A small clay tablet made in Mesopotamia about 5,000 years ago. It is covered with sums but also some of the world's earliest writing - about beer rationing. Neil MacGregor, Director of the British Museum, looks at how humans invented writing to cope with the increasing sophistication of trade and commerce. Including contributions from philosopher John Searle, on what the invention of writing does for the human mind, and Britain's top civil servant, Sir Gus O'Donnell, on the beginnings of bureaucracy. (Duration: 14 mins)