The Catechism of St. Thomas Aquinas

THE EIGHTH COMMANDMENT: “Thou Shalt Not Bear False Witness Against Thy Neighbor.”

The Lord has forbidden anyone to injure his neighbor by deed; now he forbids us to injure him by word. “Thou shalt not bear false witness against thy neighbor.”[1] This may occur in two ways, either in a court of justice or in ordinary conversation.

In the court of justice it may happen in three ways, according to the three persons who may violate this Commandment in court.[2] The first person is the plaintiff who makes a false accusation: “Thou shalt not be a detractor nor a whisperer among the people.”[3] And note well that it is not only wrong to speak falsely, but also to conceal the truth: “If thy brother shall offend against thee, go and rebuke him.”[4] The second person is the witness who testifies by lying: “A false witness shall not be unpunished.”[5] For this Commandment includes all the preceding ones, inasmuch as the false witness may himself be the murderer or the thief, etc. And such should be punished according to the law. “When after most diligent inquisition, they shall find that the false witness hath told a lie against his brother, they shall render to him as he meant to do to his brother. . . . Thou shalt not pity him, but shalt require life for life, eye for eye, tooth for tooth, hand for hand, foot for foot.”[6] And again: “A man that beareth false witness against his neighbor is like a dart and a sword and a sharp arrow.”[7] The third person is the judge who sins by giving an unjust sentence: “Thou shalt not . . . judge unjustly. Respect not the person of the poor, nor honor the countenance of the mighty. But judge thy neighbor according to justice.”[8]


In ordinary conversation one may violate this Commandment in five ways. The first is by detraction: “Detractors, hateful to God.”[9] “Hateful to God” here indicates that nothing is so dear to a man as his good name: “A good name is better than great riches.”[10] But detractors take away this good name: “If a serpent bite in silence, he is no better that backbiteth secretly.”[11] Therefore, if detractors do not restore this reputation, they cannot be saved.

Secondly, one may break this precept by listening to detractors willingly: “Hedge in thy ears with thorns, hear not a wicked tongue, and make doors and bars to thy mouth.”[12] One should not listen deliberately to such things, but ought to turn away, showing a sad and stern countenance: “The north wind driveth away rain as doth a sad countenance a backbiting tongue.”[13]

Thirdly, gossipers break this precept when they repeat whatever they hear: “Six things there are which the Lord hateth, and the seventh His soul detesteth . . . him that soweth discord among brethren.”[14] Fourthly, those who speak honied words, the flatterers: “The sinner is praised in the desires of his soul, and the unjust man is blessed.”[15] And again: “O My people, they that call thee blessed, the same shall deceive thee.”[16]


The prohibition of this Commandment includes every form of falsehood: “Be not willing to make any manner of lie; for the custom thereof is no good.”[17] There are four reasons for this. The first is that lying likens one to the devil, because a liar is as the son of the devil. Now, we know that a man’s speech betrays from what region and country he comes from, thus: “Even thy speech doth discover thee.”[18] Even so, some men are of the devil’s kind, and are called sons of the devil because they are

liars, since the devil is “a liar and the father of lies.”[19] Thus, when the devil said, “No, you shall not die the death,”[20] he lied. But, on the contrary, others are the children of God, who is Truth, and they are those who speak the truth.

The second reason is that lying induces the ruin of society. Men live together in society, and this is soon rendered impossible if they do not speak the truth to one another. “Wherefore putting away Iying, speak ye the truth, every man with his neighbor; for we are members one of another.”[21]

The third reason is that the liar loses his reputation for the truth. He who is accustomed to telling lies is not believed even when he speaks the truth: “What can be made clean by the unclean? And what truth can come from that which is false?”[22]

The fourth reason is because a liar kills his soul, for “the mouth that belieth killeth the soul.”[23] And again: “Thou wilt destroy all that speak a lie.”[24] Accordingly, it is clear that lying is a mortal sin; although it must be known that some lies may be venial.

It is a mortal sin, for instance, to lie in matters of faith. This concerns professors, prelates and preachers, and is the gravest of all other kinds of lies: “There shall be among you lying teachers, who shall bring in sects of perdition.”[25] Then there are those who lie to wrong their neighbor: “Lie not to one another.”[26] These two kinds of lies, therefore, are mortal sins.

