Summa Th. II-II EN Qu.88 a.12
Objection: 1. It would seem that the authority of a prelate is not required for the commutation or dispensation of a vow. A person may enter religion without the authority of a superior prelate. Now by entering religion one is absolved from the vows he made in the world, even from the vow of making a pilgrimage to the Holy Land [*Cap. Scripturae, de Voto et Voti redempt.]. Therefore the commutation or dispensation of a vow is possible without the authority of a superior prelate.
2. Further, to dispense anyone from a vow seems to consist in deciding in what circumstances he need not keep that vow. But if the prelate is at fault in his decision, the person who took the vow does not seem to be absolved from his vow, since no prelate can grant a dispensation contrary to the divine precept about keeping one's vows, as stated above (Article [10], ad 2; Article [11]). Likewise, when anyone rightly determines of his own authority that in his case a vow is not to be kept, he would seem not to be bound; since a vow need not be kept if it have an evil result (Article [2], ad 2). Therefore the Authority of a prelate is not required that one may be dispensed from a vow.
3. Further, if it belongs to a prelate's power to grant dispensations from vows, on the same count it is competent to all prelates, but it does not belong to all to dispense from every vow. Therefore it does not belong to the power of a prelate to dispense from vows.
On the contrary A vow binds one to do something, even as a law does. Now the superior's authority is requisite for a dispensation from a precept of the law, as stated above (FS, Question [96], Article [6]; FS, Question [97], Article [4]). Therefore it is likewise required in a dispensation from a vow.
I answer that As stated above (Articles [1],2), a vow is a promise made to God about something acceptable to Him. Now if you promise something to anyone it depends on his decision whether he accept what you promise. Again in the Church a prelate stands in God's place. Therefore a commutation or dispensation of vows requires the authority of a prelate who in God's stead declares what is acceptable to God, according to 2Co 2,10: "For . . . have pardoned . . . for your sakes . . . in the person of Christ." And he says significantly "for your sakes," since whenever we ask a prelate for a dispensation we should do so to honor Christ in Whose person he dispenses, or to promote the interests of the Church which is His Body.
Reply to Objection: 1. All other vows are about some particular works, whereas by the religious life a man consecrates his whole life to God's service. Now the particular is included in the universal, wherefore a Decretal [*Cap. Scripturae, de Voto et Voti redempt.] says that "a man is not deemed a vow-breaker if he exchange a temporal service for the perpetual service of religion." And yet a man who enters religion is not bound to fulfil the vows, whether of fasting or of praying or the like, which he made when in the world, because by entering religion he dies to his former life, and it is unsuitable to the religious life that each one should have his own observances, and because the burden of religion is onerous enough without requiring the addition of other burdens.
2. Some have held that prelates can dispense from vows at their will, for the reason that every vow supposes as a condition that the superior prelate be willing; thus it was stated above (Article [8]) that the vow of a subject, e.g. of a slave or a son, supposes this condition, if "the father or master consent," or "does not dissent." And thus a subject might break his vow without any remorse of conscience, whenever his superior tells him to.But this opinion is based on a false supposition: because a spiritual prelate being, not a master, but a dispenser, his power is given "unto edification, not for destruction" (2Co 10,8), and consequently, just as he cannot command that which is in itself displeasing to God, namely, sin, so neither can he forbid what is in itself pleasing to God, namely, works of virtue. Therefore absolutely speaking man can vow them. But it does belong to a prelate to decide what is the more virtuous and the more acceptable to God. Consequently in matters presenting no difficulty, the prelate's dispensation would not excuse one from sin: for instance, if a prelate were to dispense a person from a vow to enter the religious life, without any apparent cause to prevent him from fulfilling his vow. But if some cause were to appear, giving rise, at least, to doubt, he could hold to the prelate's decision whether of commutation or of dispensation. He could not, however, follow his own judgment in the matter, because he does not stand in the place of God; except perhaps in the case when the thing he has vowed is clearly unlawful, and he is unable to have recourse to the prelate.
