If a prisoner walks into my consulting room in the prison with a stick, he’s a sex offender; if he has gold front teeth, he’s a drug dealer; and if he’s reading Wittgenstein, he’s in for fraud: for it is virtually a law of our penal establishments that fraud and philosophy are what literary theorists like to call metonymic.

When you work in a prison as I do, white-collar criminals come as something of a light relief. At last someone with whom you can have a disinterested, abstract intellectual conversation! No more talk about alcoholic mothers, brutal stepfathers, and terrible childhoods as the fons et origo of car theft: it’s straight to the meaning of life, the social contract and the metaphysical foundation of morality (they always say that there isn’t any). It’s almost like being a student again, up till three in the morning, trying to work out what no man has ever worked out before. ...