 Retributivist theory holds not only that criminal guilt is required for punishment, but that the appropriate type and amount of punishment is also determined by the crime itself. Traditionally this is the heart of the ancient injunction “an eye for an eye”. Kant supports this measurement for punishment because all other measurements bring into consideration elements besides strict justice (6:332), such as the psychological states of others that would measure the effectiveness of various possible punishments on deterrence. As a principle, retribution grounds but does not specify the exact punishment. Kant recognizes that “like for like” is not always possible to the letter, but believes that justice requires that it be used as the principle for specific judgments of punishment.