Philip Ball tells the story of Ramon Llull, the medieval prophet of computer science. During the time of the Crusades Llull argued that truth could be automated and used logic over force to prove the existence of the Christian God. It was a dangerous idea that got him thrown into prison and threatened with execution but today he is hailed, not as a prophet of the Christian faith, but of computer science. Philip Ball talks to historian Pamela Beatty of the University of Louisville in Kentucky about Ramon Llull's life and times in 13th century Catalonia, and to mathematician and Simonyi Professor for the Public Understanding of Science at Oxford University, Marcus du Sautoy, about the legacy of Llull's ideas in combinatorics, a branch of mathematics that explores how we can arrange a set of objects. Note: Many thanks to Carter Marsh & Co for the recording of mechanical sounds.