Summa Th. II-II EN Qu.88 a.3
Objection: 1. It would seem that vows are not all binding. For man needs things that are done by another, more than God does, since He has no need for our goods (Ps 15,2). Now according to the prescription of human laws [*Dig. L. xii, de pollicitat., i] a simple promise made to a man is not binding; and this seems to be prescribed on account of the changeableness of the human will. Much less binding therefore is a simple promise made to God, which we call a vow.
2. Further, no one is bound to do what is impossible. Now sometimes that which a man has vowed becomes impossible to him, either because it depends on another's decision, as when, for instance, a man vows to enter a monastery, the monks of which refuse to receive him: or on account of some defect arising, for instance when a woman vows virginity, and afterwards is deflowered; or when a man vows to give a sum of money, and afterwards loses it. Therefore a vow is not always binding.
3. Further, if a man is bound to pay something, he must do so at once. But a man is not bound to pay his vow at once, especially if it be taken under a condition to be fulfilled in the future. Therefore a vow is not always binding.
On the contrary It is written (Qo 5,3-4): "Whatsoever thou hast vowed, pay it; and it is much better not to vow, than after a vow not to perform the things promised."
I answer that For one to be accounted faithful one must keep one's promises. Wherefore, according to Augustine [*Ep. xxxii, 2: De Mendac. xx] faith takes its name "from a man's deed agreeing with his word" [*'Fides . . . fiunt dicta' Cicero gives the same etymology (De Offic. i, 7)]. Now man ought to be faithful to God above all, both on account of God's sovereignty, and on account of the favors he has received from God. Hence man is obliged before all to fulfill the vows he has made to God, since this is part of the fidelity he owes to God. On the other hand, the breaking of a vow is a kind of infidelity. Wherefore Solomon gives the reason why vows should be paid to God, because "an unfaithful . . . promise displeaseth Him" [*Qo 5,3].
Reply to Objection: 1. Honesty demands that a man should keep any promise he makes to another man, and this obligation is based on the natural law. But for a man to be under a civil obligation through a promise he has made, other conditions are requisite. And although God needs not our goods, we are under a very great obligation to Him: so that a vow made to Him is most binding.
2. If that which a man has vowed becomes impossible to him through any cause whatsoever, he must do what he can, so that he have at least a will ready to do what he can. Hence if a man has vowed to enter a monastery, he must endeavor to the best of his power to be received there. And if his intention was chiefly to bind himself to enter the religious life, so that, in consequence, he chose this particular form of religious life, or this place, as being most agreeable to him, he is bound, should he be unable to be received there, to enter the religious life elsewhere. But if his principal intention is to bind himself to this particular kind of religious life, or to this particular place, because the one or the other pleases him in some special way, he is not bound to enter another religious house, if they are unwilling to receive him into this particular one. on the other hand, if he be rendered incapable of fulfilling his vow through his own fault, he is bound over and above to do penance for his past fault: thus if a woman has vowed virginity and is afterwards violated, she is bound not only to observe what is in her power, namely, perpetual continency, but also to repent of what she has lost by sinning.
3. The obligation of a vow is caused by our own will and intention, wherefore it is written (Dt 23,23): "That which is once gone out of thy lips, thou shalt observe, and shalt do as thou hast promised to the Lord thy God, and hast spoken with thy own will and with thy own mouth." Wherefore if in taking a vow, it is one's intention and will to bind oneself to fulfil it at once, one is bound to fulfil it immediately. But if one intend to fulfil it at a certain time, or under a certain condition, one is not bound to immediate fulfilment. And yet one ought not to delay longer than one intended to bind oneself, for it is written (Dt 23,21): "When thou hast made a vow to the Lord thy God thou shalt not delay to pay it: because the Lord thy God will require it; and if thou delay, it shall be imputed to thee for a sin."
