Nelson Mandela is 91, his athlete's body is frail and he has all but disappeared from public life.

As an icon, however, Mandela is more robust than ever. The gilded and the glamorous are gathering in London and New York to celebrate the anniversary of his birth, and at home there is saturation coverage of the day and of his foundation's initiative to get South Africans to set aside 67 minutes for community service.

Perhaps it is not surprising that, at an awkward and uncertain moment in our transition, when the ship of state seems to be drifting leaderless into the future, we turn to a figure whose legacy is a lesson in clarity of vision and purpose. That is the case just as much in global capitals, roiled by economic turbulence, as it is here.

Madiba has, in a sense, returned to centre stage in our national imagination because we need him. But there is an unattractive side, too, to the uses to which we put him.

The tussle with the ANC over who can properly claim to be heir to his legacy is not just a demonstration of its renewed power, but of what people are prepared to subject the man to himself as they grasp at his mantle.

It was most clearly demonstrated when he was flown to the Eastern Cape to endorse President Jacob Zuma ahead of the April elections. It was evident, too, in the election to Parliament of his less-than-impressive grandson, Mandla.

The parade of stars outside his Houghton door hoping to borrow a little of his light can seem to reduce him to a fairground attraction just as surely as the awful grinning statue that presides over the high-end retail ghetto of Nelson Mandela Square.

Fortunately he outshines all of that -- in the extraordinary resonance of his speeches from the dock, the force of his charisma shining through photographs more than half-a-century old and, most remarkably of all, his emergence from prison in 1990 as a statesman fully formed. Even after he quit office, his activism on HIV/Aids stood in sharp rebuke to the madness of Thabo Mbeki's denialism.

That is the force that we must harness, not by wheeling our living icon out for the rich or the politically needy, not by genuflecting at his feet, but by measuring the potential difference between what he stands for and the place we find ourselves in, and putting that voltage to work.

Here is a man who was born just five years after the implementation of the 1913 Natives' Land Act and who has lived to see the colossal failure of the state's redistribution policies; who helped to radicalise the ANC of the 1940s through its Youth League and who has lived to see Julius Malema claim that legacy; who was chief volunteer during the defiance campaign and who has lived to see service delivery protests across the country; who brought us freedom while other liberation heroes -- such as Robert Mugabe -- were stealing the fruits of revolution. All that is cause for sadness and for anger.

There is cause for celebration, too, of course, in a culture of tolerance and human rights, in new houses springing up around our cities, in law courts where the young Mandelas of today can exercise their talents in full. It is in the distance between that anger and that joy that we celebrate Madiba's birthday -- not as a commemoration but as a command to close the gap and to realise the future that he has bequeathed us.