_Rule_ XXIII. A former vow or promise is not nullified by a latter that contradicteth it.[74] Otherwise a man might disoblige himself at his pleasure. Yet he that maketh contrary vows, obligeth himself to contraries and impossibles; and bringeth a necessity of perjury on himself, for not doing the things impossible which he vowed. And in some cases a later promise to men may null a former, when we made the former with the reserve of such a power or liberty, or are justly supposed to have power, to recall a former promise: or when it is the duty of a mutable relation which we vow, (as of a physician, a schoolmaster, &c.) and by a later vow we change the relation itself (which we may still lawfully change). _Rule_ XXIV. The _actus jurandi_ must still be distinguished from the _materia juramenti_; and it very often cometh to pass that the act of swearing (or the oath as our act) is unlawfully done, and was a sin from the beginning, and yet it is nevertheless obligatory as long as the _res jurata_, the matter sworn, is lawful or necessary.[75] Dr. Sanderson instanceth in Joshua's oath to the Gibeonites. The nature of the thing is proof enough; for many a thing is sinfully done, for want of a due call, or manner, or end, that yet is done, and is no nullity. A man may sinfully enter upon the ministry, that yet is bound to do the duty of a minister; and many marriages are sinful that are no nullities. [Sidenote: What is the nullity of an oath.] _Rule_ XXV. The nullity of an oath _ab initio_, is _quando realiter vel reputative non juravimus_; when really or reputatively we did not swear. The sinfulness of an oath is when we did swear really but unlawfully as to the ground, or end, or matter, or manner, or circumstances. Really that man did not swear, 1. Who spake not (mentally nor orally) the words of an oath. 2. Who thought those words had signified no such thing, and so had no intent to swear either mentally or verbally. As if an Englishman be taught to use the words of an oath in French, and made believe that they have a contrary sense. 3. Who only narratively recited the words of an oath, as a reporter or historian, without a real or professed intent of swearing. Reputatively he did not swear, 1. Who spake the words of an oath in his sleep, or in a deliration, distraction, madness, or such prevalent melancholy as mastereth reason; when a man is not _compos mentis_, his act is not _actus humanus_. (2.) When a man's hand is forcibly moved by another against his will to subscribe the words of an oath or covenant; for if it be totally involuntary it is not a moral act. But words cannot be forced; for he that sweareth to save his life, doth do it voluntarily to save his life. The will may be moved by fear, but not forced. Yet the person that wrongfully frighteneth another into consent, or to swear, hath no right to any benefit which he thought to get by force or fraud; and so _in foro civili_ such promises, or