There are some who lie for their own advantage, and this in a variety of ways. Sometimes it is out of humility. This may be the case in confession, about which St. Augustine says: “Just as one must avoid concealing what he has committed, so also he must not mention what he has not committed.” “Hath God any need of your lie?”[27] And again: “There is one that humbleth himself wickedly, and his interior is full of deceit; and there is one that humbleth himself exceedingly with a great lowness.”[28]

There are others who tell lies out of shame, namely, when one tells a falsehood believing that he is telling the truth, and on becoming aware of it he is ashamed to retract: “In no wise speak against the truth, but be ashamed of the lie of thy ignorance.”[29] Other some lie for desired results as when they wish to gain or avoid something: “We have placed our hope in lies, and by falsehood we are protected.”[30] And again: “He that trusteth in lies feedeth the winds.”[31]

Finally, there are some who lie to benefit another, that is, when they wish to free someone from death, or danger, or some other loss. This must be avoided, as St. Augustine tells us: “Accept no person against thy own person, nor against thy soul a lie.”[32] But others lie only out of vanity, and this, too, must never be done, lest the habit of such lead us to mortal sin: “For the bewitching of vanity obscureth good things.”[33]

(For “Questions for Discussion” see Chapter 6.)


1. St. Thomas also treats of this Commandment in the “Summa Theol.,” II-II, Q. cxxii, art. 6.

2. “The Commandment specially prohibits that species of false testimony which is given on oath in a court of justice. The witness swears by the Deity and thus pledges God’s holy name for the truth of what he says, and this has very great weight and constitutes the strongest claim for credit. Such testimony, therefore, because it is dangerous, is particularly prohibited. When no legal exceptions can be taken against a sworn witness, and when he cannot be convicted of open dishonesty and malice, even the judge himself cannot reject his testimony. This is especially true since it is commanded by divine authority that ‘in the mouth of two or three witnesses every word shall stand’ ” (“Roman Catechism,” “Eighth Commandment,” 3).

3. Lev., xix. 16.

4. Matt., xviii. 15.

5. Prov., xix. 5.

6. Deut., xix. 18-21.

7. Prov., xxv. 18.

8. Lev., xix. 15. “This Commandment prohibits deceit, lying, and perjury on the part of witnesses. The same prohibition also applies to plaintiffs, defendants, promoters, representatives, procurators, and advocates; in a word, all who take any part in lawsuits. . . . Finally, God forbids all testimony which may injure others or do them injustice, whether it be a matter of legal evidence or not” (“Roman Catechism,” “loc. cit.,” 6).

9. Rom., i. 30.

10. Prov., xxii. 1.

11. Eccles., x. 11.

12. Ecclus., xxviii. 28.

13. Prov., xxv. 23. “This Commandment not only forbids false testimony, but also the abominable sin of detraction. This is a moral pestilence which is the poisoned source of many and calamitous evils. . . . That we may see the nature of the sin of detraction more clearly, we must know that reputation is injured not only by calumniating the character. but also by exaggerating the faults of others. He who makes known the secret sin of any man at any time or place unnecessarily, or before persons who have no right to know, is also rightly regarded as a detractor and evil-speaker, if his revelation seriously injures the other’s reputation” (“Roman Catechism,” “loc. cit.,” 9).

14. Prov., vi. 16, 19.

15. Ps. ix. 24

16. Isa., iii. 12. “Flatterers and sycophants are among those who violate this Commandment, for by fawning and insincere praise they gain the hearing and good will of those whose favor. money, and honors they seek” (“Roman Catechism,” “loc. cit.,” 11).

17. Ecclus, vii. 14.

18. Matt., xxvi. 73.

19. John, viii. 44.

20. Gen. iii. 4.

21. Eph., iv. 25.

22. Ecclus., xxxiv. 4.

23. Wis., i. 11.

24. Ps. v. 7.

25. II Peter, ii. 1.

26. Col., iii. 9.

27. Job, xiii. 7.

28. Ecclus., xix.

29. “Ibid.,” iv. 30.

30. Isa., xxviii. 15.

31. Prov., x. 4.

32. Eccles., iv. 26.

33. Wis., iv. 12.

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