The Duty of those which take an Oath
I. All Men agree in the Opinion, That an Oath gives a great additional Confirmation to all our Assertions, and to those Actions which depend upon our Discourse. An Oath is, A Religious Asseveration, by which we disavow the Divine Clemency, or imprecate to our selves the Wrath of God if we speak not the Truth. Now when an All-wise and an Almighty Witness and Guaranty is invok’d, it causes a strong Presumption of the Truth, because no Man can easily be thought so Wicked, as to dare rashly to call down upon himself the grievous Indignation of the Deity. Hence it is the Duty of those that take an Oath, To take the same with awful Reverence, and religiously to observe what they have sworn.
The End and Use
II. Now the End and Use of an Oath is chiefly this, To oblige Men the more firmly to speak the Truth, or to make good their Promises and Contracts out of an Awe of the Divine Being, who is infinitely Wise and Powerful; whose Vengeance they imprecate to themselves when they Swear, if they wittingly are guilty of Deceit; whereas otherwise the Fear of what Men can do may not be sufficient; because possibly they may have Hope to oppose or escape their Power, or to beguile their Understandings.
Swearing by what
III. Since GOD alone is of infinite Knowledge and of infinite Power, it is a manifest Absurdity to swear by any other Name but the Name of GOD only; that is, in such a Sense, as to invoke it for a Witness to our Speech, and for an Avenger of our Perjury: But if in the Form of Oaths any other Things, that we hold Dear, or have in Veneration or Esteem, be mention’d, it is not to be understood that such Things are invok’d as Witnesses to our Truth or Avengers of our Falsehood; but GOD only is herein invok’d, with a Desire, that if we swear falsely, he would be pleas’d to punish our Crime, in these Things especially for which we are most nearly and tenderly concern’d.
Forms how to be accommodated
IV. In Oaths the Form which is prescrib’d, (by which the Person swearing invokes GOD as a Witness and an Avenger) is to be accommodated to the Religion of the said Swearer; that is, to that Persuasion and Opinion of GOD which he is of. For ’tis to no Purpose to make a Man swear by a God, whom he does not believe, and consequently does not fear. But no Man supposes himself to take an Oath in any other Form, nor under any other Notion, than that which is consonant to the Precepts of his Religion, which, in his Opinion, is the true. Hence also it is, that he who swears by false Gods, which yet himself takes to be true ones, stands obliged, and if he falsifies is really guilty of Perjury; because whatever his peculiar Notions were, he certainly had some Sense of the Deity before his Eyes; and therefore by wilfully forswearing himself he violated, as far as he was able, that Awe and Reverence which he ow’d to Almighty GOD.
V. That an Oath may be binding, ’tis necessary it be taken with deliberate Thoughts, and a real Design: Whence he shall not be obliged by an Oath who meerly recites it; or speaking in the first Person, dictates the concept formal Words thereof to another who is to say after him. But he who shall seriously behave himself as one that is about to swear solemnly, shall be obliged, whatsoever mental Reservations he all the while may harbour in his Mind. For otherwise all Oaths, nay, all Methods of mutual Obligation by the Intervention of the plainest Significations would be of no Use to human Life, if any Man by his tacit Intention could hinder such an Act from obtaining such an Effect as it was first instituted to produce.