3. Since the Sovereign Pontiff holds the place of Christ throughout the whole Church, he exercises absolute power of dispensing from all vows that admit of dispensation. To other and inferior prelates is the power committed of dispensing from those vows that are commonly made and frequently require dispensation, in order that men may easily have recourse to someone; such are the vows of pilgrimage (Cap. de Peregin., de Voto et Voti redempt.), fasting and the like, and of pilgrimage to the Holy Land, are reserved to the Sovereign Pontiff [*Cap. Ex multa].
We must now consider those external acts of religion, whereby something Divine is taken by man: and this is either a sacrament or the Name of God. The place for treating of the taking of a sacrament will be in the Third Part of this work: of the taking of God's Name we shall treat now. The Name of God is taken by man in three ways. First, by way of oath in order to confirm one's own assertion: secondly, by way of adjuration as an inducement to others: thirdly, by way of invocation for the purpose of prayer or praise. Accordingly we must first treat of oaths: and under this head there are ten points of inquiry:
(1) What is an oath?
(2) Whether it is lawful?
(3) What are the accompanying conditions of an oath?
(4) Of what virtue is it an act?
(5) Whether oaths are desirable, and to be employed frequently as something useful and good?
(6) Whether it is lawful to swear by a creature?
(7) Whether an oath is binding?
(8) Which is more binding, an oath or a vow?
(9) Whether an oath is subject to dispensation?
(10) Who may lawfully swear, and when?
Objection: 1. It would seem that to swear is not to call God to witness. Whoever invokes the authority of Holy Writ calls God to witness, since it is His word that Holy Writ contains. Therefore, if to swear is to call God to witness, whoever invoked the authority of Holy Writ would swear. But this is false Therefore the antecedent is false also.
2. Further, one does not pay anything to a person by calling him to witness. But he who swears by God pays something to Him for it is written (Mt 5,33): "Thou shall pay [Douay: 'perform'] thy oaths to the Lord"; and Augustine says [*Serm. clxxx] that to swear [jurare] is "to pay the right [jus reddere] of truth to God." Therefore to swear is not to call God to witness.
3. Further, the duties of a judge differ from the duties of a witness, as shown above (Questions [67],70). Now sometimes a man, by swearing, implores the Divine judgment, according to Ps 7,5, "If I have rendered to them that repaid me evils, let me deservedly fall empty before my enemies." Therefore to swear is not to call God to witness.
On the contrary Augustine says in a sermon on perjury (Serm. clxxx): "When a man says: 'By God,' what else does he mean but that God is his witness?"
I answer that As the Apostle says (He 6,16), oaths are taken for the purpose of confirmation. Now speculative propositions receive confirmation from reason, which proceeds from principles known naturally and infallibly true. But particular contingent facts regarding man cannot be confirmed by a necessary reason, wherefore propositions regarding such things are wont to be confirmed by witnesses. Now a human witness does not suffice to confirm such matters for two reasons. First, on account of man's lack of truth, for many give way to lying, according to Ps 16,10, "Their mouth hath spoken lies [Vulg.: 'proudly']." Secondly, on account of this lack of knowledge, since he can know neither the future, nor secret thoughts, nor distant things: and yet men speak about such things, and our everyday life requires that we should have some certitude about them. Hence the need to have recourse to a Divine witness, for neither can God lie, nor is anything hidden from Him. Now to call God to witness is named "jurare" [to swear] because it is established as though it were a principle of law [jure] that what a man asserts under the invocation of God as His witness should be accepted as true. Now sometimes God is called to witness when we assert present or past events, and this is termed a "declaratory oath"; while sometimes God is called to witness in confirmation of something future, and this is termed a "promissory oath." But oaths are not employed in order to substantiate necessary matters, and such as come under the investigation of reason; for it would seem absurd in a scientific discussion to wish to prove one's point by an oath.
Reply to Objection: 1. It is one thing to employ a Divine witness already given, as when one adduces the authority of Holy Scripture; and another to implore God to bear witness, as in an oath.
2. A man is said to pay his oaths to God because he performs what he swears to do, or because, from the very fact that he calls upon God to witness, he recognizes Him as possessing universal knowledge and unerring truth.
