these ideas did sterling service for 30 years, increasing prosperity and freedom. But pushed too far – and combined – they have created a disaster.

The subprime mortgages that are
at the heart of the current financial crisis expanded the dream of home ownership to people who could not afford the financial burdens they were taking on. In April 2005 Mr Greenspan praised subprime mortgages for helping to widen home ownership and hailed them as “representative of the market responses that have driven the financial services industry throughout the history of our country”.

Investment bankers, the shock- troops of the Reagan-Thatcher revolution, were allowed to bet their banks on this new market, because regulators and politicians believed so firmly in the magical and self- regulating qualities of the market.

The same process of intellectual overshoot happened with other signature ideas of the Reagan- Thatcher era: privatisation, scepticism about environmentalism and democracy promotion.

When Thatcherites first mooted privatisation it was derided as an impractical dream. But early triumphs with airlines and telecommunications in Britain created a vogue that spread round the world. That emboldened the privatisers to take on new and harder challenges, such as the UK's railways. But failure there led to a backlash.

Similarly, when the conservative era started, foreign military engagements were out of fashion in the west. But Britain's Falklands war and the American invasion of Grenada began to change this. During the 1990s, a series of successful military interventions – the first Gulf war, Bosnia, Kosovo, Sierra Leone – made Anglo-American political leaders much more relaxed about the use of military force. Too relaxed. The horrors that have followed the invasions of Iraq and Afghanistan will mean that the intellectual pendulum will now swing in the opposite direction.

The idea of democracy promotion has gone through a similar boom-and-bust cycle. The collapse of the Soviet empire in 1989 was regarded as the ultimate vindication of the rightwing universalism that argued that all people did indeed desire democratic, free-market systems. Advocates of the globalisation of democracy became much more assertive. A policy of providing moral support to anti-Soviet dissidents in Europe in the 1980s had, by 2003, transmogrified into a policy of exporting democracy by force of arms to the Middle East.

Once again, a successful idea has been pushed to its logical conclusion – and beyond. And once again an intellectual backlash has begun.
David Cameron, the leader of Britain's Conservatives, captured the new conventional wisdom when he said recently: “We should accept that we cannot impose democracy at the barrel of a gun. We cannot drop democracy from 10,000 feet.”

Both Ronald Reagan and Margaret Thatcher favoured growth over greenery. Mrs Thatcher hailed “the great car-owning democracy” and Reagan mused that trees were a major source of pollution. But in Britain – and, to a lesser extent, the US – climate change has turned conservatives green with anxiety. Mr Cameron, a reliable intellectual weather-vane, ostentatiously cycles to work and has adopted a tree as the symbol of the new Tory party. Mr McCain takes climate change very seriously.

The ideological roots of the conservative era lay in a reaction to the excesses of the Keynesian consensus. Now that the intellectual cycle has swung so decisively against the rightwing ideas of the Reagan-Thatcher era, it is bound to overshoot in the other direction. The joys of government regulation will quickly pall. In a few years' time, nostalgia will set in for the go-go years on Wall Street and for the bracing moral certainties of neoconservatism.

Audacious intellectual investors should now be sniffing around. Quite soon ideas such as deregulation and democracy promotion will be a buy.