Summa Th. II-II EN Qu.69
We must now consider those sins which are committed against justice on the part of the defendant. Under this head there are four points of inquiry:
(1) Whether it is a mortal sin to deny the truth which would lead to one's condemnation?
(2) Whether it is lawful to defend oneself with calumnies?
(3) Whether it is lawful to escape condemnation by appealing?
(4) Whether it is lawful for one who has been condemned to defend himself by violence if he be able to do so?
Objection: 1. It would seem one can, without a mortal sin, deny the truth which would lead to one's condemnation. For Chrysostom says (Hom. xxxi super Ep ad Heb.): "I do not say that you should lay bare your guilt publicly, nor accuse yourself before others." Now if the accused were to confess the truth in court, he would lay bare his guilt and be his own accuser. Therefore he is not bound to tell the truth: and so he does not sin mortally if he tell a lie in court.
2. Further, just as it is an officious lie when one tells a lie in order to rescue another man from death, so is it an officious lie when one tells a lie in order to free oneself from death, since one is more bound towards oneself than towards another. Now an officious lie is considered not a mortal but a venial sin. Therefore if the accused denies the truth in court, in order to escape death, he does not sin mortally.
3. Further, every mortal sin is contrary to charity, as stated above (Question , Article ). But that the accused lie by denying himself to be guilty of the crime laid to his charge is not contrary to charity, neither as regards the love we owe God, nor as to the love due to our neighbor. Therefore such a lie is not a mortal sin.
On the contrary Whatever is opposed to the glory of God is a mortal sin, because we are bound by precept to "do all to the glory of God" (1Co 10,31). Now it is to the glory of God that the accused confess that which is alleged against him, as appears from the words of Josue to Achan, "My son, give glory to the Lord God of Israel, and confess and tell me what thou hast done, hide it not" (Jos 7,19). Therefore it is a mortal sin to lie in order to cover one's guilt.
I answer that Whoever acts against the due order of justice, sins mortally, as stated above (Question , Article ). Now it belongs to the order of justice that a man should obey his superior in those matters to which the rights of his authority extend. Again, the judge, as stated above (Question , Article ), is the superior in relation to the person whom he judges. Therefore the accused is in duty bound to tell the judge the truth which the latter exacts from him according to the form of law. Hence if he refuse to tell the truth which he is under obligation to tell, or if he mendaciously deny it, he sins mortally. If, on the other hand, the judge asks of him that which he cannot ask in accordance with the order of justice, the accused is not bound to satisfy him, and he may lawfully escape by appealing or otherwise: but it is not lawful for him to lie.
Reply to Objection: 1. When a man is examined by the judge according to the order of justice, he does not lay bare his own guilt, but his guilt is unmasked by another, since the obligation of answering is imposed on him by one whom he is bound to obey.
2. To lie, with injury to another person, in order to rescue a man from death is not a purely officious lie, for it has an admixture of the pernicious lie: and when a man lies in court in order to exculpate himself, he does an injury to one whom he is bound to obey, since he refuses him his due, namely an avowal of the truth.
3. He who lies in court by denying his guilt, acts both against the love of God to whom judgment belongs, and against the love of his neighbor, and this not only as regards the judge, to whom he refuses his due, but also as regards his accuser, who is punished if he fail to prove his accusation. Hence it is written (Ps 140,4): "Incline not my heart to evil words, to make excuses in sins": on which words a gloss says: "Shameless men are wont by lying to deny their guilt when they have been found out." And Gregory in expounding Jb 31,33, "If as a man I have hid my sin," says (Moral. xxii, 15): "It is a common vice of mankind to sin in secret, by lying to hide the sin that has been committed, and when convicted to aggravate the sin by defending oneself."
Objection: 1. It would seem lawful for the accused to defend himself with calumnies. Because, according to civil law (Cod. II, iv, De transact. 18), when a man is on trial for his life it is lawful for him to bribe his adversary. Now this is done chiefly by defending oneself with calumnies. Therefore the accused who is on trial for his life does not sin if he defend himself with calumnies.
2. Further, an accuser who is guilty of collusion with the accused, is punishable by law (Decret. II, qu. iii, can. Si quem poenit.). Yet no punishment is imposed on the accused for collusion with the accuser. Therefore it would seem lawful for the accused to defend himself with calumnies.
3. Further, it is written (Pr 14,16): "A wise man feareth and declineth from evil, the fool leapeth over and is confident." Now what is done wisely is no sin. Therefore no matter how a man declines from evil, he does not sin.
On the contrary In criminal cases an oath has to be taken against calumnious allegations (Extra, De juramento calumniae, cap. Inhaerentes): and this would not be the case if it were lawful to defend oneself with calumnies. Therefore it is not lawful for the accused to defend himself with calumnies.
I answer that It is one thing to withhold the truth, and another to utter a falsehood. The former is lawful sometimes, for a man is not bound to divulge all truth, but only such as the judge can and must require of him according to the order of justice; as, for instance, when the accused is already disgraced through the commission of some crime, or certain indications of his guilt have already been discovered, or again when his guilt is already more or less proven. On the other hand it is never lawful to make a false declaration.As regards what he may do lawfully, a man can employ either lawful means, and such as are adapted to the end in view, which belongs to prudence; or he can use unlawful means, unsuitable to the proposed end, and this belongs to craftiness, which is exercised by fraud and guile, as shown above (Question , Articles , seqq.). His conduct in the former case is praiseworthy, in the latter sinful. Accordingly it is lawful for the accused to defend himself by withholding the truth that he is not bound to avow, by suitable means, for instance by not answering such questions as he is not bound to answer. This is not to defend himself with calumnies, but to escape prudently. But it is unlawful for him, either to utter a falsehood, or to withhold a truth that he is bound to avow, or to employ guile or fraud, because fraud and guile have the force of a lie, and so to use them would be to defend oneself with calumnies.
