astrologers. You know there are many scientists who are deeply religious, even though the evidence for God is not strong. And look at Intelligent Design, which is very popular in the United States. Is the human race becoming more or less rational? We're not too bad at looking for internal consistency, but we are very bad indeed at looking at the evidence. Especially on complex things. Also it's jolly difficult. Do you realise that the first proper clinical trials in relation to treating medicine only happened 150 years ago? Before that doctors were treating people with blood-letting and such like without ever trying to find out if it worked. And in relation to witchcraft, people don't test whether the witchcraft is working or not. You touch wood for good luck. Why? I don't know why. I just do. For the same reason I never boast of being well because the gods won't like it. But you don't believe in any gods. I know. This is an edited extract from 'Six Impossible Things Before Breakfast', by Lewis Wolpert (Faber). The book is available for 11.99 + 1.25 p&p from Telegraph Books on 0870 428 4115