Obj. 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 (Matt. 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 (Q. 88, A. 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 Obj. 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.
Reply Obj. 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."
Reply Obj. 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.