Oaths how obliging
VI.We ought likewise carefully to observe, that Oaths do not of themselves produce a new and peculiar Obligation, but are only apply’d as an Accessional Strength, and an additional Bond to an Obligation, in its nature valid before. For whenever we swear, we always suppose some Matter, upon non-performance of which we thus imprecate the Vengeance of Heaven. But now this would be to no purpose, unless the Omission of the Thing suppos’d had been before unlawful, and consequently, unless we had before been obliged. Tho’ indeed it frequently happens, that we comprehend in one Speech, both the principal Obligation and the additional Bond of the Oath; as thus, As God help me, I’ll give you a hundred Pounds. Where the Oath is not superfluous, albeit ’tis added to a Promise that might have been valid of it self. Because tho’ every good Man believes a bare Promise to oblige, yet ’tis look’d upon to be the more firm when ’tis reinforced with an Imprecation of Vengeance from above upon a Failure. Hence it follows, that any Acts which were before attended with some inward Flaw, hindring any Obligation to arise from them, cannot be made obligatory by the Accession of an Oath; as neither can a subsequent Oath avoid a former legitimate Engagement, or annul that Right which another may claim thereby; thus a Man would swear in vain not to pay another Person what is justly due to him: Nor will an Oath be of any Validity, where it appears, that ’twas made by the Juror upon Supposition of a Thing to be done which was not really so; and that he would not have so sworn, had not he believ’d it to be done;  especially if he were cajol’d into such his Error by the Craft of him to whom the Oath was made: Neither shall he, who by setting me under panick Fear forces me to take an Oath, have any good Title to require my Performance. Farthermore, an Oath shall have no Obligation upon me to do any unlawful Act, or to omit the performing any Duty enjoyn’d by the Laws of God or Man. Lastly, an Oath cannot alter the Nature or Substance of the Contract or Promise to which it is annex’d: Hence it cannot oblige to Impossibilities. Again, a Conditional Promise, by the Addition of an Oath, is not changed into a Positive and Absolute Promise: In like manner, it is no less requisite to Promises confirm’d by Oaths, than to others which are not so confirm’d, that they be accepted by the other Party: So that he who obtains a Right by any Covenant, may equally release the Performance of it, whether it was sworn to or= not.
VII. But the taking of an Oath has this Effect among Men, for the sake of that Invocation of God which is therein made use of, whose Wisdom no Man’s Cunning can elude, and who suffers not the Man that mocks Him to escape unpunish’d; that not only a heavier Punishment is assign’d to him who forswears himself, than to him who barely breaks his Word; but it puts them in mind to avoid all Deceit and Prevarication in the Matters which it is added to confirm.
VIII. Not yet that all Oaths are to be consider’d in their greatest Latitude, but that sometimes they must be interpreted in the narrowest Sense, if so it be, that the Subject-matter seem to require it: For instance; if the Oath be made to promote some malicious Design against another, to execute something threatned, and not to perform somewhat promis’d. Neither does an Oath exclude tacit Conditions and Limitations, provided they are such as plainly result from the Nature of the Thing; as suppose, I have sworn to give another whatsoever he shall request, if he ask what it is wicked or absurd for me to grant, I am not at all obliged. For he who indefinitely promises any Thing to him that desires, before he knows what he is like to ask, presupposes the other will crave nothing but what is honest, and morally possible, not Things absurd or mischievous to himself or any Body else.
Sense of an Oath
IX. This is also to be noted, that in Oaths the Sense of all the Words thereof is to be such as he shall acknowledge himself to take them in, who accepts the Oath, that is, to whom the other Party swears. For the Oath is to be look’d upon to be made for his sake, and not for the sake of the Juror. Whence it is his Part to dictate the Form of the Oath, and this to do in Words as plain as is possible, so that himself may signify in what Sense he conceives them; and the Person swearing may profess that he well understands his Meaning, and then those Words are distinctly to be express’d, that so no room may be left for Cavils or Shuffling.
X.Oaths may most fitly be distinguish’d according to the Use they are apply’d to in Human Life. Some are annex’d to Promises and Contracts, thereby to procure a strict and religious Observance of the same; others are apply’d to the Confirmation of any Man’s Assertion concerning a Matter of Fact not altogether evident, and where the Truth cannot by other Means be more conveniently search’d out; such are the Oaths administred to Witnesses, and those who are privy to another Man’s doings; sometimes also two Adversaries, or Litigants, may, with the Consent of the Judge, or the Concession of one Party, by taking such or such an Oath put an end to their Law-Suit.
Source: Samuel von Pufendorf, The Whole Duty of Man According to the Law of Nature