Thomas Aquinas

“ 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. ”

“ 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." ”

“ The other thing to be considered is on the part of the man, whose assertion is confirmed by oath. For a man's assertion needs no confirmation save because there is a doubt about it. Now it derogates from a person's dignity that one should doubt about the truth of what he says, wherefore "it becomes not persons of great dignity to swear." ”

“ One man may promise something under oath to another in two ways. First, when he promises something for his benefit: for instance, if he promise to serve him, or to give him money: and from such a promise he can be released by the person to whom he made it: for he is understood to have already kept his promise to him when he acts towards him according to his will. Secondly, one man promises another something pertaining to God's honor or to the benefit of others: for instance, if a man promise another under oath that he will enter religion, or perform some act of kindness. ”

“ 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. ”

“ 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.
Reply to Objection 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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 (2 Cor. 1:23) , saying: "I call God to witness upon my soul." ”

“ 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 ”

“ Yet the contrary may be the case on account of the manner of swearing for instance, an oath by the Gospels might be taken with deliberation and solemnity, and an oath by God frivolously and without deliberation.
Reply to Objection 3: Nothing prevents the same thing from arising out of contrary causes, by way of superabundance and defect. It is in this way that some are debarred from swearing, through being of so great authority that it is unbecoming for them to swear; while others are of such little authority that their oaths have no standing. ”

“ Deliberation of the mind gives a vow its stability, on the part of the person who takes the vow: but it has a greater cause of stability on the part of God, to Whom the vow is offered.
Art. 9 - Whether anyone can dispense from an oath? Objection 1: It would seem that no one can dispense from an oath. Just as truth is required for a declaratory oath, which is about the past or the present, so too is it required for a promissory oath, which is about the future. Now no one can dispense a man from swearing to the truth about present or past things. ”

“ 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. ”

“ 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.
Objection 3: Further, an oath is an act of religion, as stated above (A [4] ) . But religious worship is not due to any creature, according to Rom. 1:23, 25. Therefore it is not lawful to swear by a creature.
On the contrary, Joseph swore "by the health of Pharaoh" (Gn. 42:16) . ”

“ Further, the duties of a judge differ from the duties of a witness, as shown above (QQ [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 (Heb. 6:16) , oaths are taken for the purpose of confirmation. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ Art. 5 - Whether oaths are desirable and to be used frequently as something useful and good? Objection 1: It would seem that oaths are desirable and to be used frequently as something useful and good. ”