Chapter VIII — Under what conditions an oath is lawful; a threefold condition is necessary for it: truth, judgment, justice. Other rules of swearing are assigned by St. Thomas

I. From Chapter II of this Dissertation it has been demonstrated against the heretics that an oath is an honorable act, and lawful, indeed an act of virtue — namely, of religion, or of latria — provided it is attended by the necessary conditions. In this chapter we undertake to explain these conditions, which ought to accompany an oath. But, before we examine each singly, it must be noted, from St. Thomas, that an oath is not absolutely to be sought after, and frequently employed. For, among those goods which are to be sought only when necessity presses, one may reckon that which is medicine — a good thing, given the illness which it meets. Thus an oath has been instituted to provide for weakness — namely, of the incredulous man. "Just as medicine (says the Angelic Doctor, q. 89, art. 5, ad 3) is useful for health, but the more virtuous it is, the greater harm it induces, if not duly taken — so too an oath is indeed useful for confirmation by divine testimony; yet the more it is to be venerated, the more dangerous it is, unless duly employed."

II. Now, in order for an oath to be opportunely employed, three conditions must accompany it, as Jeremiah 4 has it: "Thou shalt swear: As the Lord liveth, in truth, and in judgment, and in justice." St. Thomas assigns the reason why these three conditions are required (loc. cit., art. 3, in the body): "An oath is not good, except for one who uses it well. For a good use of an oath, two things are required. The first is that one swear not lightly, but from a necessary cause, and discreetly, as regards that which is confirmed by the oath — namely, that it be neither false, nor anything unlawful; and as regards this, judgment is required. The second is required on the part of the swearer's discretion. First, one swears truly by an oath who confirms that which is true; but an oath lacking judgment is called rash. Second, one swears rightly by an oath who confirms by it what is lawful. As regards this, truth is required, whereby it is true, and not a lie; and justice, by which one confirms what is lawful."

From this angelic teaching, the sense of the aforesaid conditions plainly shines forth.

III. In every oath, then, truth is first necessary, in confirmation of which divine testimony is invoked. This truth, or certainty of the truth, can be considered both on the part of the thing sworn, and on the part of the swearer. On the part of the thing sworn, it is required that the thing sworn be true according to common understanding and custom. Thus one who swears that a student frequented the schools swears truly, even if he was sometimes absent — since this is commonly understood in this sense. Certainty on the part of the swearer consists in this: that a man confirm, by oath, a matter such as he holds, or believes, in his mind to be true. If he knows the matter to be certain, he cannot swear it as doubtful; and if he judges it to be doubtful, he cannot swear it as certain. In a promissory oath, a twofold truth is implied: one, of the intention now had of fulfilling what he promises; the other, future, or of the deed, in the execution of what is promised. But of this twofold truth, more below.

Second, justice is required for an oath: that truth is necessary for an oath which excludes from the matter to be confirmed anything unjust, or any dishonorable thing — that is, an unlawful, culpable saying or deed, whether contrary to justice, or to any other virtue. He is said to swear falsely who confirms, by oath, present or future matter, contrary to what he presently has in mind. He is said to swear unjustly and wickedly who is opposed to justice. The former is opposed to truth by oath; the latter, to justice.

Third, judgment is necessary, which is not taken for the exercise of justice, but for prudent consideration; and it consists in this: that a man, before he swears, maturely and seriously weigh the utility and necessity of swearing. For, just as prudence is the guide, or rule, of all the virtues, especially insofar as it directs the oath, this third condition is supremely necessary, so that the oath may be lawful, and an act of virtue.

The first two conditions, however, are required so that it may not be a sin. Under this "judgment," in a certain way, are contained all the other conditions. As to truth and justice, this is clear: for, just as this judgment prescribes that one ought to swear, having regard to the utility and necessity of the matters, so too it forbids swearing to something false, or vicious. Other conditions too — chiefly theological faith, whereby one believes that God knows all things, and devotion, whereby, on the part of the swearer, the reverence owed to God is shown — are, St. Thomas openly teaches, comprehended under "judgment" (loc. cit., art. 5, ad 2): "Devotion, and faith, and all such things as are required for the due manner of swearing, are understood as included in judgment."

St. Augustine speaks excellently of this "judgment" (Sermon 28, on the words of the Apostle, ch. 9): "As far as it depends on me, I do not wish to swear; but, when I see that I am not believed, and that it is not expedient that he does not believe, who does not believe him to whom I am a witness — this reason having been weighed, and this consideration balanced, I swear, compelled by great necessity, as it seems to me, saying, with great fear, in the presence of God, or, as Christ knows in my soul, or such things."

IV. The Angelic Doctor not only explains the three common conditions which ought to accompany any oath whatsoever, but furthermore pursues the occasions in which an oath is free of fault, indeed possesses the merit of virtue. He assigns six such cases. First, when an oath is a conclusion of peace. Second, the necessity of preserving one's own reputation. Third, as a pledge of fidelity to be maintained. Fourth, for rendering obedience to a superior. Fifth, when the necessity presses of securing a promised thing. Sixth, so that the truth, which it greatly matters be believed, may be rendered credible. Here are the words of the holy Doctor, in his Epistle to the Hebrews, ch. 6: "The causes, however, in which it is lawful to swear, are these. First, for the confirmation of peace, as Laban swore (Genesis 30). Second, for the preservation of one's reputation. Third, for keeping fidelity, as vassals swear to their lords. Fourth, for fulfilling obedience, if something honorable is commanded by a superior. Fifth, for effecting security. Sixth, for attesting truth." Thus swore the Apostle (Romans 1): "God is my witness."