Objection: 1. It would seem that it is not expedient to take vows. It is not expedient to anyone to deprive himself of the good that God has given him. Now one of the greatest goods that God has given man is liberty whereof he seems to be deprived by the necessity implicated in a vow. Therefore it would seem inexpedient for man to take vows.
2. Further, no one should expose himself to danger. But whoever takes a vow exposes himself to danger, since that which, before taking a vow, he could omit without danger, becomes a source of danger to him if he should not fulfil it after taking the vow. Hence Augustine says (Ep. cxxvii, ad Arment. et Paulin.): "Since thou hast vowed, thou hast bound thyself, thou canst not do otherwise. If thou dost not what thou hast vowed thou wilt not be as thou wouldst have been hadst thou not vowed. For then thou wouldst have been less great, not less good: whereas now if thou breakest faith with God (which God forbid) thou art the more unhappy, as thou wouldst have been happier, hadst thou kept thy vow." Therefore it is not expedient to take vows.
3. Further, the Apostle says (1Co 4,16): "Be ye followers of me, as I also am of Christ." But we do not read that either Christ or the Apostles took any vows. Therefore it would seem inexpedient to take vows.
On the contrary It is written (Ps 75,12): "Vow ye and pay to the Lord your God."
I answer that As stated above (Articles ,2), a vow is a promise made to God. Now one makes a promise to a man under one aspect, and to God under another. Because we promise something to a man for his own profit; since it profits him that we should be of service to him, and that we should at first assure him of the future fulfilment of that service: whereas we make promises to God not for His but for our own profit. Hence Augustine says (Ep. cxxvii, ad Arment. et Paulin.): "He is a kind and not a needy exactor, for he does not grow rich on our payments, but makes those who pay Him grow rich in Him." And just as what we give God is useful not to Him but to us, since "what is given Him is added to the giver," as Augustine says (Ep. cxxvii, ad Arment. et Paulin.), so also a promise whereby we vow something to God, does not conduce to His profit, nor does He need to be assured by us, but it conduces to our profit, in so far as by vowing we fix our wills immovably on that which it is expedient to do. Hence it is expedient to take vows.
Reply to Objection: 1. Even as one's liberty is not lessened by one being unable to sin, so, too, the necessity resulting from a will firmly fixed to good does not lessen the liberty, as instanced in God and the blessed. Such is the necessity implied by a vow, bearing a certain resemblance to the confirmation of the blessed. Hence, Augustine says (Ep. cxxvii, ad Arment. et Paulin.) that "happy is the necessity that compels us to do the better things."
2. When danger arises from the deed itself, this deed is not expedient, for instance that one cross a river by a tottering bridge: but if the danger arise through man's failure in the deed, the latter does not cease to be expedient: thus it is expedient to mount on horseback, though there be the danger of a fall from the horse: else it would behoove one to desist from all good things, that may become dangerous accidentally. Wherefore it is written (Qo 11,4): "He that observeth the wind shall not sow, and he that considereth the clouds shall never reap." Now a man incurs danger, not from the vow itself, but from his fault, when he changes his mind by breaking his vow. Hence, Augustine says (Ep. cxxvii, ad Arment. et Paulin.): "Repent not of thy vow: thou shouldst rather rejoice that thou canst no longer do what thou mightest lawfully have done to thy detriment."
3. It was incompetent for Christ, by His very nature, to take a vow, both because He was God, and because, as man, His will was firmly fixed on the good, since He was a "comprehensor." By a kind of similitude, however, He is represented as saying (Ps 21,26): "I will pay my vows in the sight of them that fear Him," when He is speaking of His body, which is the Church.The apostles are understood to have vowed things pertaining to the state of perfection when "they left all things and followed Christ."
Objection: 1. It would seem that a vow is not an act of latria or religion. Every act of virtue is matter for a vow. Now it would seem to pertain to the same virtue to promise a thing and to do it. Therefore a vow pertains to any virtue and not to religion especially.