3. A person is called to give witness, in order that he may make known the truth about what is alleged. Now there are two ways in which God makes known whether the alleged facts are true or not. In one way He reveals the truth simply, either by inward inspiration, or by unveiling the facts, namely, by making public what was hitherto secret: in another way by punishing the lying witness, and then He is at once judge and witness, since by punishing the liar He makes known his lie. Hence oaths are of two kinds: one is a simple contestation of God, as when a man says "God is my witness," or, "I speak before God," or, "By God," which has the same meaning, as Augustine states [*See argument On the contrary]; the other is by cursing, and consists in a man binding himself or something of his to punishment if what is alleged be not true.
Objection: 1. It would seem that it is not lawful to swear. Nothing forbidden in the Divine Law is lawful. Now swearing is forbidden (Mt 5,34), "But I say to you not to swear at all"; and (Jc 5,12), "Above all things, my brethren, swear not." Therefore swearing is unlawful.
2. Further, whatever comes from an evil seems to be unlawful, because according to Mt 7,18, "neither can an evil tree bring forth good fruit." Now swearing comes from an evil, for it is written (Mt 5,37): "But let your speech be: Yea, yea: No, no. And that which is over and above these is of evil." Therefore swearing is apparently unlawful.
3. Further, to seek a sign of Divine Providence is to tempt God, and this is altogether unlawful, according to Dt 6,16, "Thou shalt not tempt the Lord thy God." Now he that swears seems to seek a sign of Divine Providence, since he asks God to bear witness, and this must be by some evident effect. Therefore it seems that swearing is altogether unlawful.
On the contrary It is written (Dt 6,13): "Thou shalt fear the Lord thy God . . . and shalt swear by His name."
I answer that Nothing prevents a thing being good in itself, and yet becoming a source of evil to one who makes use thereof unbecomingly: thus to receive the Eucharist is good, and yet he that receives it "unworthily, eateth and drinketh judgment to himself" (1Co 11,29). Accordingly in answer to the question in point it must be stated that an oath is in itself lawful and commendable. This is proved from its origin and from its end. From its origin, because swearing owes its introduction to the faith whereby man believes that God possesses unerring truth and universal knowledge and foresight of all things: and from its end, since oaths are employed in order to justify men, and to put an end to controversy (He 6,16).Yet an oath becomes a source of evil to him that makes evil use of it, that is who employs it without necessity and due caution. For if a man calls God as witness, for some trifling reason, it would seemingly prove him to have but little reverence for God, since he would not treat even a good man in this manner. Moreover, he is in danger of committing perjury, because man easily offends in words, according to Jc 3,2, "If any man offend not in word, the same is a perfect man." Wherefore it is written (Si 23,9): "Let not thy mouth be accustomed to swearing, for in it there are many falls."
Reply to Objection: 1. Jerome, commenting on Mt 5,34, says: "Observe that our Saviour forbade us to swear, not by God, but by heaven and earth. For it is known that the Jews have this most evil custom of swearing by the elements." Yet this answer does not suffice, because James adds, "nor by any other oath." Wherefore we must reply that, as Augustine states (De Mendacio xv), "when the Apostle employs an oath in his epistles, he shows how we are to understand the saying, 'I say to you, not to swear at all'; lest, to wit, swearing lead us to swear easily and from swearing easily, we contract the habit, and, from swearing habitually, we fall into perjury. Hence we find that he swore only when writing, because thought brings caution and avoids hasty words."
2. According to Augustine (De Serm. Dom. in Monte i. 17): "If you have to swear, note that the necessity arises from the infirmity of those whom you convince, which infirmity is indeed an evil. Accordingly He did not say: 'That which is over and above is evil,' but 'is of evil.' For you do no evil; since you make good use of swearing, by persuading another to a useful purpose: yet it 'comes of the evil' of the person by whose infirmity you are forced to swear."
3. He who swears tempts not God, because it is not without usefulness and necessity that he implores the Divine assistance. Moreover, he does not expose himself to danger, if God be unwilling to bear witness there and then: for He certainly will bear witness at some future time, when He "will bring to light the hidden things of darkness, and will make manifest the counsels of hearts" (1Co 4,5). And this witness will be lacking to none who swears, neither for nor against him.
