2208. Case in Which One Has Taken Two Opposite Vows.—(a) If the vows are equally good, or if it is doubtful which is better, the first prevails; the second being impossible does not oblige. (b) If the second is certainly better, it prevails, and the first does not oblige, being impossible. Thus, if one has first vowed to go on a pilgrimage and next to stay at home and attend the sick during an epidemic, the pilgrimage should not stand in the way of the more urgent good of caring for those in distress.
2209. The Obligation of a Vow.—Every valid vow obliges to fulfillment, for it is a promise, and loyalty to promises is a moral duty. Scripture declares that one must pay one’s vows, and that it is better not to vow than to vow and not fulfill (Eccles., v. 4), that God will hold as sin the neglect of a vow (Deut., xxiii. 21), that a faithless promise displeases Him (Eccles., V. 3).
(a) The Quality of the Obligation.—A vow is an act of religion, since it promises to God a tribute of honor, even though the thing promised (e.g., a fast, virginity) does not belong to worship but to some other virtue. Hence, the violation of a vow is a sin against fidelity and also against religion. But it seems that sacrilege is committed only by sins against certain vows, namely, those whose matter is a sacred thing dedicated to God; for example, the violation of the vow to fast would not be sacrilegious, while the violation of a public vow of chastity is a sacrilege. All transgressions of vows, as such, are sins of the same species, namely, sins against religion.
(b) The Quantity of the Obligation.—A vow, as being a duty of religion (see 2146, 2148), obliges under grave sin. But in an individual case the sin committed may be only venial on account of lightness of matter or imperfection of the act.
2210. Gravity of the Obligation of a Vow.—A vow has the nature of a private law, since it is an obligation which the vow-maker voluntarily imposes on himself. But a law obliges under grave sin when the subject-matter itself and the intention of the lawgiver require strict obligation (see 381 sqq.). Hence, gravity of matter in a vow depends on the great importance of the thing vowed and on the will of the vower to assume a grave obligation.
(a) Thus, the thing vowed must be of great importance, either in itself (e.g., chastity) or from its relation to divine worship (e.g., a fast, a communion). One cannot oblige oneself to a grave obligation under a vow whose matter is absolutely and relatively of minor importance (e.g., a daily Hail Mary, an alms of twenty-five cents; see 382).
(b) The intention of the vower must be to bind himself under grave sin. He is free not to oblige himself by vow at all, and hence, should he elect to vow, he may bind himself, even though the matter of the vow is of great importance, either under grave or under light sin (or only to penalty), as he wishes. Exception must be made, however, for public vows taken in religious institutes and for the vow of chastity and celibacy taken in the reception of subdeaconship, for the law of the Church on account of the public good decrees that these vows oblige under grave sin.
2211. Rules for Determining What Is Important Matter in a Vow.—(a) In personal vows, an act of omission may be made the subject of grave obligation, if it may be made the subject of grave precept by the Church (e.g., the hearing of Mass, Confession, Communion, a fast or abstinence, a Rosary).
(b) In real vows, some fix grave matter according to the standards for commutative justice (see 1896 sqq.). But such a rule seems unsuitable. The amounts absolutely and relatively grave in theft are determined by the wealth of the person stolen from, but, since God is owner of all things, we do not see how those amounts could be fixed in reference to Him. On the other hand, the duty of religion obliges more strictly than that of commutative justice, and hence it does not seem that grave matter should be the same for both. In practice, the matter is not grave when it is quite inconsiderable (say less than a dollar), or when the vower intends only a light obligation. If the matter is not inconsiderable and the intent of the vower is uncertain, one may decide as to the obligation of the vow from presumptions based on custom, the circumstances of the vow, or the rule that grave obligation is not to be taken for granted (see 709, 714, 659).
2212. Coalescence of Light into Grave Matter.—(a) If the vower has determined the relation of the items of the vow one to another, coalescence can be judged from his intention. Thus, if the vower intends that the items shall be parts of one whole, there is coalescence (e.g., if he vows to give fifty cents in alms daily and neglects this for a year, there is grave sin); but if he intends the items as separate promises, there is no coalescence (e.g., if he vows to say a Hail Mary every day and neglects this for a year, there are many venial sins). If some of the items have been performed, others omitted, and the omissions coalesce, there is grave sin according to some as soon as a notable quantity is reached, but others believe that there is grave sin only when a notable percentage (say one-third or one-fourth) of the matter vowed has been neglected.