205. This consideration limits the kind of punishment which the state may justly inflict. It ought not in punishing to sacrifice unnecessarily to the maintenance of rights in general what might be called the reversionary rights of the criminal, rights which, if properly treated, he might ultimately become capable of exercising for the general good. Punishment therefore either by death or by perpetual imprisonment is justifiable only on one of two grounds; either that association of the extremest terror with certain actions is under certain conditions necessary to preserve the possibility of a social life based on the observance of rights, or that the crime punished affords a presumption of a permanent incapacity for rights on the part of the criminal. The first justification may be pleaded for the executions of men concerned in treasonable outbreaks, or guilty of certain breaches of discipline in war (on the supposition that the war is necessary for the safety of the state and that such punishments are a necessary incident of war). Whether the capital punishment is really just in such cases must depend, not only on its necessity as an incident in the defence of a certain state, but on the question whether that state itself is fulfilling its function as a sustainer of true rights. For the penalty of death for murder both justifications may be urged. It cannot be defended on any other ground, but it may be doubted whether the presumption of permanent incapacity for rights is one which in our ignorance we can ever be entitled to make. As to the other plea, the question is whether, with a proper police system and sufficient certainty of detection and conviction, the association of this extremest terror with the murderer is necessary to the security of life. Where the death-penalty, however, is unjustifiable, so must be that of really permanent imprisonment; one as much as the other is an absolute deprivation of free social life, and of the possibilities of moral development which that life affords. The only justification for a sentence of permanent imprisonment in a case where there would be none for capital punishment would be that, though inflicted as permanent, the imprisonment might be brought to an end in the event of any sufficient proof appearing of the criminal's amendment. But such proof could only be afforded if the imprisonment were so modified as to allow the prisoner a certain amount of liberty.