Today’s Catechism sections begin the discussion on the Eighth Commandment with an exploration of the truth. Supporting material comes from the “Summa Theologica”.
THE EIGHTH COMMANDMENT
You shall not bear false witness against your neighbor.252
It was said to the men of old, "You shall not swear falsely, but shall perform to the Lord what you have sworn."253
2464 The eighth commandment forbids misrepresenting the truth in our relations with others. This moral prescription flows from the vocation of the holy people to bear witness to their God who is the truth and wills the truth. Offenses against the truth express by word or deed a refusal to commit oneself to moral uprightness: they are fundamental infidelities to God and, in this sense, they undermine the foundations of the covenant.
I. Living in the Truth
2465 The Old Testament attests that God is the source of all truth. His Word is truth. His Law is truth. His "faithfulness endures to all generations."254 Since God is "true," the members of his people are called to live in the truth.255
2466 In Jesus Christ, the whole of God's truth has been made manifest. "Full of grace and truth," he came as the "light of the world," he is the Truth.256 "Whoever believes in me may not remain in darkness."257 The disciple of Jesus continues in his word so as to know "the truth [that] will make you free" and that sanctifies.258 To follow Jesus is to live in "the Spirit of truth," whom the Father sends in his name and who leads "into all the truth."259 To his disciples Jesus teaches the unconditional love of truth: "Let what you say be simply 'Yes or No.'"260
2467 Man tends by nature toward the truth. He is obliged to honor and bear witness to it: "It is in accordance with their dignity that all men, because they are persons . . . are both impelled by their nature and bound by a moral obligation to seek the truth, especially religious truth. They are also bound to adhere to the truth once they come to know it and direct their whole lives in accordance with the demands of truth."261
2468 Truth as uprightness in human action and speech is called truthfulness, sincerity, or candor. Truth or truthfulness is the virtue which consists in showing oneself true in deeds and truthful in words, and in guarding against duplicity, dissimulation, and hypocrisy.
2469 "Men could not live with one another if there were not mutual confidence that they were being truthful to one another."262 The virtue of truth gives another his just due. Truthfulness keeps to the just mean between what ought to be expressed and what ought to be kept secret: it entails honesty and discretion. In justice, "as a matter of honor, one man owes it to another to manifest the truth."263
2470 The disciple of Christ consents to "live in the truth," that is, in the simplicity of a life in conformity with the Lord's example, abiding in his truth. "If we say we have fellowship with him while we walk in darkness, we lie and do not live according to the truth."264
2504 "You shall not bear false witness against your neighbor" (⇒ Ex 20:16). Christ's disciples have "put on the new man, created after the likeness of God in true righteousness and holiness" (⇒ Eph 4:24).
2505 Truth or truthfulness is the virtue which consists in showing oneself true in deeds and truthful in words, and guarding against duplicity, dissimulation, and hypocrisy.
In the “Summa Theologica” (Secunda Secundæ Partis, 109, 1), St. Thomas Aquinas discusses truth.
Article 1. Whether truth is a virtue?
Objection 1. It seems that truth is not a virtue. For the first of virtues is faith, whose object is truth. Since then the object precedes the habit and the act, it seems that truth is not a virtue, but something prior to virtue.
Objection 2. Further, according to the Philosopher (Ethic. iv, 7), it belongs to truth that a man should state things concerning himself to be neither more nor less than they are. But this is not always praiseworthy--neither in good things, since according to Proverbs 27:2, "Let another praise thee, and not thy own mouth"--nor even in evil things, because it is written in condemnation of certain people (Isaiah 3:9): "They have proclaimed abroad their sin as Sodom, and they have not hid it." Therefore truth is not a virtue.
Objection 3. Further, every virtue is either theological, or intellectual, or moral. Now truth is not a theological virtue, because its object is not God but temporal things. For Tully says (De Invent. Rhet. ii) that by "truth we faithfully represent things as they are were, or will be." Likewise it is not one of the intellectual virtues, but their end. Nor again is it a moral virtue, since it is not a mean between excess and deficiency, for the more one tells the truth, the better it is. Therefore truth is not a virtue.
On the contrary, The Philosopher both in the Second and in the Fourth Book of Ethics places truth among the other virtues.
I answer that, Truth can be taken in two ways. First, for that by reason of which a thing is said to be true, and thus truth is not a virtue, but the object or end of a virtue: because, taken in this way, truth is not a habit, which is the genus containing virtue, but a certain equality between the understanding or sign and the thing understood or signified, or again between a thing and its rule, as stated in I, 16, 1; I, 21, 2. Secondly, truth may stand for that by which a person says what is true, in which sense one is said to be truthful. This truth or truthfulness must needs be a virtue, because to say what is true is a good act: and virtue is "that which makes its possessor good, and renders his action good."
Reply to Objection 1. This argument takes truth in the first sense.
Reply to Objection 2. To state that which concerns oneself, in so far as it is a statement of what is true, is good generically. Yet this does not suffice for it to be an act of virtue, since it is requisite for that purpose that it should also be clothed with the due circumstances, and if these be not observed, the act will be sinful. Accordingly it is sinful to praise oneself without due cause even for that which is true: and it is also sinful to publish one's sin, by praising oneself on that account, or in any way proclaiming it uselessly.
Reply to Objection 3. A person who says what is true, utters certain signs which are in conformity with things; and such signs are either words, or external actions, or any external thing. Now such kinds of things are the subject-matter of the moral virtues alone, for the latter are concerned with the use of the external members, in so far as this use is put into effect at the command of the will. Wherefore truth is neither a theological, nor an intellectual, but a moral virtue. And it is a mean between excess and deficiency in two ways. First, on the part of the object, secondly, on the part of the act. On the part of the object, because the true essentially denotes a kind of equality, and equal is a mean between more and less. Hence for the very reason that a man says what is true about himself, he observes the mean between one that says more than the truth about himself, and one that says less than the truth. On the part of the act, to observe the mean is to tell the truth, when one ought, and as one ought. Excess consists in making known one's own affairs out of season, and deficiency in hiding them when one ought to make them known.
252 ⇒ EX 20:16; Cf. ⇒ Deut 5:20.
253 ⇒ Mt 5:33.
254 ⇒ PS 119:90; Cf. ⇒ Prov 8:7; ⇒ 2 Sam 7:28; ⇒ PS 119:142; ⇒ Lk 1:50.
255 ⇒ Rom 3:4; Cf. ⇒ PS 119:30.
256 ⇒ Jn 1:14; ⇒ 8:12; Cf. ⇒ 14:6.
257 ⇒ Jn 12:46.
258 ⇒ Jn 8:32; Cf. ⇒ 17:17.
259 ⇒ Jn 16:13.
260 ⇒ Mt 5:37.
261 DH 2 # 2.
262 St. Thomas Aquinas, STh II-II 109, 3 ad 1.
263 St. Thomas Aquinas, STh II-II, 109, 3, corp. art.
264 ⇒ 1 Jn 1:6.