Objection: 1. It would seem that justice, judgment and truth are unsuitably assigned as the conditions accompanying an oath. Things should not be enumerated as diverse, if one of them includes the other. Now of these three, one includes another, since truth is a part of justice, according to Tully (De Invent. Rhet. ii, 53): and judgment is an act of justice, as stated above (Question [60], Article [1]). Therefore the three accompanying conditions of an oath are unsuitably assigned.
2. Further, many other things are required for an oath, namely, devotion, and faith whereby we believe that God knows all things and cannot lie. Therefore the accompanying conditions of an oath are insufficiently enumerated.
3. Further, these three are requisite in man's every deed: since he ought to do nothing contrary to justice and truth, or without judgment, according to 1Tm 5,21, "Do nothing without prejudice," i.e. without previous judgment [*Vulg.: 'Observe these things without prejudice, doing nothing by declining to either side.']. Therefore these three should not be associated with an oath any more than with other human actions.
On the contrary It is written (Jr 4,2): "Thou shalt swear: As the Lord liveth, in truth, and in judgment, and in justice": which words Jerome expounds, saying: "Observe that an oath must be accompanied by these conditions, truth, judgment and justice."
I answer that As stated above (Article [2]), an oath is not good except for one who makes good use of it. Now two conditions are required for the good use of an oath. First, that one swear, not for frivolous, but for urgent reasons, and with discretion; and this requires judgment or discretion on the part of the person who swears. Secondly, as regards the point to be confirmed by oath, that it be neither false, nor unlawful, and this requires both truth, so that one employ an oath in order to confirm what is true, and justice, so that one confirm what is lawful. A rash oath lacks judgment, a false oath lacks truth, and a wicked or unlawful oath lacks justice.
Reply to Objection: 1. Judgment does not signify here the execution of justice, but the judgment of discretion, as stated above. Nor is truth here to be taken for the part of justice, but for a condition of speech.
2. Devotion, faith and like conditions requisite for the right manner of swearing are implied by judgment: for the other two regard the things sworn to as stated above. We might also reply that justice regards the reason for swearing.
3. There is great danger in swearing, both on account of the greatness of God Who is called upon to bear witness, and on account of the frailty of the human tongue, the words of which are confirmed by oath. Hence these conditions are more requisite for an oath than for other human actions.
Objection: 1. It would seem that an oath is not an act of religion, or latria. Acts of religion are about holy and divine things. But oaths are employed in connection with human disputes, as the Apostle declares (He 6,16). Therefore swearing is not an act of religion or latria.
2. Further, it belongs to religion to give worship to God, as Tully says (De Invent. Rhet. ii, 53). But he who swears offers nothing to God, but calls God to be his witness. Therefore swearing is not an act of religion or latria.
3. Further, the end of religion or latria is to show reverence to God. But the end of an oath is not this, but rather the confirmation of some assertion. Therefore swearing is not an act of religion.
On the contrary It is written (Dt 6,13): "Thou shalt fear the Lord thy God, and shalt serve Him only, and thou shalt swear by His name." Now he speaks there of the servitude of religion. Therefore swearing is an act of religion.
I answer that As appears from what has been said above (Article [1]), he that swears calls God to witness in confirmation of what he says. Now nothing is confirmed save by what is more certain and more powerful. Therefore in the very fact that a man swears by God, he acknowledges God to be more powerful, by reason of His unfailing truth and His universal knowledge; and thus in a way he shows reverence to God. For this reason the Apostle says (He 6,16) that "men swear by one greater than themselves," and Jerome commenting on Mt 5,34, says that "he who swears either reveres or loves the person by whom he swears." The Philosopher, too, states (Metaph. i, 3) that "to swear is to give very great honor." Now to show reverence to God belongs to religion or latria. wherefore it is evident that an oath is an act of religion or latria.
Reply to Objection: 1. Two things may be observed in an oath. The witness adduced, and this is Divine: and the thing witnessed to, or that which makes it necessary to call the witness, and this is human. Accordingly an oath belongs to religion by reason of the former, and not of the latter.
