I am 35,000ft above Afghanistan. Beneath me, in the snowy hills, an insurgency is raging. In front of me sits David Miliband, Britain’s foreign secretary, who is leaning forward in his cream-coloured leather seat on a flight from Kabul to Dhaka, the capital of Bangladesh.

“Amartya Sen is a brilliant man,” remarks Miliband. “I think his argument that there is a fusion tradition – a liberal tradition that is concerned with social justice – is right. And I admire his work on capabilities, and on freedom as capability.”

At 42, Miliband is one of Britain’s youngest-ever foreign secretaries. As his musings on Amartya Sen [the Harvard academic and Nobel prize winner] suggest, he is also one of the most intellectual. The son of Ralph Miliband, a famous Marxist academic, he worked in a think-tank before serving as head of Tony Blair’s policy unit at 10 Downing Street. With the predictable British reaction to anybody who might seem a bit clever, Miliband’s colleagues in Downing Street nicknamed him “Brains”. He entered parliament in 2001 and rose swiftly. When Tony Blair was forced to step down as Labour leader and prime minister in 2007, despairing Blairites appealed to Miliband to run for the leadership against Gordon Brown. But Miliband resisted the temptation, and was rewarded with the job of foreign secretary when Brown formed a government.

A rise from the backbenches to one of the great offices of state in just six years demands real political skill. And a couple of days in Miliband’s company have convinced me that he is much more than a jumped-up intellectual. He has a politician’s knack for rarely saying the wrong thing – which makes him a tricky man to interview when there is a tape recorder running. He is also formidably energetic. His day began with an early morning visit to British troops in Kabul. It will end with a late-evening meeting in Dhaka, with politicians and businessmen. His schedule includes no “down time”. He doesn’t seem to sleep much, and his staff claim that they have to remind him to eat.