Further, we include under the idea of binding engagements not merely verbal promises, but also what are called ‘implied contracts’ or ‘tacit understandings.’ But this latter term is a difficult one to keep precise: and, in fact, is often used to include not only the case where _A_ has in some way positively implied a pledge to _B_, but also the case where _B_ has certain expectations of which _A_ is aware. Here, however, the obligation is not so clear: for it would hardly be said that a man is bound to dispel all erroneous expectations that he may know to be formed respecting his conduct, at the risk of being required to fulfil them. Still, if the expectation was such as most persons would form under the circumstances, there seems to be some sort of moral obligation to fulfil it, if it does not conflict with other duties, though the obligation seems less definite and stringent than that arising out of contract. Indeed I think we may say that Justice is generally, though somewhat vaguely, held to prescribe the fulfilment of all such expectations (of services, etc.) as arise naturally and normally out of the relations, voluntary or involuntary, in which we stand towards other human beings. But the discussions in the preceding chapter have shown the difficulty of defining even those duties of this kind which, in an indefinite form, seemed certain and indisputable: while others are only defined by customs which to reflection appear arbitrary. And though while these customs persist, the expectations springing from them are in a certain sense natural, so that a just man seems to be under a kind of obligation to fulfil them, this obligation cannot be regarded as clear or complete, for two reasons that were given in the last chapter; first, because customs are continually varying, and as long as any one is in a state of variation, growing or decaying, the validity of the customary claim is obviously doubtful; and secondly, because it does not seem right that an irrational and inexpedient custom should last for ever, and yet it can only be abolished by being “more honoured in the breach than in the observance.”