Article II: On the Liceity of an Oath

S. Th. II-II, q. 89, a. 2, 3, 5, 7, 10.

749. Thesis. An oath according to itself is licit, under the due conditions. - It is of faith from Lateran Council IV, c. 3, against the Waldensians, and from the Council of Constance, against Wyclif, sess. 8, prop. 43 and a. 12.

The Beguines and Beghards, Fraticelli, Hussites, Anabaptists, Mennonites, and Quakers also denied the thesis.

The first part is proved: a) From the origin and end of an oath. From origin, because it has been introduced from faith by which men believe God to have infinite truth and universal knowledge and providence; from end, because it is introduced for justifying men and for ending controversies (Hebr. VI, 16), and because it is an efficacious means for obtaining the necessary security in human affairs, especially in contracts and judgments.

b) From many places both of the Old and the New Testament (Gen. XXII; Deut. VI, 13; X, 20; Ps. XIV, 4; LXII, 12; CIV, 9; CIX, 4; Eccli. XLIV; Is. XLV, 23; LXV, 15; Jer. IV, 2; Rom. I, 9; Phil. I, 8; II Cor. I, 23; Gal. I, 20; Hebr. VI), although Scripture proscribes its abuse, for example, in ordinary conversation (Exod. XX, 7; Lev. XIX, 12; Eccli. XXIII, 9-14; Mt. V, 34 sq.).

c) From the constant practice both of the faithful and of the Church, even in the earlier centuries (a. 2).

The second part is proved: An oath is not good through itself, but as a means to another thing; therefore it is not licit except under due conditions. For that which is sought only as a remedy for helping a defect is not good through itself, as is clear concerning medicine; but an oath is sought only for helping a defect, namely, that one man distrusts another; therefore an oath is not good through itself, nor licit except in necessity, but under due conditions (a. 2 and 5).

750. Three conditions are necessary for the liceity of an oath: judgment, truth, and justice, as Jeremiah IV, 2 indicates: "And you shall swear: The Lord lives, in truth and in judgment and in justice," which St. Jerome calls the companions of an oath (can. 1316).

For the good use of an oath two things are required: 1. that someone swear not lightly, but from a necessary cause and discreetly, namely, judgment of discretion on the part of the one swearing; 2. that the thing which is confirmed by oath be neither false nor illicit, namely, truth and justice on the part of the thing sworn. An incautious oath lacks judgment, a lying oath or perjury lacks truth, an iniquitous or illicit oath lacks justice (a. 3).

751. Judgment of discretion signifies a sufficient cause for swearing. For it is against the reverence due to the divine majesty to invoke it as witness of a thing of no or little moment (a. 2); which inordination nevertheless is of itself venial. In an assertory oath, a sufficient cause is a true utility for an honest end of sufficiently great moment, such as the public good, the peace of the family, security in a contract, one's own or another's reputation (condemned prop. Quesnel 101; Pistoia 73): in a promissory oath, it is required besides that the thing be in the power of the one swearing (a. 7).

752. Truth essentially requires conformity of the statement, not indeed with the thing sworn (material and objective truth), but with the mind of the one swearing (formal truth). For it is of itself a very grave irreverence toward God, infinite truth, to invoke Him as witness of falsehood or even as patron.

In an assertory oath there is only the truth of the assertion concerning the present, or the conformity of the statement with knowledge of the fact concerning which one swears, whence the certain is to be sworn as certain, the doubtful as doubtful, the probable as probable; in a promissory oath there is conformity of the statement with the present intention of doing that which is promised, and besides another truth is required, namely, conformity of the deed with the present promise, or truth of execution in the future (a. 7 and ad 1).

Therefore truth excludes a lie, but not error: thus he swears in truth who swears to something false which he prudently judges to be true; on the contrary, he commits perjury who swears what he thinks is false, although on the part of the thing it is true. Therefore in order that something be sworn as certain, practical and prudential certitude is required, or moral certitude improperly so called, which excludes every prudent doubt, greater however than for simply affirming; but for swearing judicially, experiential knowledge is required, because it is demanded by law.

753. Justice here signifies honesty. It requires 1. in an assertory oath, not indeed that the thing asserted, but that its assertion be honest: attestation of a crime before a judge is licit; the disclosure of a secret, profession of heresy, boasting of a crime committed, detraction, confirmed under oath, are dishonest; 2. in a promissory oath, not only that the promise itself, but also that the action or omission promised, insofar as it is sworn, be honest. For it is irreverence toward God's sanctity to call Him into testimony for a purpose or assertion which He prohibits (a. 7 and ad 2); which irreverence is grave from its genus.

754. Whether to swear befits all? - In an oath two things must be considered: one on the part of God whose testimony is invoked, and as to this the greatest reverence is due to an oath, and therefore non-pubescent persons and perjurers or those who are in danger of perjury are not admitted to it;¹ the other on the part of man whose statement is confirmed by oath, which he does not need except if there is doubt concerning his truth: but this derogates from the dignity of a person, that there be doubt concerning the truth of those things which he says, and therefore it does not befit persons of great dignity to swear, except on account of great utility or necessity (a. 10). Whence the Church has permitted an oath more sparingly to priests and religious, and prohibits a cleric from swearing before a lay judge unless the permission of the Apostolic See or of the bishop has been obtained (can. 120).

¹ Cf. can. 1757; and Belgian Penal Code, a. 31-34; Code of Civil Procedure, a. 285.