Most laws and rules are backed up by some form of punishment if not followed, even if the punishment is not fully regularized. When is punishment legitimate? And what kind of punishment is legitimate? John Locke gives an answer to that question in section 8 of his 2d Treatise on Government: “On Civil Government”:
And thus, in the state of nature, one man comes by a power over another; but yet no absolute or arbitrary power, to use a criminal, when he has got him in his hands, according to the passionate heats, or boundless extravagancy of his own will; but only to retribute to him, so far as calm reason and conscience dictate, what is proportionate to his transgression, which is so much as may serve for reparation and restraint: for these two are the only reasons, why one man may lawfully do harm to another, which is that we call punishment. In transgressing the law of nature, the offender declares himself to live by another rule than that of reason and common equity, which is that measure God has set to the actions of men, for their mutual security; and so he becomes dangerous to mankind, the tye, which is to secure them from injury and violence, being slighted and broken by him. Which being a trespass against the whole species, and the peace and safety of it, provided for by the law of nature, every man upon this score, by the right he hath to preserve mankind in general, may restrain, or where it is necessary, destroy things noxious to them, and so may bring such evil on any one, who hath transgressed that law, as may make him repent the doing of it, and thereby deter him, and by his example others, from doing the like mischief. And in this case, and upon this ground, every man hath a right to punish the offender, and be executioner of the law of nature.
John puts down serious limitations on punishment: First, punishment is only legitimate when someone has violated the preexisting “law of nature,” not when someone has violated an arbitrary rule that has been established against their opposition or otherwise without their consent, and without any promise they have freely made coming into play. The law of nature is given a new description in this section as “the tye, which is to secure them from injury and violence.”
Second, punishment should not be in proportion to the extreme anger it is easy to feel when someone crosses one of us in some regard. Third, punishment should be governed by the three legitimate purposes of punishment:
Although John uses the word “retribute” he seems to be excluding simply “getting back at someone” as a legitimate ground of punishment–a ground or motive that is sometimes called “retribution.”
By contrast, reparation, which improves the condition of the person originally harmed from its low ebb after that injury is an excellent purpose of punishment. It is important to search for ways to punish that accomplish at least some reparation at the same time that they work toward restraint or deterrence.
John’s concern about legitimate vs. illegitimate punishment is clear in his care to make the case for punishment at all instead of no punishment: those who violate “reason and common equity” are “dangerous to mankind,” and “every man upon this score, by the right he hath to preserve mankind in general, may restrain, or where it is necessary, destroy things noxious to them.” The concern to justify punishment that John exhibits here is a model for all of us. A minimum (though often far from sufficient) requirement for justifying punishment is this: Whenever one argues for punishment of an individual, or executes punishment on one’s own account, one should be prepared to point to some significant bad consequence that would occur if there were not a policy of punishment in a situation like that. That bad consequence needs to be “bad” in a reasonably objective sense, and greater than the badness of the punishment itself. If nothing bad would happen in the absence of punishment in a given type of situation, punishment should not be undertaken. (Note that this is a different standard than “absence of punishment in this one particular instance would do no harm, taking people’s expectation of the probability of punishment in similar future instances as fixed.”)