To solve one of the great mathematical problems of his day, Alan Turing proposed an imaginary programmable calculating machine. But the idea of actually producing a 'thinking machine' did not crystallise until he and his brilliant Bletchley Park colleagues built devices to crack the Nazis' Enigma code, thus ensuring the Allied victory in the Second World War. In so doing, Turing became a champion of artificial intelligence, formulating the famous (and still unbeaten) Turing test that challenges ...