On a chilly Friday afternoon, a shirt-sleeved Neil MacGregor is in his element, beaming and basking amid the coated throng in his Enlightenment Gallery. People are handling objects at one of the curator-manned tables, puzzling at the labels in the large cases, gazing at the statuary. They're like children playing with a new toy, trying all the knobs to see what they do and then having another go. "You see, it works!" MacGregor exclaims, clasping his hands in something like glee, as a man with a child pouched on his chest brushes past to get to a case of archaeological fragments. "They're discovering!"
Yet this permanent exhibition is just as it would have looked 200 years ago. The newest British Museum gallery, around which MacGregor is showing me so enthusiastically, is also the oldest, sited as it is in the glorious neoclassical room that once held the King's Library, George III's collection of 600,000 antiquarian books. The display is set in 1800, an age of discovery that MacGregor, the museum's director, has set out to recreate.
And not just here in Bloomsbury, in London. On Monday, in tandem with a spectacular new acquisition, he will announce the latest step of that mission, a programme to take objects on tour around the country. But his broader plans for the museum place it, not just at the centre of Britain, but at the centre of the world as the leading international museum. That aim was given the highest stamp of approval when Tony Blair, opening the Enlightenment Gallery in December, announced that the Government would give the museum £1m for its Africa Project.
That was a glittering end to the museum's 250th anniversary year, and one that MacGregor couldn't have dreamt of at the start of 2003, a year that saw an extraordinary turnaround in fortunes.
"What we want to show in this room is the way in which the study of things in a museum shapes our understanding of the world," he says, showing me a long, detailed label. "These labels demanded a certain amount of input from the reader, and that's what we've done here - you bring elements of your own experience to what you see.
"The point was to put into one room what Europeans could know about the world in 1800 - the organisation, the comparison and the study of things that actually allowed the Europe of the time to completely rewrite the way the world functions."
In 1700, such a room would have explained the systems of the world only by the Bible and the texts of the classical world. Then empirical study based entirely on research taught by the Enlightenment in the mid-18th century slowly destroyed the authority of those texts. "It must have been terrifying for them," MacGregor says.
The British Museum, both a product of the Enlightenment and a conduit of it, was the first national museum in the world. Created by an Act of Parliament - "So it's the people's museum, not a royal possession," MacGregor says - it was based on material sold to the nation by the physician, scientist and collector Sir Hans Sloane, who died aged 93 in 1753, the year of the museum's foundation.
Sir Hans had been George II's physician and had attended Pepys on his death bed. He made a fortune importing cocoa from the West Indies to make chocolate, and used his money to indulge his passion for collecting. He was an indefatigable traveller throughout Europe, and knew Newton and Voltaire. He had offered his collection to France, Sweden and Germany; he didn't mind where it ended up, as long as everyone had free access to it.
The 18th century was a neoclassical age, but the sculptures are Roman copies of antique Greek works, because that is what visitors would have known as classical before Lord Elgin brought the real thing from the Parthenon in Athens in 1802.
We stand in front of the giant bowl that is the gallery's centrepiece, supposedly a piece of Roman marble, although most of it was done by Piranesi in the 1760s. "And this marvellous Roman statue," MacGregor says: "It had been a Dionysus, and when it was restored in that Age of Reason 250 years ago, they turned it into an Apollo because Dionysus was not a god of reason, and Apollo was."
What wasn't known until 1802 was that Roman sculpture was a bowdlerised version of ancient Greek - contrary to popular belief, the Romans were prudes. Then came the Elgin Marbles, "and for the first time ever people knew they were looking at Greek sculpture, real Greek sculpture," MacGregor says. "And they were stunned because they're so naturalistic; the sheer physical vigour and muscular delineation of the Greek work is so utterly amazing."
So the Marbles are on the other side of the museum, evidence of how thinking changed again after the Enlightenment. The Italian neoclassical sculptor Canova came to see them and declared them to be reale carne - real flesh.
