§5. We have next to treat of Good Faith, or Fidelity to Promises; which it is natural to consider in this place, because, as has been seen, the Duty of Law-observance has by some thinkers been based upon a prior duty of fulfilling a contract. The Social Contract however, as above examined, seems at best merely a convenient fiction, a logical artifice, by which the mutual jural relations of the members of a civilised community may be neatly expressed: and in stating the ethical principles of Common Sense, such a fiction would seem to be out of place. It must, however, be allowed that there has frequently been a close historical connection between the Duty of Law-observance and the duty of Good Faith. In the first place, a considerable amount of Constitutional Law at least, in certain ages and countries, has been established or confirmed by compacts expressly made between different sections of the community; who agree that for the future government shall be carried on according to certain rules. The duty of observing these rules thus presents itself as a Duty of Fidelity to compact. Yet more is this the case, when the question is one of imposing not a law, but a law-giver; whose authority is strengthened by the exaction of an oath of allegiance from his subjects generally or a representative portion of them. Still, even in such cases, it can only be by a palpable fiction that the mass of the citizens can be regarded as bound by an engagement which only a few of them have actually taken.
We may begin our examination of the duty of Keeping Promises by noticing that some moralists have classified or even identified it with Veracity. From one point of view there certainly seems to be an analogy, between the two; as we fulfil the obligations of Veracity and Good Faith alike by effecting a correspondence between words and facts---in the one case by making fact correspond with statement, and in the other by making statement correspond with fact. But the analogy is obviously superficial and imperfect; for we are not bound to make our actions correspond with our assertions generally, but only with our promises. If I merely assert my intention of abstaining from alcohol for a year, and then after a week take some, I am (at worst) ridiculed as inconsistent: but if I have pledged myself to abstain, I am blamed as untrustworthy. Thus the essential element of the Duty of Good Faith seems to be not conformity to my own statement, but to expectations that I have intentionally raised in others.
On this view, however, the question arises whether, when a promise has been understood in a sense not intended by the promiser, he is bound to satisfy expectations which he did not voluntarily create. It is, I think, clear to Common Sense that he is so bound in some cases, if the expectation was natural and such as most men would form under the circumstances: but this would seem to be one of the more or less indefinite duties of Justice, and not properly of Good Faith, as there has not been, strictly speaking, any promise at all. The normal effect of language is to convey the speaker's meaning to the person addressed (here the promiser's to the promisee), and we always suppose this to have taken place when we speak of a promise. If through any accident this normal effect is missed, we may say that there is no promise, or not a perfect promise.
The moral obligation, then, of a promise is perfectly constituted when it is understood by both parties in the same sense. And by the term `promise' we include not words only, but all signs and even tacit understandings not expressly signified in any way, if such clearly form a part of the engagement. The promiser is bound to perform what both he and the promisee understood to be undertaken.