I. NOTION AND DIVISIONS
An oath is an invocation to God to witness the truth of a statement. It may be express and direct, as when one swears by God Himself; or implicit and tacit, as when we swear by creatures, since they bear a special relation to the Creator and manifest His majesty and the supreme Truth in a special way: for instance, if one swears by heaven, the throne of God ( Matthew 5:34 ), by the Holy Cross, or by the Gospels. Imprecatory oaths are also tacit (see below). To have an oath in foro interno , there must be the intention, at least virtual, of invoking the testimony of God, and a word or sign by which the intention is manifested. Oaths may be: (1) assertory—or affirmative—if we call God to witness the assertion of a past or present fact; promissory, if we call Him to witness a resolution which we bind ourselves to execute, or a vow made to Him, or an agreement entered into with our neighbour, or a vow made to God in favour of a third party; every promissory oath includes of necessity an assertory oath (see below). A promissory oath accompanied by a threat against a third party is said to be comminatory; (2) contestatory—or simple—if there is a mere invocation of the Divine testimony; imprecatory—or execratory—as in the formula "So help me God "; if at the same time we call upon God as a judge and avenger of perjury, offering Him our property and especially our life and eternal salvation, or those of our friends, as a pledge of our sincerity. Thus the expression: "Upon my soul ", often used without any intention of swearing, may be either contestatory—the soul being in a special manner the image of God — or execratory—if we wish to call down upon our soul Divine punishment, either temporal or eternal, in case we be wanting in sincerity; (3) private, if used between private individuals ; public, if exacted by public authorities; public oaths are divided into: (a) doctrinal, by which one declares that he holds a given doctrine, or promises to be faithful, to teach, and to defend a given doctrine in the future; (b) political, which have as their object the exercise of any authority whatsoever, or submission to such an authority or laws ; (c) judicial, which are taken in courts of justice either by the parties to the suit or the witnesses thereof.
II. LAWFULNESS AND CONDITIONS
An oath is licit, and an act of virtue, under certain conditions. It is, in effect, an act of homage rendered by the creature to the wisdom and omnipotence of the Creator—it is therefore an act of the virtue of religion; moreover, it is an excellent way of affording men security in their mutual intercourse. It is justified in the Old and New Testament ; the faithful and the Church from Apostolic times to the present day have employed oaths; and canonical legislation and doctrinal decrees have affirmed their lawfulness. Improper use is often made of oaths, and the habit of swearing may easily lead to abuses and even to perjury. In counselling men "not to swear at all" ( Matthew 5:34 ) Christ meant, as the Fathers and ecclesiastical writers explain, to be so truthful that men could believe them without need of oath to confirm what they say. He did not forbid the use of oaths under proper conditions, when necessary to satisfy others of our truthfulness. These conditions are ( Jeremiah 4:2 ): (1) Judgment, or careful and reverent consideration of the necessity or utility of the oath; for it would be showing a want of the respect due to God, to invoke Him as witness in trivial matters; on the other hand, it would be wrong to require a grave or extreme necessity. To swear without a sufficient reason, being an idle use of God's name, is a venial sin ; (2) truth, for what we affirm should be in conformity with the truth. Consequently in case of an assertory oath, our affirmation must be truthful, and in a promissory oath we must have the intention of doing what we are promising. To swear falsely constitutes the sin of perjury, always mortal in its nature : for it is an insult to the Divine Truth to call God in witness to a lie; besides, such an act is likely to do injury to the common good; see the propositions condemned by Innocent XI , prop. xxiv; (3) justice requiring: (a) in the case of an assertory oath, that it be lawful to make the affirmation which one wishes to corroborate; failure to observe this condition is a venial sin, as when boasting of some evil deed one should swear to it; it is a grievous sin, if one employs an oath as the means and instrument of sin, at least of mortal sin, for example, to make a person believe a grave detraction ; (b) in the case of a promissory oath, justice requires that one be able to assume licitly the obligation of doing the thing promised. It is a mortal sin to promise an oath to do a grievously illicit thing; and it is, in the opinion of St. Alphonsus Liguori, a mortal sin to swear to do a thing which is illicit though not grievously so.
III. OBLIGATION ARISING FROM A PROMISSORY OATH
In a promissory oath, we call on God not only as a witness of our desire to fulfil the promise we make, but also as a guarantee and pledge for its future execution; for at the proper moment He will require us, under pain of sin against the virtue of religion, to do what we have promised in His presence; whence it follows that it is a sin against religion not to perform, when we can, what we promised under oath: a mortal sin if the matter is grave; a venial sin (according to the more common and more probable opinion), if the matter is not grave. Certain conditions are requisite before a promissory oath entails the obligation of fulfilling it, notably the intention of swearing and of binding oneself, full deliberation, the lawfulness of making the promise, as well as the lawfulness and possibility of executing it, etc. Several causes may put an end to this obligation : intrinsic causes, such as a notable change occurring after the taking of the oath, the cessation of the final cause of the oath; or extrinsic causes, such as annulment, dispensation, commutation, or relaxation granted by a competent authority, a release, express or tacit, either by the person in whose favour the obligation was undertaken, or by a competent authority to whom the beneficiary is subject.
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