Chapter II. On Oath

306. Definition. An oath is an invocation ¹ of the divine Name as testimony of the truth ².

¹ By this chiefly an oath differs from a vow, whose essence consists in the promise alone.

² Others, in order to explain better the twofold end of an oath, say that an oath is an invocation of the divine Name as testimony, either for giving credibility to words, or for confirming a promise.

Division. An oath is distinguished:

1. Into assertory or promissory, according as the truth ³ alone is asserted, or a promise or agreement is confirmed.

³ Others, in order to include both a true and a false oath, define an assertory oath as that by which present or past things are affirmed or denied; for a promissory oath, as is clear, extends itself only to future things. Although not every oath concerning the future is promissory. For a twofold oath concerning the future is distinguished by the doctors: one bare and simple, which contains no promise toward another, for example if you swear that you will not enter this house or tavern, etc.; another truly promissory, which is confirmatory of a promise either toward God, which is a sworn vow, or toward one's neighbor, which adds to the obligation of justice or fidelity arising from the promise an obligation from the virtue of religion.

2. Into solemn or simple, according as it is made with the form required by law, or with solemn ceremonies, or with these set aside ⁴.

⁴ Clerics, especially priests, cannot interpose a solemn oath except by permission of the Bishop, which is granted only for a grave cause.

3. Into explicit or implicit, according as God is expressly invoked as witness, or creatures are employed as testimony, in which the divine attributes shine forth in a special way ⁵.

⁵ The doctors warn that an oath by creatures, by heaven, for example by earth, etc., should be used only by more perfect and spiritual persons, for whom it is easy to apply the due reverence by which we honor God in creatures. Besides, among us that custom is not in force; and if someone, for example, swears by the sun, he is judged to be jesting.

There is added also an imprecatory oath, by which God is invoked at the same time as witness and as avenger of perjury ⁶.

⁶ This distinction of an oath into contestatory and execratory or imprecatory is also common. But it is disputed whether, when someone confirms a falsehood by an imprecatory oath, by which he obliges himself or something pertaining to himself (as St. Thomas says, 2. 2, q. 89, art. 1 ad 3) to punishment, another species of malice is added, namely against charity toward oneself or one's neighbor. The doctors more commonly deny this, because a person is not accustomed to imprecate such an evil upon himself or his own from his heart, but rather thinks that God will not do it. Thus St. Alphonsus (lib. 3, n. 140), who soon after (n. 142) notes that, according to the common opinion, all oaths are of the same species.

An oath clothed with due conditions is honorable and lawful, and if it is promissory, it obliges in conscience, unless a just cause prevents it, or its obligation has lawfully ceased. - Thus all, and it is clear both from the definition of the Council of Constance, sess. 8, against Wyclif, and from the universal practice of the Church and of peoples, and also from various places of Sacred Scripture. Deuteronomy 6. 13: "You shall fear the Lord your God... and by His name you shall swear." - St. Alphonsus, n. 133, 439 and following.