XX. There is also another kind of interpretation, arising from conjectures, which apply exactly to the signification of the words containing a promise or engagement; and that is of a twofold description, either extending or limiting the meaning. But it is more difficult to extend than to limit the acceptation of expressions. For as in all matters the want of one essential requisite is sufficient to defeat their effect; so in engagements, those conjectures, which extend the obligation are not readily to be admitted. And it is much more difficult here than in the case above mentioned; where words allow a more extensive but less familiar acceptation. For here it is seeking a conjecture to extend the words of a promise: the conjecture therefore, which is to create an obligation, ought to be very certain. Nor is it sufficient that there is some resemblance in the motives; for the motive produced to confirm an obligation must be exactly the same as that of the case under consideration. Neither is it always proper to allege a motive for extending an obligation; because, as it has been already said, motives, in actuating us to form engagements, may sometimes be swayed by the will which often acts independently of any just motive. To authorise therefore such an extension, it must be evident that the motive, produced as an example and authority, was the sole and effectual cause, which influenced the promiser, and that he considered it in the same extensive view; for otherwise it would have been unjust and prejudicial. The ancients in their treatises on rhetoric follow the same rule, when, in speaking of the LETTER and DESIGN, they give us one invariable form of expressing the same sentiment, but in their syllogisms or arts of reasoning they point out a way of interpreting what is not written, by what is written. In the same manner too legal writers lay down rules for avoiding frauds. Now if at a time, when there was no other mode of fortifying towns, than by surrounding them with walls, it were stipulated that a certain place should not be so surrounded, it is evident that to employ any other means of fortification would be a breach of that treaty. As in the above case the interpretation must be extended to guard against every possible evasion, so in the following example, the prohibition to assemble an armed force to assail us includes all kinds of violence and force, by which our lives and security may be endangered.[48]