2. Further, according to Tully (De Invent. ii, 53) it belongs to religion to offer God worship and ceremonial rites. But he who takes a vow does not yet offer something to God, but only promises it. Therefore, a vow is not an act of religion.
3. Further, religious worship should be offered to none but God. But a vow is made not only to God, but also to the saints and to one's superiors, to whom religious vow obedience when they make their profession. Therefore, a vow is not an act of religion.
On the contrary It is written (Is 19,21): "(The Egyptians) shall worship Him with sacrifices and offerings and they shall make vows to the Lord, and perform them." Now, the worship of God is properly the act of religion or latria. Therefore, a vow is an act of latria or religion.
I answer that As stated above (Question , Article , ad 1), every act of virtue belongs to religion or latria by way of command, in so far as it is directed to the reverence of God which is the proper end of latria. Now the direction of other actions to their end belongs to the commanding virtue, not to those which are commanded. Therefore the direction of the acts of any virtue to the service of God is the proper act of latria.Now, it is evident from what has been said above (Articles ,2) that a vow is a promise made to God, and that a promise is nothing else than a directing of the thing promised to the person to whom the promise is made. Hence a vow is a directing of the thing vowed to the worship or service of God. And thus it is clear that to take a vow is properly an act of latria or religion.
Reply to Objection: 1. The matter of a vow is sometimes the act of another virtue, as, for instance, keeping the fast or observing continency; while sometimes it is an act of religion, as offering a sacrifice or praying. But promising either of them to God belongs to religion, for the reason given above. Hence it is evident that some vows belong to religion by reason only of the promise made to God, which is the essence of a vow, while others belong thereto by reason also of the thing promised, which is the matter of the vow.
2. He who promises something gives it already in as far as he binds himself to give it: even as a thing is said to be made when its cause is made, because the effect is contained virtually in its cause. This is why we thank not only a giver, but also one who promises to give.
3. A vow is made to God alone, whereas a promise may be made to a man also: and this very promise of good, which is fore made to a man, may be the matter of a vow, and in so far as it is a virtuous act. This is how we are to understand vows whereby we vow something to the saints or to one's superiors: so that the promise made to the saints or to one's superiors is the matter of the vow, in so far as one vows to God to fulfil what one has promised to the saints or one's superiors.
Objection: 1. It would seem that it is more praiseworthy and meritorious to do a thing without a vow than in fulfilment of a vow. Prosper says (De Vita Contempl. ii): "We should abstain or fast without putting ourselves under the necessity of fasting, lest that which we are free to do be done without devotion and unwillingly." Now he who vows to fast puts himself under the necessity of fasting. Therefore it would be better for him to fast without taking the vow.
2. Further, the Apostle says (2Co 9,7): "Everyone as he hath determined in his heart, not with sadness, or of necessity: for God loveth a cheerful giver." Now some fulfil sorrowfully what they have vowed: and this seems to be due to the necessity arising from the vow, for necessity is a cause of sorrow according to Metaph. v [*Ed. Did. iv, 5]. Therefore, it is better to do something without a vow, than in fulfilment of a vow.
3. Further, a vow is necessary for the purpose of fixing the will on that which is vowed, as stated above (Article ). But the will cannot be more fixed on a thing than when it actually does that thing. Therefore it is no better to do a thing in fulfilment of a vow than without a vow.
On the contrary A gloss on the words of Ps 75,12, "Vow ye and pay," says: "Vows are counseled to the will." But a counsel is about none but a better good. Therefore it is better to do a deed in fulfilment of a vow than without a vow: since he that does it without a vow fulfils only one counsel, viz. the counsel to do it, whereas he that does it with a vow, fulfils two counsels, viz. the counsel to vow and the counsel to do it.
