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 (Q[88], A[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.
Objection 2: Further, Jerome, commenting on Mat. 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.
Objection 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 ( Ecclus. 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.
Reply to Objection 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.
Reply to Objection 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 ( Ecclus. 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. lxiii, 7] declares]: "and if he swear in vain," i.e. without due cause and necessity, "he shall not be justified."
Whether it is lawful to swear by creatures?
Objection 1: It would seem that it is not lawful to swear by creatures. It is written ( Mat. 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.
Objection 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.
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 ). Moreover it is customary to swear by the Gospel, by relics, and by the saints.
I answer that, As stated above (A[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 (Q[1], A[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 ( 2 Cor. 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, OBJ[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."
This suffices for the Reply to the Second Objection.
Reply to 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.
Whether an oath has a binding force?
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 ( 2 Cor. 1:15, seqq.) though he went not to Corinth, as he had said he would ( 1 Cor. 16:5 ). Therefore it seems that an oath is not binding.
Objection 2: Further, virtue is not contrary to virtue (Categ. viii, 22). Now an oath is an act of virtue, as stated above (A[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.
Objection 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.
Objection 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.