Secunda Secundæ, Question 89 — Summa Theologiae
Article 1. Whether to swear is to call God to witness?
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.
Objection 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 (Matthew 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.
Objection 3. Further, the duties of a judge differ from the duties of a witness, as shown above (Questions 67 and 70). Now sometimes a man, by swearing, implores the Divine judgment, according to Psalm 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 (Hebrews 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 Psalm 16:10, “Their mouth hath spoken lies [Vulgate: ‘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.
Reply to Objection 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.
Reply to Objection 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. On 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.
Article 2. Whether it is lawful to swear?
Objection 1. It would seem that it is not lawful to swear. Nothing forbidden in the Divine Law is lawful. Now swearing is forbidden (Matthew 5:34), “But I say to you not to swear at all”; and (James 5:12), “Above all things, my brethren, swear not.” Therefore swearing is unlawful.
Objection 2. Further, whatever comes from an evil seems to be unlawful, because according to Matthew 7:18, “neither can an evil tree bring forth good fruit.” Now swearing comes from an evil, for it is written (Matthew 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.
Objection 3. Further, to seek a sign of Divine Providence is to tempt God, and this is altogether unlawful, according to Deuteronomy 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 (Deuteronomy 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” (1 Corinthians 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 (Hebrews 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 James 3:2, “If any man offend not in word, the same is a perfect man.” Wherefore it is written (Sirach 23:9): “Let not thy mouth be accustomed to swearing, for in it there are many falls.”
Reply to Objection 1. Jerome, commenting on Matthew 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.”
Reply to Objection 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.”
Reply to Objection 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” (1 Corinthians 4:5). And this witness will be lacking to none who swears, neither for nor against him.