Article III: On the Obligation of a Promissory Oath

S. Th. II-II, q. 89, a. 7-9.

755. The intrinsic effect of a promissory oath is the obligation of executing the thing sworn from the virtue of religion. - It is certain, both from Scripture (Num. XXX, 3; Mt. V, 33), and from reason, because he who does not fulfill a promise confirmed in His name inflicts injury upon the authority of God (can. 1317, § 1).

The obligation is grave from its genus, yet probably admits lightness of matter, just as a vow does.

It is proved: 1. directly. Because the obligation of a vow is graver than the obligation of an oath as promissory.¹ For the former is caused from the fidelity which we owe to God Himself, so that we pay Him what was promised; the latter is caused from reverence toward God, from which we are bound to verify that which through His name we promised before Him. But the obligation of fidelity regards God more directly than the obligation of reverence, for infidelity contains irreverence, not conversely, and infidelity toward a lord is the greatest irreverence. Therefore we are more strictly bound to do that which we promised to God Himself than that which we promised with God only as witness (a. 8). Whence if the obligation of a vow admits lightness of matter, a fortiori the obligation of a promissory oath does.

¹ We say: as promissory. For the obligation of an oath as assertory is greater: there it is a negative precept not to swear falsely, which is directly contrary to God; here it is an affirmative precept of fulfilling a sworn promise. But a negative precept obliges more than an affirmative.

2. indirectly. Because the violation of a promissory oath is not proved to be perjury strictly so called, as the adversaries² contend, saying: He who does not fulfill an oath makes it be false and lying, because he makes God witness of a lie. For he who does not fulfill what he sincerely promised does not make a lie; because he does not say or do against his mind, but only changes his mind, inconstant and unfaithful, as St. Thomas himself explains (II-II, q. 110, a. 3, ad 5). Therefore he who does not fulfill what he sincerely swore does not make God witness of a lie, provided he had the intention of fulfilling the oath; but he only acts inconstantly or unfaithfully in a thing promised under divine authority, and thus contracts irreverence toward God, which is a grave or light sin according to the gravity or lightness of his inconstancy or infidelity.

² For example, Cajetan, Toledo, Valentia, Lessius, Gabriel, Bonacina, etc. On the contrary, St. Antoninus, Sylvester, Soto, Suarez, Laymann, Navarrus, Sylvius, Azor, the Salmanticenses, Billuart, and almost all moderns.

In other words, violation of an oath as promissory is not a sin which directly opposes a divine attribute, and therefore it is not necessarily and always mortal.

Nevertheless it does not follow that someone can oblige himself by oath under light obligation in grave matter, as for a vow. For the disparity is that to promise is nothing other than to will to oblige oneself, whence the whole obligation of a promise immediately depends upon the will of the agent, who can promise or oblige himself more or less. But an oath is not directly for obliging but for the confirmation of truth by divine testimony, from which confirmation obligation naturally follows, because of the reverence of God; whence this obligation of an oath does not depend only and immediately upon the will of the one swearing, but upon the law of reverence owed to God, because it is a consequence of the testimony invoked. For someone cannot swear more or less, or invoke God as witness more or less, but he either invokes or does not invoke, and once He is invoked, God is patron of the obligation and urges fulfillment from the law of reverence owed to Him, which is greater or less not indeed according to the intention of the one swearing, but according to the gravity of the promise or of the promised matter sworn.

756. The conditions required for the validity of a promissory oath, both on the part of the act and on the part of the matter, are almost the same as for a vow.

Yet there are two exceptions:

1. An oath made in favor of a third party need not be made concerning a better good, but it suffices that it be concerning a good useful to one's neighbor. The direct end of a vow is the greater worship and honor of God; but the direct end of an oath is the confirmation of truth, which can also be had in a lesser good.

2. Even grave and unjust fear does not invalidate an extorted oath, as is now altogether certain from canon 1317, § 2, because although it renders the act involuntary in a certain respect, nevertheless it does not simply remove the voluntary. Whence an oath remains a human act and consequently is capable of inducing obligation; which nevertheless can, in the name of God, be relaxed by an ecclesiastical superior.

Nor should you say that God does not accept an oath extorted by injury, because an oath is not made in favor of God nor must it be accepted by Him, but is a simple appeal to His testimony; that God as patron gives authority to the promise is only a consequence of the invocation. Yet in the case the one swearing can use a restriction: I will give insofar as you have a right; but if he did not do this but truly invoked, he is bound to pay the promise, yet he can immediately seek back in judgment what he paid (a. 7, ad 3).

If a condition required for the validity of an oath is lacking, sin is committed, grave or light, in the same way as was said concerning a vow (n. 721). But he who swears fictitiously or merely externally is not obliged by force of the oath from religion, but can sometimes be held because of scandal or harm inflicted.

757. The obligation of a promissory oath must be fulfilled and ceases in the same way as was said concerning the fulfillment of a vow (n. 727) or its release from obligation (n. 732). Yet the obligation of an oath ceases not only by all the things by which the obligation of a vow ceases, but also by the renunciation of the third party or by condonation made by him in whose favor it principally redounds (a. 7, ad 2 and a. 9; can. 1319).

But three things should be noted: 1. that he who can irritate, dispense, or commute a vow can by that very fact do this concerning a promissory oath (can. 1320); 2. but if the dispensation of an oath tends to the prejudice of others who refuse to remit the obligation, the Apostolic See alone can dispense, and only because of necessity or utility of the Church or the common good (ibid.); 3. probably no oaths are reserved, even if they confirm reserved vows.

758. The obligation of a promissory oath is to be interpreted strictly, because the one swearing is judged to have wished to oblige himself as little as he could. Thus especially canon 1321, conformably to St. Thomas, a. 7, ad 4.

Hence: a) An oath per se, and unless the one swearing intended otherwise, follows the nature of the act to which it is joined: the accessory follows the principal; therefore if it is added to a vow, promise, or contract, its obligation will have the same extension, the same limits, and the same expressed or tacit conditions (can. 1318, § 1). Thus, for example, an oath of keeping a secret, residence, a contract such as betrothal, the statutes of a college, fidelity toward a regime, per se does not oblige beyond what is demanded by the matter of the secret, or the law of residence, or the nature of the contract, for example, of betrothal, or the custom had concerning the statutes of a college not to be published but already published, or the fidelity of a regime insofar as it is not opposed to a superior divine and ecclesiastical right.

b) "If an oath is added to an act directly tending to the harm of another or to the prejudice of the public good or of eternal salvation, the act obtains no firmness from it" (can. 1318, § 2).

c) Although an oath does not have the force of validating and confirming invalid or rescindable contracts, nevertheless, if the contract is invalid, it can induce an obligation of religion, if it is emitted absolutely and independently of the obligation of the contract, so that the one swearing is prohibited by religion from seeking the annulment of the contract, and, with the contract irritated, the obligation of religion remains until there is dispensation in it. Then, namely, there is a mere purpose confirmed by oath and a promissory oath broadly so called or standing of itself.

d) Oaths very often used: of not selling for less or not buying for more, of not granting or accepting a favor, of chastising a subject, etc., generally do not oblige, because the intention of swearing is lacking, or the matter is vain and useless, or finally the condition understood is lacking (for example, if the matter is not otherwise more opportune), or the state of things is sufficiently changed.