185. The idea of punishment implies on the side of the person punished at once a capacity for determination by the conception of a common or public good, or in other words a practical understanding of the nature of rights as founded on relations to such public good, and an actual violation of a right or omission to fulfil an obligation, the right or obligation being one of which the agent might have been aware and the violation or omission one which he might have prevented. On the side of the authority punishing, it implies equally a conception of right founded on relation to public good, and one which, unlike that on the part of the criminal, is realised in act; a conception of which the punitive act, as founded on a consideration of what is necessary for the maintenance of rights, is the logical expression. A punishment is unjust if either element is absent; if either the act punished is not a violation of known rights or an omission to fulfil known obligations of a kind which the agent might have prevented, or the punishment is one that is not required for the maintenance of rights, or (which comes to the same thing), if the ostensible rights for the maintenance of which the punishment is required are not real rights, are not liberties of action or acquisition which there is any real public interest in maintaining. 186. When the specified conditions of just punishment are fulfilled, the person punished himself recognises it as just, as his due or desert, and it is so recognised by the onlooker who thinks himself into the situation. The criminal, being susceptible to the idea of public good, and through it to the idea of rights, though this idea has not been strong enough to regulate his actions, sees in the punishment its natural expression. He sees that the punishment is his own act returning on himself, in the sense that it is the necessary outcome of his act in a society governed by the conception of rights, a conception which he appreciates and to which he does involuntary reverence.