"We are all heirs of the Greeks," MacGregor says, in response to the inevitable question about returning the Elgin Marbles to Athens. "Nobody in the ancient world talks about the Parthenon Frieze, not once, and only in London do they become great works of art."
To the north side of the gallery is the 18th century's "other" world: the discoveries made by Captain James Cook and the explorer and naturalist Joseph Banks, often funded by commissions from the museum. From there, MacGregor takes me through the Aztec Gallery into another new space, the Wellcome Gallery of Life and Death.
"And here, the non-European world isn't 'other' any more, it's part of us," he says. "It's about how the world addresses the common problems of humanity, and how we negotiate our wellbeing." North American Indians developed relationships with animals to avoid famine; Papua New Guineans negotiated with neighbours to avoid war; in South America, the natural ally is the land, hence the importance of harvest-based festivals.
The 21st-century European notion of wellbeing lies in prescription drugs, and a case half the length of the gallery has all the prescribed pills the average human will consume in a lifetime - 14,000 for a man, and 17,000 for a woman "because they live so much longer". One case is devoted to how Aids has become a universal issue, with a political message about effective but expensive Western drugs being unaffordable in Third World countries. "I think this gallery shows that the museum is moving into a new place," MacGregor says.
He had arrived in September 2002 with the museum in turmoil, still smarting from its first-ever curatorial strike that summer. The overstaffed institution was divided into two camps, curatorial and administrative. The Great Court development was controversial and difficult to assimilate into the management system; the ambitious scheme for a £100m study and storage centre had been cancelled; and the finances were spinning into the red.
So the anniversary year dawned bleakly. "You can't celebrate in an organisation when you're still asking people to leave," MacGregor says. "You can't talk encouragingly about the future of the place to a group of people, some of whom are not going to be part of that future.
"Luckily, the actual birthday didn't happen till 7 June, and by then we were in a very different position." The debt was cancelled, at the cost of 130 jobs out of 1,050, or 14 per cent of the staff. The museum is now organised behind a single director.
MacGregor hadn't been destined for a curatorial career. The son of a Scottish barrister turned GP, he did degrees in languages, philosophy and the law at Oxford, the Ecole Normale Supérieure in Paris and Edinburgh University. He was even called to the Bar before he wrenched himself back to his first love, art history, and continued his studies at the Courtauld.
MacGregor came under the influence of two art historians who were to achieve fame for other reasons: Sir Anthony Blunt, who was unmasked as a Cambridge spy in 1979, and the 1984 Booker-winning novelist, Anita Brookner. "From Blunt, I got the link between a work of art and the intellectual world the work came from. I worked with him on Poussin. With Anita, it was the fact that a work of art is a means of defining your own experience. I was terribly lucky to have two such completely different views of the engagement with an object."
Too old to embark on a doctorate that would have given him a specialism - "I actually know nothing, you see" - he taught art history at Reading University for four years before becoming the editor of The Burlington Magazine. Then, in 1987, he was the unexpected choice to succeed Michael Levey at the National Gallery. He took to the work as if it were the most natural career move, handling the controversy over the design of the gallery's extension with some skill, and opening the new wing in 1993 to general approval.
At the museum, MacGregor had the tough task of realigning the staff and sacking many of them. He created two new deputy directorships, one curatorial and one administrative. "There had to be one * * person responsible, and that can only be the director. Absolutely."
His predecessor, Robert Anderson, had blamed government underfunding for the museum's plight, but MacGregor puts the cause further back, to the departure of the British Library to its new building at St Pancras in 1998. "It's very difficult to grasp the scale of disruption of the British Library moving. It was like the separation of Siamese twins, who had been born conjoined in 1753. Then it was like having plasterers in your house for five years and everything having to be moved, re-moved, dusted and dusted again. And building the Great Court caused enormous financial and administrative strains. The real problem here was simply exhaustion."
