Neil MacGregor's world history told through the things that time has left behind. Throughout this week, Neil has been looking at Europe's discoveries around the world and its engagement with different cultures during the 18th century - the European Enlightenment project. Today he describes what was happening in China during this period, as the country was experiencing its own Enlightenment under the Qianlong Emperor. He tells the story through a jade disc (called a Bi) that was probably made around 1500 BC and then written over by the Emperor himself. What does this prehistoric piece of jade tell us about life in 18th century China and how they explored their past? The historian Jonathan Spence and the poet Yang Lian find meaning in this intriguing object. Producer: Anthony Denselow