I answer that For three reasons it is better and more meritorious to do one and the same deed with a vow than without. First, because to vow, as stated above (Article ) is an act of religion which is the chief of the moral virtues. Now the more excellent the virtue the better and more meritorious the deed. Wherefore the act of an inferior virtue is the better the more meritorious for being commanded by a superior virtue, whose act it becomes through being commanded by it, just as the act of faith or hope is better if it be commanded by charity. Hence the works of the other moral virtues (for instance, fasting, which is an act of abstinence; and being continent, which is an act of chastity) are better and more meritorious, if they be done in fulfilment of a vow, since thus they belong to the divine worship, being like sacrifices to God. Wherefore Augustine says (De Virg. viii) that "not even is virginity honorable as such, but only when it is consecrated to God, and cherished by godly continence."Secondly, because he that vows something and does it, subjects himself to God more than he that only does it; for he subjects himself to God not only as to the act, but also as to the power, since in future he cannot do something else. Even so he gives more who gives the tree with its fruit, than he that gives the fruit only, as Anselm [*Eadmer] observes (De Simil. viii). For this reason, we thank even those who promise, as stated above (Article , ad 2).Thirdly, because a vow fixes the will on the good immovably and to do anything of a will that is fixed on the good belongs to the perfection of virtue, according to the Philosopher (Ethic. ii, 4), just as to sin with an obstinate mind aggravates the sin, and is called a sin against the Holy Ghost, as stated above (Question , Article ).
Reply to Objection: 1. The passage quoted should be understood as referring to necessity of coercion which causes an act to be involuntary and excludes devotion. Hence he says pointedly: "Lest that which we are free to do be done without devotion and unwillingly." On the other hand the necessity resulting from a vow is caused by the immobility of the will, wherefore it strengthens the will and increases devotion. Hence the argument does not conclude.
2. According to the Philosopher, necessity of coercion, in so far as it is opposed to the will, causes sorrow. But the necessity resulting from a vow, in those who are well disposed, in so far as it strengthens the will, causes not sorrow but joy. Hence Augustine says (Ep ad Arment. et Paulin. cxxcii): "Repent not of thy vow: thou shouldst rather rejoice that thou canst no longer do what thou mightest lawfully have done to thy detriment." If, however, the very deed, considered in itself, were to become disagreeable and involuntary after one has taken the vow, the will to fulfil it remaining withal, it is still more meritorious than if it were done without the vow, since the fulfilment of a vow is an act of religion which is a greater virtue than abstinence, of which fasting is an act.
3. He who does something without having vowed it has an immovable will as regards the individual deed which he does and at the time when he does it; but his will does not remain altogether fixed for the time to come, as does the will of one who makes a vow: for the latter has bound his will to do something, both before he did that particular deed, and perchance to do it many times.
Objection: 1. It would seem that a vow is not solemnized by the reception of holy orders and by the profession of a certain rule. As stated above (Article ), a vow is a promise made to God. Now external actions pertaining to solemnity seem to be directed, not to God, but to men. Therefore they are related to vows accidentally: and consequently a solemnization of this kind is not a proper circumstance of a vow.
2. Further, whatever belongs to the condition of a thing, would seem to be applicable to all in which that thing is found. Now many things may be the subject of a vow, which have no connection either with holy orders, or to any particular rule: as when a man vows a pilgrimage, or something of the kind. Therefore the solemnization that takes place in the reception of holy orders or in the profession of a certain rule does not belong to the condition of a vow.
3. Further, a solemn vow seems to be the same as a public vow. Now many other vows may be made in public besides that which is pronounced in receiving holy orders or in professing a certain rule; which latter, moreover, may be made in private. Therefore not only these vows are solemn.
On the contrary These vows alone are an impediment to the contract of marriage, and annul marriage if it be contracted, which is the effect of a solemn vow, as we shall state further on in the Third Part of this work [*XP, Question , Article ].
