Imagine Martin Luther King never had a dream. Imagine that instead of thinking outside the narrow confines of his time and place he had resolved to work only within them. Imagine he had, instead, risen to the steps of the Lincoln monument and announced a five-point plan that he thought he could both sell to the black community and win a majority for in both houses of Congress, that would bring civil rights legislation that one step closer.
Imagine, in short, that he had been just a realist rather than an idealist. Who could have blamed him? The year that four black girls were bombed to death in their Sunday school class in Birmingham, Alabama, was arguably not the most propitious time to head off in search of a mythical future when his children would be judged "not by the colour of their skin, but by the content of their character".

It was one of the most memorable speeches ever delivered. And yet, if he gave it today he would be called naive. Commentators might praise his oratory, but the consensus would be that King should stop dreaming, wake up and get a grip on reality. Realism has no time for dreamers. Realism comes with the adjectives "hard-headed", "unpalatable" and "harsh". If some are to believed, it not only sets out the parameters of our politics. It also defines the outcome. When they say face up to political reality, they really mean accept political destiny.

For proof, look no further than the polls about our entry to the euro. Between July 1999 and December last year, the percentage of those against the euro fell slightly, from 62% to 58%. Meanwhile, when asked the question: "Leaving aside how you would vote, in 10 years time which of the following do you think is most likely?" the number of those who thought Britain would be in the euro anyway leapt dramatically: from 36% in July 1999 to 62% in December.

In other words most British people still do not want the euro, but most think we are going to end up with it anyway. A growing number has decided that, irrespective of their wishes in a referendum, the euro's arrival on our shores is inevitable. I'm not focusing here on the relative benefits or drawbacks of signing up to the single currency, but on what these polls say about the powerlessness and dislocation in our political culture. For it suggests that many have decided there are forces at work that are far more powerful than whatever we may decide. In short, democracy has its place, but it cannot compete with destiny.

It explains why, in western Europe, those who do vote are now increasingly likely to opt for parties offering extreme solutions: there can be no tinkering with fate. It also offers some reasons as to why voter turnout, even in newer and hard-won democracies like South Africa has plunged so dramatically - why bother if it will make no difference?

Indeed this feeling of political impotence is far more prevalent in poorer countries, where economic priorities are so clearly decided not by the needs or wants of their citizens or even their elected governments, but by the IMF, World Bank or the WTO. Given a choice of either facing down the might of international capital or imposing market reforms on reluctant populations, neo-liberalism does gain an air not only of invincibility but also inevitability.

It is not difficult to see where this came from. In Britain, with Labour out of power for 18 years a large proportion of both its members and supporters wanted it to stop dreaming, wake up and get back into government. By the end of their electoral exile realism had become at least as much of a dogma for New Labour as socialism had been for the old. Internationally, since the end of the cold war we have been left with only one superpower which is intent on getting precisely what it wants. Nowhere has this been more evident than during the ongoing war in Afghanistan, where any suggestion of subjecting the United States to international law in its military conduct or treatment of prisoners is regarded as laughable. Indeed, the British government's entire foreign policy strategy is based on the notion that American aggression is inevitable - all we can hope to do is hold it back from its excesses.

The trouble is that destiny and democracy do not sit well together. Politics demands action while destiny denies it. Democracy depends on the notion that through our participation in the process, either individually or collectively, we can make a difference. Destiny, meanwhile, states that the outcome is already a foregone conclusion and that participation is meaningless. Sooner or later the two have to face off.

Take Argentina. According to Pedro Lacoste, an Argentine economic consultant, the assumption behind the decision of Argentina's currency board's plan to peg the peso to the dollar in 1991 was that "globalisation was unstoppable". For six years the country's interest rates were effectively set by the United States Federal Reserve, leaving the government little leeway to redefine its priorities.

When times got hard and people wanted a change of direction, the electoral system offered them only realists who embraced a reality they did not want. Last October, in a congressional election, 40% of voters cast blank or spoiled ballots. In December alone they got rid of two presidents. For the past two months they have been trashing banks and beating up politicians in the street. Suddenly it is not globalisation but the Argentine people who look unstoppable.

But while destiny may be the antithesis of democracy, realism itself is the kernel of political activity. King would have achieved little if he had been dreaming every day. But there is a difference between acknowledging reality and accepting it, between trying to shape it and readily submitting to fate.

The left is no use to anyone unless it can be pivotal in the movement for social change. It has to position itself where it thinks it can make a difference. But that demands not a malleability of principle but a flexibility in application. A willingness to engage with the world as it is rather than as we would like it to be. Similarly, it has to be competent to be credible. Most people derive their political views from their day-to-day experiences. Those in power have to fix the banal before they can start talking about the abstract. People do not want to hear their council leaders talk about world peace if they cannot even get the rubbish collected on time. Nor do they want to hear about healing the world while waiting three hours for a train. Competence matters. That, in fact is one of the biggest problems with New Labour at the moment. It claims it dumped ideology in favour of "what works" - but nothing much, from the railways to the hospitals, is working. So they, and we, are left with nothing.

Realism is vital, but realism also has its limits. Without realism there is only the utopian; but without idealism or ideology there is no vision or ambition. If politics is the art of the possible, then radicalism must entail the desire to imagine other possibilities.