2. In the very fact that a man takes God as witness by way of an oath, he acknowledges Him to be greater: and this pertains to the reverence and honor of God, so that he offers something to God, namely, reverence and honor.
3. Whatsoever we do, we should do it in honor of God: wherefore there is no hindrance, if by intending to assure a man, we show reverence to God. For we ought so to perform our actions in God's honor that they may conduce to our neighbor's good, since God also works for His own glory and for our good.
Objection: 1. It would seem that oaths are desirable and to be used frequently as something useful and good. Just as a vow is an act of religion, so is an oath. Now it is commendable and more meritorious to do a thing by vow, because a vow is an act of religion, as stated above (Question [88], Article [5]). Therefore for the same reason, to do or say a thing with an oath is more commendable, and consequently oaths are desirable as being good essentially.
2. Further, Jerome, commenting on Mt 5,34, says that "he who swears either reveres or loves the person by whom he swears." Now reverence and love of God are desirable as something good essentially. Therefore swearing is also.
3. Further, swearing is directed to the purpose of confirming or assuring. But it is a good thing for a man to confirm his assertion. Therefore an oath is desirable as a good thing.
On the contrary It is written (Si 23,12): "A man that sweareth much shall be filled with iniquity": and Augustine says (De Mendacio xv) that "the Lord forbade swearing, in order that for your own part you might not be fond of it, and take pleasure in seeking occasions of swearing, as though it were a good thing."
I answer that Whatever is required merely as a remedy for an infirmity or a defect, is not reckoned among those things that are desirable for their own sake, but among those that are necessary: this is clear in the case of medicine which is required as a remedy for sickness. Now an oath is required as a remedy to a defect, namely, some man's lack of belief in another man. Wherefore an oath is not to be reckoned among those things that are desirable for their own sake, but among those that are necessary for this life; and such things are used unduly whenever they are used outside the bounds of necessity. For this reason Augustine says (De Serm. Dom. in Monte i, 17): "He who understands that swearing is not to be held as a good thing," i.e. desirable for its own sake, "restrains himself as far as he can from uttering oaths, unless there be urgent need."
Reply to Objection: 1. There is no parity between a vow and an oath: because by a vow we direct something to the honor of God, so that for this very reason a vow is an act of religion. On the other hand, in an oath reverence for the name of God is taken in confirmation of a promise. Hence what is confirmed by oath does not, for this reason, become an act of religion, since moral acts take their species from the end.
2. He who swears does indeed make use of his reverence or love for the person by whom he swears: he does not, however, direct his oath to the reverence or love of that person, but to something else that is necessary for the present life.
3. Even as a medicine is useful for healing, and yet, the stronger it is, the greater harm it does if it be taken unduly, so too an oath is useful indeed as a means of confirmation, yet the greater the reverence it demands the more dangerous it is, unless it be employed aright; for, as it is written (Si 23,13), "if he make it void," i.e. if he deceive his brother, "his sin shall be upon him: and if he dissemble it," by swearing falsely, and with dissimulation, "he offendeth double," [because, to wit, "pretended equity is a twofold iniquity," as Augustine [*Enarr. in Ps 63,7] declares]: "and if he swear in vain," i.e. without due cause and necessity, "he shall not be justified."
Objection: 1. It would seem that it is not lawful to swear by creatures. It is written (Mt 5,34-36): "I say to you not to swear at all, neither by heaven . . . nor by the earth . . . nor by Jerusalem . . . nor by thy head": and Jerome, expounding these words, says: "Observe that the Saviour does not forbid swearing by God, but by heaven and earth," etc.
2. Further, punishment is not due save for a fault. Now a punishment is appointed for one who swears by creatures: for it is written (22, qu. i, can. Clericum): "If a cleric swears by creatures he must be very severely rebuked: and if he shall persist in this vicious habit we wish that he be excommunicated." Therefore it is unlawful to swear by creatures.
3. Further, an oath is an act of religion, as stated above (Article [4]). But religious worship is not due to any creature, according to Rm 1,23 Rm 1,25. Therefore it is not lawful to swear by a creature.
