*31* X. But occasions often arise, when those duties which seem most becoming to the just man and to the "good man," as we call him, undergo a change and take on a contrary aspect. It may, for example, not be a duty to restore a trust or to fulfil a promise, and it may become right and proper sometimes to evade and not to observe what truth and honour would usually demand. For we may well be guided by those fundamental principles of justice which I laid down at the outset: first, that no harm be done to anyone; second, that the common interests be conserved. When these are modified under changed circumstances, moral duty also undergoes a change, and it does not always remain the same. *32* #Non-fulfilment of promises.# For a given promise or agreement may turn out in such a way that its performance will prove detrimental either to the one to whom the promise has been made or to the one who has made it. If, for example, Neptune, in the drama, had not carried out his promise to Theseus, Theseus would not have lost his son Hippolytus; for, as the story runs, of the three wishes[F] that Neptune had promised to grant him the third was this: in a fit of anger he prayed for the death of Hippolytus, and the granting of this prayer plunged him into unspeakable grief. Promises are, therefore, not to be kept, if the keeping of them is to prove harmful to those to whom you have made them; and, if the fulfilment of a promise should do more harm to you than good to him to whom you have made it, it is no violation of moral duty to give the greater good precedence over the lesser good. For example, if you have made an appointment with anyone to appear as his advocate in court, and if in the meantime your son should fall dangerously ill, it would be no breach of your moral duty to fail in what you agreed to do; nay, rather, he to whom your promise was given would have a false conception of duty, if he should complain that he had been deserted in his time of need. Further than this, who fails to see that those promises are not binding which are extorted by intimidation or which we make when misled by false pretences? Such obligations are annulled in most cases by the praetor's edict in equity,[G] in some cases by the laws. #Chicanery.#