I answer that The manner in which a thing is solemnized depends on its nature [conditio]: thus when a man takes up arms he solemnizes the fact in one way, namely, with a certain display of horses and arms and a concourse of soldiers, while a marriage is solemnized in another way, namely, the array of the bridegroom and bride and the gathering of their kindred. Now a vow is a promise made to God: wherefore, the solemnization of a vow consists in something spiritual pertaining to God; i.e. in some spiritual blessing or consecration which, in accordance with the institution of the apostles, is given when a man makes profession of observing a certain rule, in the second degree after the reception of holy orders, as Dionysius states (Eccl. Hier. vi). The reason of this is that solemnization is not wont to be employed, save when a man gives himself up entirely to some particular thing. For the nuptial solemnization takes place only when the marriage is celebrated, and when the bride and bridegroom mutually deliver the power over their bodies to one another. In like manner a vow is solemnized when a man devotes himself to the divine ministry by receiving holy orders, or embraces the state of perfection by renouncing the world and his own will by the profession of a certain rule.
Reply to Objection: 1. This kind of solemnization regards not only men but also God in so far as it is accompanied by a spiritual consecration or blessing, of which God is the author, though man is the minister, according to Nb 6,27, "They shall invoke My name upon the children of Israel, and I will bless them." Hence a solemn vow is more binding with God than a simple vow, and he who breaks a solemn vow sins more grievously. When it is said that a simple vow is no less binding than a solemn vow, this refers to the fact that the transgressor of either commits a mortal sin.
2. It is not customary to solemnize particular acts, but the embracing of a new state, as we have said above. Hence when a man vows particular deeds, such as a pilgrimage, or some special fast, such a vow is not competent to be solemnized, but only such as the vow whereby a man entirely devotes himself to the divine ministry or service: and yet many particular works are included under this vow as under a universal.
3. Through being pronounced in public vows may have a certain human solemnity, but not a spiritual and divine solemnity, as the aforesaid vows have, even when they are pronounced before a few persons. Hence the publicity of a vow differs from its solemnization.
Objection: 1. It would seem that those who are subject to another's power are not hindered from taking vows. The lesser bond is surpassed by the greater. Now the obligation of one man subject to another is a lesser bond than a vow whereby one is under an obligation to God. Therefore those who are subject to another's power are not hindered from taking vows.
2. Further, children are under their parents' power. Yet children may make religious profession even without the consent of their parents. Therefore one is not hindered from taking vows, through being subject to another's power.
3. Further, to do is more than to promise. But religious who are under the power of their superiors can do certain things such as to say some psalms, or abstain from certain things. Much more therefore seemingly can they promise such things to God by means of vows.
4. Further, whoever does what he cannot do lawfully sins. But subjects do not sin by taking vows, since nowhere do we find this forbidden. Therefore it would seem that they can lawfully take vows.
On the contrary It is commanded (Nb 30,4-6) that "if a woman vow any thing . . . being in her father's house, and yet but a girl in age," she is not bound by the vow, unless her father consent: and the same is said there (Nb 30,7-9) of the woman that has a husband. Therefore in like manner other persons that are subject to another's power cannot bind themselves by vow.
I answer that As stated above (Article ), a vow is a promise made to God. Now no man can firmly bind himself by a promise to do what is in another's power, but only to that which is entirely in his own power. Now whoever is subject to another, as to the matter wherein he is subject to him, it does not lie in his power to do as he will, but it depends on the will of the other. And therefore without the consent of his superior he cannot bind himself firmly by a vow in those matters wherein he is subject to another.
Reply to Objection: 1. Nothing but what is virtuous can be the subject of a promise made to God, as stated above (Article ). Now it is contrary to virtue for a man to offer to God that which belongs to another, as stated above (Question , Article ). Hence the conditions necessary for a vow are not altogether ensured, when a man who is under another's power vows that which is in that other's power, except under the condition that he whose power it concerns does not gainsay it.
2. As soon as a man comes of age, if he be a freeman he is in his own power in all matters concerning his person, for instance with regard to binding himself by vow to enter religion, or with regard to contracting marriage. But he is not in his own power as regards the arrangements of the household, so that in these matters he cannot vow anything that shall be valid without the consent of his father.A slave, through being in his master's power, even as regards his personal deeds, cannot bind himself by vow to enter religion, since this would withdraw him from his master's service.