On the contrary Joseph swore "by the health of Pharaoh" (Gn 42,16). Moreover it is customary to swear by the Gospel, by relics, and by the saints.
I answer that As stated above (Article [1], ad 3), there are two kinds of oath. One is uttered a simple contestation or calling God as witness: and this kind of oath, like faith, is based on God's truth. Now faith is essentially and chiefly about God Who is the very truth, and secondarily about creatures in which God's truth is reflected, as stated above (Question [1], Article [1]). In like manner an oath is chiefly referred to God Whose testimony is invoked; and secondarily an appeal by oath is made to certain creatures considered, not in themselves, but as reflecting the Divine truth. Thus we swear by the Gospel, i.e. by God Whose truth is made known in the Gospel; and by the saints who believed this truth and kept it.The other way of swearing is by cursing and in this kind of oath a creature is adduced that the judgment of God may be wrought therein. Thus a man is wont to swear by his head, or by his son, or by some other thing that he loves, even as the Apostle swore (2Co 1,23), saying: "I call God to witness upon my soul."As to Joseph's oath by the health of Pharaoh this may be understood in both ways: either by way of a curse, as though he pledged Pharao's health to God; or by way of contestation, as though he appealed to the truth of God's justice which the princes of the earth are appointed to execute.
Reply to Objection: 1. Our Lord forbade us to swear by creatures so as to give them the reverence due to God. Hence Jerome adds that "the Jews, through swearing by the angels and the like, worshipped creatures with a Divine honor."In the same sense a cleric is punished, according to the canons (22, qu. i, can. Clericum, Objection [2]), for swearing by a creature, for this savors of the blasphemy of unbelief. Hence in the next chapter, it is said: "If any one swears by God's hair or head, or otherwise utter blasphemy against God, and he be in ecclesiastical orders, let him be degraded."
2. This suffices for the Reply to the Second Objection.
3. Religious worship is shown to one whose testimony is invoked by oath: hence the prohibition (Ex 23,13): "By the name of strange gods you shall not swear." But religious worship is not given to creatures employed in an oath in the ways mentioned above.
Objection: 1. It would seem that an oath has no binding force. An oath is employed in order to confirm the truth of an assertion. But when a person makes an assertion about the future his assertion is true, though it may not be verified. Thus Paul lied not (2Co 1,15, seqq.) though he went not to Corinth, as he had said he would (1Co 16,5). Therefore it seems that an oath is not binding.
2. Further, virtue is not contrary to virtue (Categ. viii, 22). Now an oath is an act of virtue, as stated above (Article [4]). But it would sometimes be contrary to virtue, or an obstacle thereto, if one were to fulfil what one has sworn to do: for instance, if one were to swear to commit a sin, or to desist from some virtuous action. Therefore an oath is not always binding.
3. Further, sometimes a man is compelled against his will to promise something under oath. Now, "such a person is loosed by the Roman Pontiffs from the bond of his oath" (Extra, De Jurejur., cap. Verum in ea quaest., etc.). Therefore an oath is not always binding.
4. Further, no person can be under two opposite obligations. Yet sometimes the person who swears and the person to whom he swears have opposite intentions. Therefore an oath cannot always be binding.
On the contrary It is written (Mt 5,33): "Thou shalt perform thy oaths to the Lord."