MacGregor was already 56 when he took over the British Museum after 15 years running the National Gallery, during which time he had won unassailable respect as a scholar and administrator. Five years ago, quietly, he turned down a knighthood.
He could have seen out his career happily in Trafalgar Square. Or, if he had wanted to boost his pension, as a multilingualist he could have earnt five times as much working in Europe. "But this is the most important museum in the world, with the richest, most complete collection in the world," he says. "What I find fascinating is the role a collection like this can play now, in a city such as London has become - a complete world city, and in a world that's become so close and so small."
So he came ready with a big idea, a well-developed view of the international stature of the British Museum. His Enlightenment approach of teaching through discovery was first manifested at the National Gallery, where he had the collections rehung in a chronological narrative, rather than by country or school. "The one place where, in an hour, you can get a measure of what European culture has achieved in terms of painting is the National Gallery," he says. "The same is true of the British Museum - what mankind has actually done over four or five thousand years."
Only when the financial deficit and job cuts were behind him could MacGregor begin to think positively about the anniversary, which he chose to mark not so much with the exact 7 June date (though he had a series of events on that Saturday), but with the opening of the Enlightenment Gallery. The outline plan was in place, but the detail could now be developed.
Then, on 12 April last year, the entire vision was suddenly cast in a new and sharp focus - "a black-edged relief" - while he was in Iran discussing a planned Persepolis exhibition. That day, looters broke into the Baghdad Museum and devastated the primary source of learning about the birth of civilisation. "We realised that something had to be done, quickly, and the British Museum was the only institution that could do it," he says. "Suddenly, what this place is for became blindingly clear."
He flew back the following day, a Sunday, and drove straight from Heathrow airport to Bloomsbury. He rang the Prime Minister and got tanks - American ones - deployed in front of the Baghdad Museum, and secured an anonymous benefactor to fund a rescue mission. John Curtis, the museum's Near East curator, rang friends in Iraq for detailed information, and MacGregor used his reports to mobilise French, German, Italian, Russian and American scholars and get United Nations support. Only under the auspices of the British Museum could colleagues from France, Germany, the United States and Britain work together in the political climate that prevailed after the war against Iraq.
A press conference scheduled for the Tuesday to reveal the 250th birthday plans was exploited to announce the mobil- ising of governments, agencies and Mesopotamia scholars worldwide into a task force to help the Iraqi curators deal with the catastrophe of the looting. "It was a recall to our international responsibilities of a particularly disagreeable and forceful kind," MacGregor says. Within a week, British Museum curators were on a flight to Baghdad, and Iraqi conservators are now in Bloomsbury learning the technological developments of their craft that have been denied them by sanctions.
Throughout the Saddam years, British Museum curators had been visiting Baghdad with a freedom that politicians and diplomats could not, and Iraqi colleagues had become close friends. It is part of the museum's mission to draw Iraq back into the international museum family.
It is about to do the same for Sudan now that the civil war there is over. This is separate from the Africa Project, which is itself not to be confused with Africa 2005, a multimuseum joint exhibition at the Hayward Gallery in which the British Museum is also taking part. All these initiatives chime with last week's announcement of the Prime Minister's Commission for Africa.
MacGregor says: "Our role in that is to work with colleagues in Africa, particularly at museums, to help to develop their sense of history" - just as the British Museum had done for Europeans in the 18th and 19th centuries. "We need to engage Africans in the presentation, not just of objects, but also of the non-material culture that surrounds the objects - storytelling, music, and so on - that is right for them.
"But you can't make sense of contemporary Africa without thinking about what part the world plays in Africa. Nor can you make sense of contemporary London without thinking about the African population here. There's no 'other' any more," MacGregor says.
"We tend to forget that it was Newton and Darwin who completely rewrote the way the modern world understands itself, and it's a British achievement, an achievement that comes out of this endless comparison and collecting. So we are renewing that mission, comparing with others and sharing our collections - the Enlightenment still, but worldwide."