An acutely difficult question of principle arises when a person commits an offence because the police incite him to do so. For the civil libertarian, “undercover agents” (as the prosecution will seek to call them) usually go by another and nastier name: secret police. Even for authoritarians entrapment is of dubious value, because policemen find the crimes they have themselves incited particularly easy to solve—and if we condone entrapment too readily we risk having a police force which expends its energy on committing new crimes instead of solving old ones. But regrettably, the use of entrapment is something effective crime prevention occasionally requires.