There is a nice story about how Calvin Coolidge went to hear a clergyman preach on sin. "What did he say?" he was asked. 'He said he was against it', Coolidge replied. The history teacher or student, well used by now to the normal form of questions at GCSE, might be forgiven for asking mischievously, 'Ah, but he was a clergyman. Was he a reliable source? HOw do we know he wasn't biased?'