WHEN criminals are deprived of their liberty, as by imprisonment (or banishment, or very tight control in the community), their ability to commit offenses against citizens is ended. We say these persons have been “incapacitated,” and we try to estimate the amount by which crime is reduced by this incapacitation. Incapacitation cannot be the sole purpose of the criminal justice system; if it were, we would put everybody who has committed one or two offenses in prison until they were too old to commit another.  And if we thought prison too costly, we would simply cut off their hands or their heads. Justice, humanity, and proportionality, among other goals, must also be served by the courts.