(a) The object of this intention is the obligation itself, not its fulfillment. Hence, he who makes a vow, but intends not to oblige himself, vows invalidly; for he has two contrary intentions and (unless the intention to vow is stronger) the substance of the vow is excluded. On the contrary, he who makes a vow, intending not to fulfill it, vows validly but illicitly, since he really intends to oblige himself, but he sins by his purpose not to keep his vow (see 1883).
(b) The quality of the intention must be such that the character of the vow as a deliberate act and a sincere agreement to obligation will be preserved. Hence, an habitual intention (e.g., Claudius intending to take a vow on the morrow pronounces the words of promise while asleep) does not suffice, because the act made with such an intention is not deliberate or human. Likewise, an external intention (e.g., Balba forced by her parents to enter a convent takes the vows, intending only the external rite) and an indirect intention (e.g., Sempronius, foreseeing that if he drinks certain liquors he will bind himself by vow to a number of things, takes the drinks and makes the vows) are not sufficient, because with them there is no real agreement to obligation. On the other hand, it suffices to have an actual but implicit or tacit intention (e.g., Titus receives subdeaconship intending the obligations annexed to the office, but not knowing that celibacy is a duty vowed by subdeacons), or a virtual intention (e.g., Caius intended to make religious profession, but at the moment of pronouncing the vows he is distracted and gives no attention to the words), for in either case there is a human act and real agreement to obligation (see 2164, 2165).
2200. The Matter of a Vow.—(a) A vow is a free promise, and hence its matter must not be something necessary. (b) A vow is made to God, and hence its matter must not be something that is not pleasing to Him.
2201. Vows that Promise Something Necessary.—(a) If the necessity is absolute, because a certain thing must be or cannot be, a vow is invalid. A vow to die is null, because death is a necessity; a vow to avoid venial sin, deliberate and indeliberate, is null, because it is impossible without a special privilege from God to keep such a vow; a vow that one’s child shall enter religion is also null, because one has no power over that which depends on the will of another. The vows made by communities do not oblige their successors or posterity as vows, but only as laws or customs having the force of law, or as contracts to which agreement is given, etc.
(b) If the necessity is hypothetical, because a certain thing must be done or omitted if one is to observe the natural or positive law, the vow is valid. For though it is necessary to observe a commandment (e.g., to avoid intoxication), it is not necessary to add to the existent obligation the new obligation of religion. The most suitable matter for a vow, however, is something that is of counsel, but not of precept, for example, to practise celibacy.
2202. When Fulfillment of Vow Is Only Partly Possible.—(a) If the vower intended that his promise should be an entire one obliging him to fulfillment of all the items, the vow is invalid, since its fulfillment as intended is impossible. Thus, if one vowed to go on foot to a place of pilgrimage but was unable to accomplish the journey on foot, or vowed to go to Rome and became unable to go the full distance, there would be no obligation.
(b) If the vower intended that his promise should be severable, the difficulty can be settled as follows: he is held to nothing if the matter is severable but the principal part impossible (e.g., if he vowed to go on a pilgrimage and also to go barefooted, he is not bound to go barefooted an equal distance if the pilgrimage becomes impossible). He is held to the part that is possible, if it is really severable and was intended as the principal part of the vow (e.g., if he vowed to go on a pilgrimage and to go barefooted, but is unable to go barefooted).
(c) If the intention of the vow-maker is uncertain, it seems he is held to perform what is possible (e.g., if one has vowed to pay for the erection of a church but becomes unable to pay for more than a part of the expense); but if there is a good reason to presume that he intended an entire vow, or a severable vow whose chief part has become impossible, one may decide the doubt in the sense of that presumption (cfr. 465).
2203. Vows that Promise Something Displeasing to God.—(a) Vows that promise what is always evil (e.g., to steal) are invalid and, on account of the irreverence, gravely sinful, at least if the sin promised is mortal. (b) Vows that promise something that may turn out either evil or good (e.g., Jepthe’s vow to immolate the first living being that came before him) are imprudent, and should not be kept as to the part that is sinful.
2204. What should be said of vows that promise something good, but that have an evil end or other evil circumstances?