Afterwards a sentiment of morals concurs with interest, and becomes a new obligation upon mankind.  This sentiment of morality, in the performance of promises, arises from the same principles as that in the abstinence from the property of others.  Public interest, education, and the artifices of politicians, have the same effect in both cases.  The difficulties, that occur to us, in supposing a moral obligation to attend promises, we either surmount or elude.  For instance; the expression of a resolution is not commonly supposed to be obligatory; and we cannot readily conceive how the making use of a certain form of words should be able to cause any material difference.  Here, therefore, we feign a new act of the mind, which we call the willing an obligation; and on this we suppose the morality to depend.  But we have proved already, that there is no such act of the mind, and consequently that promises impose no natural obligation.

To confirm this, we may subjoin some other reflections concerning that will, which is supposed to enter into a promise, and to cause its obligation.  It is evident, that the will alone is never supposed to cause the obligation, but must be expressed by words or signs, in order to impose a tye upon any man.  The expression being once brought in as subservient to the will, soon becomes the principal part of the promise; nor will a man be less bound by his word, though he secretly give a different direction to his intention, and with-hold himself both from a resolution, and from willing an obligation.  But though the expression makes on most occasions the whole of the promise, yet it does not always so; and one, who should make use of any expression, of which he knows not the meaning, and which he uses without any intention of binding himself, would not certainly be bound by it.  Nay, though he knows its meaning, yet if he uses it in jest only, and with such signs as shew evidently he has no serious intention of binding himself, he would not lie under any obligation of performance; but it is necessary, that the words be a perfect expression of the will, without any contrary signs.  Nay, even this we must not carry so far as to imagine, that one, whom, by our quickness of understanding, we conjecture, from certain signs, to have an intention of deceiving us, is not bound by his expression or verbal promise, if we accept of it; but must limit this conclusion to those cases, where the signs are of a different kind from those of deceit.  All these contradictions are easily accounted for, if the obligation of promises be merely a human invention for the convenience of society; but will never be explained, if it be something real and natural, arising from any action of the mind or body.