 Theorists have distinguished ‘positive’ and ‘negative’ forms of retributivism. Positive retributivism holds that an offender’s desert provides a reason in favour of punishment; essentially, the state should punish those found guilty of criminal offences to the extent that they deserve, because they deserve it. Penal desert constitutes not just a necessary, but an in-principle sufficient reason for punishment (only in principle, however, since there are good reasons — to do with the costs, both material and moral, of punishment — why we should not even try to punish all the guilty). Negative retributivism, by contrast, provides not a positive reason to punish, but rather a constraint on punishment: punishment should be imposed only on those who deserve it, and only in proportion with their desert. Because negative retributivism represents only a constraining principle, not a positive reason to punish, it has been employed in various mixed accounts of punishment, which endorse punishment for consequentialist reasons but only insofar as the punishment is no more than is deserved (see s. 6 below).