I answer that An obligation implies something to be done or omitted; so that apparently it regards neither the declaratory oath (which is about something present or past), nor such oaths as are about something to be effected by some other cause (as, for example, if one were to swear that it would rain tomorrow), but only such as are about things to be done by the person who swears.Now just as a declaratory oath, which is about the future or the present, should contain the truth, so too ought the oath which is about something to be done by us in the future. Yet there is a difference: since, in the oath that is about the past or present, this obligation affects, not the thing that already has been or is, but the action of the swearer, in the point of his swearing to what is or was already true; whereas, on the contrary, in the oath that is made about something to be done by us, the obligation falls on the thing guaranteed by oath. For a man is bound to make true what he has sworn, else his oath lacks truth.Now if this thing be such as not to be in his power, his oath is lacking in judgment of discretion: unless perchance what was possible when he swore become impossible to him through some mishap. as when a man swore to pay a sum of money, which is subsequently taken from him by force or theft. For then he would seem to be excused from fulfilling his oath, although he is bound to do what he can, as, in fact, we have already stated with regard to the obligation of a vow (Question [88], Article [3], ad 2). If, on the other hand, it be something that he can do, but ought not to, either because it is essentially evil, or because it is a hindrance to a good, then his oath is lacking in justice: wherefore an oath must not be kept when it involves a sin or a hindrance to good. For in either case "its result is evil" [*Cf. Bede, Homil. xix, in Decoll. S. Joan. Bapt.]Accordingly we must conclude that whoever swears to do something is bound to do what he can for the fulfilment of truth; provided always that the other two accompanying conditions be present, namely, judgment and justice.
Reply to Objection: 1. It is not the same with a simple assertion, and with an oath wherein God is called to witness: because it suffices for the truth of an assertion, that a person say what he proposes to do, since it is already true in its cause, namely, the purpose of the doer. But an oath should not be employed, save in a matter about which one is firmly certain: and, consequently, if a man employ an oath, he is bound, as far as he can, to make true what he has sworn, through reverence of the Divine witness invoked, unless it leads to an evil result, as stated.
2. An oath may lead to an evil result in two ways. First, because from the very outset it has an evil result, either through being evil of its very nature (as, if a man were to swear to commit adultery), or through being a hindrance to a greater good, as if a man were to swear not to enter religion, or not to become a cleric, or that he would not accept a prelacy, supposing it would be expedient for him to accept, or in similar cases. For oaths of this kind are unlawful from the outset: yet with a difference: because if a man swear to commit a sin, he sinned in swearing, and sins in keeping his oath: whereas if a man swear not to perform a greater good, which he is not bound to do withal, he sins indeed in swearing (through placing an obstacle to the Holy Ghost, Who is the inspirer of good purposes), yet he does not sin in keeping his oath, though he does much better if he does not keep it.Secondly, an oath leads to an evil result through some new and unforeseen emergency. An instance is the oath of Herod, who swore to the damsel, who danced before him, that he would give her what she would ask of him. For this oath could be lawful from the outset, supposing it to have the requisite conditions, namely, that the damsel asked what it was right to grant. but the fulfilment of the oath was unlawful. Hence Ambrose says (De Officiis i, 50): "Sometimes it is wrong to fulfil a promise, and to keep an oath; as Herod, who granted the slaying of John, rather than refuse what he had promised."
3. There is a twofold obligation in the oath which a man takes under compulsion: one, whereby he is beholden to the person to whom he promises something; and this obligation is cancelled by the compulsion, because he that used force deserves that the promise made to him should not be kept. The other is an obligation whereby a man is beholden to God, in virtue of which he is bound to fulfil what he has promised in His name. This obligation is not removed in the tribunal of conscience, because that man ought rather to suffer temporal loss, than violate his oath. He can, however, seek in a court of justice to recover what he has paid, or denounce the matter to his superior even if he has sworn to the contrary, because such an oath would lead to evil results since it would be contrary to public justice. The Roman Pontiffs, in absolving men from oaths of this kind, did not pronounce such oaths to be unbinding, but relaxed the obligation for some just cause.
4. When the intention of the swearer is not the same as the intention of the person to whom he swears, if this be due to the swearer's guile, he must keep his oath in accordance with the sound understanding of the person to whom the oath is made. Hence Isidore says (De Summo Bono ii, 31): "However artful a man may be in wording his oath, God Who witnesses his conscience accepts his oath as understood by the person to whom it is made." And that this refers to the deceitful oath is clear from what follows: "He is doubly guilty who both takes God's name in vain, and tricks his neighbor by guile." If, however, the swearer uses no guile, he is bound in accordance with his own intention. Wherefore Gregory says (Moral. xxvi, 7): "The human ear takes such like words in their natural outward sense, but the Divine judgment interprets them according to our inward intention."
Summa Th. II-II EN Qu.88 a.12