3. A religious is subject to his superior as to his actions connected with his profession of his rule. Wherefore even though one may be able to do something now and then, when one is not being occupied with other things by one's superior, yet since there is no time when his superior cannot occupy him with something, no vow of a religious stands without the consent of his superior, as neither does the vow of a girl while in (her father's) house without his consent; nor of a wife, without the consent of her husband.
4. Although the vow of one who is subject to another's power does not stand without the consent of the one to whom he is subject, he does not sin by vowing; because his vow is understood to contain the requisite condition, providing, namely, that his superior approve or do not gainsay it.
Objection: 1. It would seem that children cannot bind themselves by vow to enter religion. Since a vow requires deliberation of the mind, it is fitting that those alone should vow who have the use of reason. But this is lacking in children just as in imbeciles and madmen. Therefore just as imbeciles and madmen cannot bind themselves to anything by vow, so neither, seemingly, can children bind themselves by vow to enter religion.
2. Further, that which can be validly done by one cannot be annulled by another. Now a vow to enter religion made by a boy or girl before the age of puberty can be revoked by the parents or guardian (20, qu. ii, cap. Puella). Therefore it seems that a boy or girl cannot validly make a vow before the age of fourteen.
3. Further, according to the rule of Blessed Benedict [*Ch. 58] and a statute of Innocent IV, a year's probation is granted to those who enter religion, so that probation may precede the obligation of the vow. Therefore it seems unlawful, before the year of probation, for children to be bound by vow to enter religion.
On the contrary That which is not done aright is invalid without being annulled by anyone. But the vow pronounced by a maiden, even before attaining the age of puberty, is valid, unless it be annulled by her parents within a year (20, qu. ii, cap. Puella). Therefore even before attaining to puberty children can lawfully and validly be bound by a vow to enter religion.
I answer that As may be gathered from what has been said above (Article ), vows are of two kinds, simple and solemn. And since, as stated in the same article, the solemnization of a vow consists in a spiritual blessing and consecration bestowed through the ministry of the Church, it follows that it comes under the Church's dispensation. Now a simple vow takes its efficacy from the deliberation of the mind, whereby one intends to put oneself under an obligation. That such an obligation be of no force may happen in two ways. First, through defect of reason, as in madmen and imbeciles, who cannot bind themselves by vow so long as they remain in a state of madness or imbecility. Secondly, through the maker of a vow being subject to another's power, as stated above (Article ). Now these two circumstances concur in children before the age of puberty, because in most instances they are lacking in reason, and besides are naturally under the care of their parents, or guardians in place of their parents: wherefore in both events their vows are without force. It happens, however, through a natural disposition which is not subject to human laws, that the use of reason is accelerated in some, albeit few, who on this account are said to be capable of guile: and yet they are not, for this reason, exempt in any way from the care of their parents; for this care is subject to human law, which takes into account that which is of most frequent occurrence.Accordingly we must say that boys or girls who have not reached the years of puberty and have not attained the use of reason can nowise bind themselves to anything by vow. If, however, they attain the use of reason, before reaching the years of puberty, they can for their own part, bind themselves by vow; but their vows can be annulled by their parents, under whose care they are still subject.Yet no matter how much they be capable of guile before the years of puberty, they cannot be bound by a solemn religious vow, on account of the Church's decree [*Sext. Decret. cap. Is qui, de Reg. et transeunt. ad Relig.] which considers the majority of cases. But after the years of puberty have been reached, they can bind themselves by religious vows, simple or solemn, without the consent of their parents.
Reply to Objection: 1. This argument avails in the case of children who have not yet reached the use of reason: for their vows then are invalid, as stated above.
2. The vows of persons subject to another's power contain an implied condition, namely, that they be not annulled by the superior. This condition renders them licit and valid if it be fulfilled, as stated above.
