Even arch-rationalists believe in spirits

Richard Dawkins' The God Delusion has many passages that demand blind faith, but one of the most remarkable seems entirely natural and obvious - even when we know it isn't. 'Douglas, I miss you,' writes Dawkins. 'You are my cleverest, funniest, most open-minded, wittiest, tallest and possibly only convert.'

So where is 'Douglas', and why does Dawkins miss him? The second question is easier to answer. Douglas Adams (right) has been dead since 2001. He's not going to read the sentence I have quoted. So why does it seem so natural that Dawkins wrote it?

There is now a growing body of scientific evidence to suggest an answer: Dawkins writes to the ghost of Douglas Adams because his genes have given him a brain that makes it seem to make sense.

Our brains are not general-purpose thinking machines. They are built by our genes, as Dawkins would say, to process particular kinds of information, and these processes go on independently all the time.

One part of our mind is constantly generating stories about the other people around us - what they might do and how they might react. Another part distinguishes very clearly between live beings and dead things. When someone we love has died, the second part tells us they are gone. A corpse is a corpse. There is no-one there.

But if we care about the dead person, the part of our mind which generated expectations about them can't just stop working. We keep knowing what they would have liked, or laughed at.

So, encountering the spirit of a dead person is a result of these two experiences; it is a being with feelings and desires, but no body. Clearly it is a rationalisation to which even rationalists are not immune.

Andrew Brown presents Analysis: Revealing Religion, BBC Radio 4, March 20, 8.30pm ·