Thomas Aquinas, Summa Theologiae

Objection 1: It would seem that it is not lawful to swear. Nothing forbidden in the Divine Law is lawful. Now swearing is forbidden (Mt. 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 Mt. 7:18, "neither can an evil tree bring forth good fruit." Now swearing comes from an evil, for it is written (Mt. 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 Dt. 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 (Dt. 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 Cor. 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 (Heb. 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 (Ecclus. 23:9): "Let not thy mouth be accustomed to swearing, for in it there are many falls."

Reply to Objection 1: Jerome, commenting on Mt. 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 Cor. 4:5). And this witness will be lacking to none who swears, neither for nor against him.

Articulus 2

Argumentum 1

Ad secundum sic proceditur. Videtur quod non sit licitum iurare.

Nihil enim quod prohibetur in lege divina est licitum.

Sed iuramentum prohibetur Matth. V, ego dico vobis, non iurare omnino, et Iac. V dicitur, ante omnia, fratres mei, nolite iurare.

Ergo iuramentum est illicitum.

Argumentum 2

Praeterea, id quod est a malo videtur esse illicitum, quia, ut dicitur Matth. VII, non potest arbor mala fructus bonos facere.

Sed iuramentum est a malo, dicitur enim Matth. V, sit autem sermo vester, est, est; non, non. Quod autem his abundantius est a malo est.

Ergo iuramentum videtur esse illicitum.

Argumentum 3

Praeterea, exquirere signum divinae providentiae est tentare deum, quod est omnino illicitum, secundum illud Deut. VI, non tentabis dominum deum tuum.

Sed ille qui iurat videtur exquirere signum divinae providentiae, dum petit divinum testimonium, quod est per aliquem evidentem effectum. Ergo videtur quod iuramentum sit omnino illicitum.

Sed Contra

Sed contra est quod dicitur Deut. VI, dominum deum tuum timebis, et per nomen eius iurabis.

Corpus

Respondeo dicendum quod nihil prohibet aliquid esse secundum se bonum quod tamen cedit in malum eius qui non utitur eo convenienter, sicut sumere eucharistiam est bonum, et tamen qui indigne sumit sibi iudicium manducat et bibit, ut dicitur I ad Cor. XI.

Sic ergo in proposito dicendum est quod iuramentum secundum se est licitum et honestum. Quod patet ex origine et ex fine.

Ex origine quidem, quia iuramentum est introductum ex fide qua homines credunt deum habere infallibilem veritatem et universalem omnium cognitionem et provisionem. Ex fine autem, quia iuramentum inducitur ad iustificandum homines, et ad finiendum controversias, ut dicitur ad Heb. VI.

Sed iuramentum cedit in malum alicui ex eo quod male utitur eo, idest sine necessitate et cautela debita. Videtur enim parvam reverentiam habere ad deum qui eum ex levi causa testem inducit, quod non praesumeret etiam de aliquo viro honesto. Imminet etiam periculum periurii, quia de facili homo in verbo delinquit, secundum illud Iac. III, si quis in verbo non offendit, hic perfectus est vir.

Unde et Eccli. XXIII dicitur, iurationi non assuescat os tuum, multi enim casus in illa.

Ad 1

Ad primum ergo dicendum quod Hieronymus, super Matth., dicit, considera quod salvator non per deum iurare prohibuerit, sed per caelum et terram. Hanc enim per elementa iurandi pessimam consuetudinem habere Iudaei noscuntur.

Se ista responsio non sufficit, quia Iacobus addit, neque per aliud quodcumque iuramentum.

Et ideo dicendum est quod, sicut Augustinus dicit, in libro de mendacio, quod apostolus, in epistolis suis iurans, ostendit quomodo accipiendum esset quod dictum est, dico vobis non iurare omnino, ne scilicet iurando ad facilitatem iurandi veniatur, ex facilitate iurandi ad consuetudinem, a consuetudine in periurium decidatur.

Et ideo non invenitur iurasse nisi scribens, ubi consideratio cautior non habet linguam praecipitem.

Ad 2

Ad secundum dicendum quod, sicut Augustinus dicit, in libro de serm. Dom. In monte, si iurare cogeris, scias de necessitate venire infirmitatis eorum quibus aliquid suades, quae utique infirmitas malum est. Itaque non dixit, quod amplius est malum est; tu enim non malum facis qui bene uteris iuratione, ut alteri persuadeas quod utiliter persuades, sed, a malo est illius cuius infirmitate iurare cogeris.

Ad 3

Ad tertium dicendum quod ille qui iurat non tentat deum, quia non implorat divinum auxilium absque utilitate et necessitate; et praeterea non exponit se alicui periculo si deus testimonium adhibere noluerit in praesenti. Adhibebit autem pro certo testimonium in futuro, quando illuminabit abscondita tenebrarum et manifestabit consilia cordium, ut dicitur I ad Cor. IV.

Et illud testimonium nulli iuranti deficiet, vel pro eo vel contra eum.