3. This argument avails in the case of solemn vows which are taken in profession.
Objection: 1. It would seem that vows are not subject to dispensation. It is less to have a vow commuted than to be dispensed from keeping it. But a vow cannot be commuted, according to Lv 27,9-10, "A beast that may be sacrificed to the Lord, if anyone shall vow, shall be holy, and cannot be changed, neither a better for a worse, nor a worse for a better." Much less, therefore, do vows admit of dispensation.
2. Further, no man can grant a dispensation in matters concerning the natural law and in the Divine precepts, especially those of the First Table, since these aim directly at the love of God, which is the last end of the precepts. Now the fulfilment of a vow is a matter of the natural law, and is commanded by the Divine law, as shown above (Article ), and belongs to the precepts of the First Table since it is an act of religion. Therefore vows do not admit of dispensation.
3. Further, the obligation of a vow is based on the fidelity which a man owes to God, as stated above (Article ). But no man can dispense in such a matter as this. Neither, therefore, can any one grant a dispensation from a vow.
On the contrary That which proceeds from the common will of many has apparently greater stability than that which proceeds from the individual will of some one person. Now the law which derives its force from the common will admits of dispensation by a man. Therefore it seems that vows also admit of dispensation by a man.
I answer that The dispensation from a vow is to be taken in the same sense as a dispensation given in the observance of a law because, as stated above (I-II 96,6; I-II 97,4), a law is made with an eye to that which is good in the majority of instances. But since, in certain cases this is not good, there is need for someone to decide that in that particular case the law is not to be observed. This is properly speaking to dispense in the law: for a dispensation would seem to denote a commensurate distribution or application of some common thing to those that are contained under it, in the same way as a person is said to dispense food to a household.In like manner a person who takes a vow makes a law for himself as it were, and binds himself to do something which in itself and in the majority of cases is a good. But it may happen that in some particular case this is simply evil, or useless, or a hindrance to a greater good: and this is essentially contrary to that which is the matter of a vow, as is clear from what has been said above (Article ). Therefore it is necessary, in such a case, to decide that the vow is not to be observed. And if it be decided absolutely that a particular vow is not to be observed, this is called a "dispensation" from that vow; but if some other obligation be imposed in lieu of that which was to have been observed, the vow is said to be "commuted." Hence it is less to commute a vow than to dispense from a vow: both, however, are in the power of the Church.
Reply to Objection: 1. An animal that could be lawfully sacrificed was deemed holy from the very moment that it was the subject of a vow, being, as it were, dedicated to the worship of God: and for this reason it could not be changed: even so neither may one now exchange for something better, or worse, that which one has vowed, if it be already consecrated, e.g. a chalice or a house. On the other hand, an animal that could not be sacrificed, through not being the lawful matter of a sacrifice, could and had to be bought back, as the law requires. Even so, vows can be commuted now, if no consecration has intervened.
2. Even as man is bound by natural law and Divine precept to fulfil his vow, so, too, is he bound under the same heads to obey the law or commands of his superiors. And yet when he is dispensed from keeping a human law, this does not involve disobedience to that human law, for this would be contrary to the natural law and the Divine command; but it amounts to this---that what was law is not law in this particular case. Even so, when a superior grants a dispensation, that which was contained under a vow is by his authority no longer so contained, in so far as he decides that in this case such and such a thing is not fitting matter for a vow. Consequently when an ecclesiastical superior dispenses someone from a vow, he does not dispense him from keeping a precept of the natural or of the Divine law, but he pronounces a decision on a matter to which a man had bound himself of his own accord, and of which he was unable to consider every circumstance.
3. The fidelity we owe to God does not require that we fulfil that which it would be wrong or useless to vow, or which would be an obstacle to the greater good whereunto the dispensation from that vow would conduce. Hence the dispensation from a vow is not contrary to the fidelity due to God.
Summa Th. II-II EN Qu.88 a.3