CHAPTER XI
On the Duty of Those Who Take Oaths

1. By an oath it is held that a striking confirmation is added to our speech, and all the acts in which speech has a part. For it is a religious affirmation, in which we renounce our claim upon the Divine mercy, or call down the Divine punishment upon ourselves, if we do not tell the truth. And while the omniscient and omnipotent witness, who is at the same time the avenger, is being invoked, the presumption of truth is created by the fact that hardly anyone is thought so impious that he will dare thus insolently to call down upon himself the dire wrath of the Deity. Hence it is understood to be the duty of oath-takers to approach the oath with reverence, and then religiously to observe their oath.

2. Moreover the end and use of an oath consists chiefly in this, that men may be bound all the more firmly to declare the truth, or to keep promise or contract, by fear of the omniscient and omnipotent Deity, whose vengeance, if they knowingly deceive, they call down upon themselves by the oath, whereas the fear of men seemed ineffectual. For they hoped to be able to despise or elude men's power, or escape their knowledge.

3. But because, aside from God, there is nothing omniscient or omnipotent, it is absurd to take an oath by something which is not believed to be divine, with the idea that that thing is being invoked as witness and punisher of perjury. And yet it is common in oaths to name a certain thing and swear by it, with the understanding that, in case of perjury, God may wreak His vengeance on that particular thing, as very dear to the deponent, and highly esteemed by him.

4. In oaths the formula, in which God is described, while invoked as witness and avenger, must be adapted to that belief or religion which the taker of the oath cherishes in regard to God. For it is vain to require of a man an oath by a god whom he does not believe in, and so does not fear. But no one thinks he is swearing by God, if under another formula or under another name than that contained in the teachings of his own religion, — the true religion in the opinion of the deponent. Hence also he who swears by false gods, whom he however holds to be true, is certainly bound, and in case he deceives, really commits perjury. For under some special concept he had before his eyes a general notion of deity, and so, in knowingly forswearing himself, he has violated, so far as in him lay, the reverence due the majesty of God.

5. To the binding force of an oath it is necessary that it be undertaken with a deliberate intention. Hence one who has read an oath aloud, or dictated its formal wording in the first person to another, is not bound at all by an oath. But one who gave the appearance of intending seriously to swear, certainly will be bound, no matter what he had in mind while swearing. For otherwise the whole practice of oaths, or rather every means of binding one's self by recourse to outward signs, would be taken away from human life, if by mental reservation one could prevent a formal act from having the effect which it was designed to produce.

6. Oaths do not in themselves produce a new and particular obligation, but are added as a sort of accessory bond to an obligation in itself valid. For always in swearing we suppose something, failing to perform which, we call down upon us the Divine punishment. And this would be foolish, if it were not unlawful to fail to perform the thing in question, in other words, if we were not already bound. Hence it follows that formal arts, to which attaches a defect nullifying the obligation, do not become obligatory by the addition of an oath. In the same way a previous valid agreement is not destroyed by a subsequent oath, nor is a right which another had thereby acquired now taken away from him. Therefore it is vain to swear that one will not pay one's debt to another. And an oath is not binding when it is established that the swearer supposed some fact which in reality was not such, and would not have sworn, if he had not so believed; especially if he was led into error by a trick on the part of him to whom he swore. Also the man who by an unjust fear makes me take an oath acquires in consequence of that oath no right legally to demand anything of me. Of no effect also is an oath concerning the performance of an unlawful act; or even that which concerns the omission of some good deed enjoined by laws divine or human. Finally, an oath does not change the nature and substance of a promise or contract to which it is added. Hence it is in vain that one swears to impossible things. And a conditional promise is not changed into an absolute or simple one by an oath. And acceptance is required just as truly in the case of a promise under oath.

7. Moreover, by reason of the added appeal to God, whom none may deceive by cunning nor mock with impunity, oaths have this effect, that not only is a severer penalty thought to await him who has proved false to a sworn than an unsworn promise; but also that trickery and equivocation must be excluded from dealings to which oaths are added.

8. Not always, however, are oaths to be interpreted in a broad sense, but also sometimes strictly, if indeed the subject-matter seems to require it; for example, if the oath took the direction of hatred toward another, and was added to threats, rather than to a promise. In fact even an oath does not exclude tacit conditions and limitations, which properly flow from the very nature of the thing; for example, if under oath I have given a man the privilege of asking anything that he pleases, in case he asks unjust or absurd things, I shall not be bound at all. For whoever at another's request makes an indefinite promise, not yet knowing what he is going to ask, presupposes that the other will ask things honorable, morally possible, and not absurd or dangerous for himself or others.

9. We must also note that in oaths the sense of the whole text prevails as avowedly understood by the man who tenders the oath, that is, the man to whom we swear. For it is in his interest especially, not that of the swearer, that an oath is given. Hence it is his privilege also to frame the words of the oath, and with all possible clearness. And he must show how he understands them, while the swearer should declare that he clearly understands the meaning; and he must clearly pronounce the words, so that he can in no way equivocate or evade.

10. The division of oaths is conveniently based upon the use to which they are put in the common life. For some are added to promises and contracts, that they may be observed the more scrupulously. And some are employed to confirm a man's assertion concerning a fart that is not dear, and where the truth cannot be discovered more conveniently by other means. Such an oath is demanded of witnesses, that is, persons thought to have knowledge of another's deed. Sometimes also those who are at odds settle their suit by taking an oath, as proposed by the judge or the other party.

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