As a teacher I have sometimes offered a million pounds to any student who can form any one of the following beliefs: that they can fly; that they were born on the moon; or that sheep are carnivorous. Needless to say, I have never had to pay up. The Queen in Lewis Carroll’s *Through the Looking Glass* might have been able to believe six impossible things before breakfast, but that is a feat few of us can match. In fact, it is doubtful whether the formation of belief is under voluntary control at all. Adopting a belief seems to be more like digesting or metabolizing and rather unlike looking or speaking—it seems to be something that happens to one rather than something that one does.