In the United States, a criminal defendant can show himself to be not guilty of the crime of which he is accused by showing that he was “entrapped” by agents of the government. Entrapment is not merely a mitigating factor to be considered in sentencing a defendant who is acknowledged to be guilty of a crime; it is not treated in the law in the way, for instance, a defendant’s poverty is often treated, as providing reason to give a guilty defendant a lighter sentence than an equally guilty but rich defendant, who also stole for money. Rather, entrapment constitutes a complete defense: An entrapped defendant is simply not guilty of violating the law under which he is being prosecuted. Those who think that there would be something wrong with a...