Reply to Objection: 1. Human laws leave many things unpunished, which according to the Divine judgment are sins, as, for example, simple fornication; because human law does not exact perfect virtue from man, for such virtue belongs to few and cannot be found in so great a number of people as human law has to direct. That a man is sometimes unwilling to commit a sin in order to escape from the death of the body, the danger of which threatens the accused who is on trial for his life, is an act of perfect virtue, since "death is the most fearful of all temporal things" (Ethic. iii, 6). Wherefore if the accused, who is on trial for his life, bribes his adversary, he sins indeed by inducing him to do what is unlawful, yet the civil law does not punish this sin, and in this sense it is said to be lawful.
2. If the accuser is guilty of collusion with the accused and the latter is guilty, he incurs punishment, and so it is evident that he sins. Wherefore, since it is a sin to induce a man to sin, or to take part in a sin in any way---for the Apostle says (Rm 1,32), that "they . . . are worthy of death . . . that consent" to those who sin---it is evident that the accused also sins if he is guilty of collusion with his adversary. Nevertheless according to human laws no punishment is inflicted on him, for the reason given above.
3. The wise man hides himself not by slandering others but by exercising prudence.
Objection: 1. It would seem unlawful for the accused to escape judgment by appealing. The Apostle says (Rm 13,1): "Let every soul be subject to the higher powers." Now the accused by appealing refuses to be subject to a higher power, viz. the judge. Therefore he commits a sin.
2. Further, ordinary authority is more binding than that which we choose for ourselves. Now according to the Decretals (II, qu. vi, cap. A judicibus) it is unlawful to appeal from the judges chosen by common consent. Much less therefore is it lawful to appeal from ordinary judges.
3. Further, whatever is lawful once is always lawful. But it is not lawful to appeal after the tenth day [*Can. Anteriorum, caus. ii, qu. 6], nor a third time on the same point [*Can. Si autem, caus. ii, qu. 6]. Therefore it would seem that an appeal is unlawful in itself.
On the contrary Paul appealed to Caesar (Ac 25).
I answer that There are two motives for which a man appeals. First through confidence in the justice of his cause, seeing that he is unjustly oppressed by the judge, and then it is lawful for him to appeal, because this is a prudent means of escape. Hence it is laid down (Decret. II, qu. vi, can. Omnis oppressus): "All those who are oppressed are free, if they so wish, to appeal to the judgment of the priests, and no man may stand in their way." Secondly, a man appeals in order to cause a delay, lest a just sentence be pronounced against him. This is to defend oneself calumniously, and is unlawful as stated above (Article ). For he inflicts an injury both on the judge, whom he hinders in the exercise of his office, and on his adversary, whose justice he disturbs as far as he is able. Hence it is laid down (II, qu. vi, can. Omnino puniendus): "Without doubt a man should be punished if his appeal be declared unjust."
Reply to Objection: 1. A man should submit to the lower authority in so far as the latter observes the order of the higher authority. If the lower authority departs from the order of the higher, we ought not to submit to it, for instance "if the proconsul order one thing and the emperor another," according to a gloss on Rm 13,2. Now when a judge oppresses anyone unjustly, in this respect he departs from the order of the higher authority, whereby he is obliged to judge justly. Hence it is lawful for a man who is oppressed unjustly, to have recourse to the authority of the higher power, by appealing either before or after sentence has been pronounced. And since it is to be presumed that there is no rectitude where true faith is lacking, it is unlawful for a Catholic to appeal to an unbelieving judge, according to Decretals II, qu. vi, can. Catholicus: "The Catholic who appeals to the decision of a judge of another faith shall be excommunicated, whether his case be just or unjust." Hence the Apostle also rebuked those who went to law before unbelievers (1Co 6,6).
2. It is due to a man's own fault or neglect that, of his own accord, he submits to the judgment of one in whose justice he has no confidence. Moreover it would seem to point to levity of mind for a man not to abide by what he has once approved of. Hence it is with reason that the law refuses us the faculty of appealing from the decision of judges of our own choice, who have no power save by virtue of the consent of the litigants. On the other hand the authority of an ordinary judge depends, not on the consent of those who are subject to his judgment, but on the authority of the king or prince who appointed him. Hence, as a remedy against his unjust oppression, the law allows one to have recourse to appeal, so that even if the judge be at the same time ordinary and chosen by the litigants, it is lawful to appeal from his decision, since seemingly his ordinary authority occasioned his being chosen as arbitrator. Nor is it to be imputed as a fault to the man who consented to his being arbitrator, without adverting to the fact that he was appointed ordinary judge by the prince.
3. The equity of the law so guards the interests of the one party that the other is not oppressed. Thus it allows ten days for appeal to be made, this being considered sufficient time for deliberating on the expediency of an appeal. If on the other hand there were no fixed time limit for appealing, the certainty of judgment would ever be in suspense, so that the other party would suffer an injury. The reason why it is not allowed to appeal a third time on the same point, is that it is not probable that the judges would fail to judge justly so many times.
Objection: 1. It would seem that a man who is condemned to death may lawfully defend himself if he can. For it is always lawful to do that to which nature inclines us, as being of natural right, so to speak. Now, to resist corruption is an inclination of nature not only in men and animals but also in things devoid of sense. Therefore if he can do so, the accused, after condemnation, may lawfully resist being put to death.
2. Further, just as a man, by resistance, escapes the death to which he has been condemned, so does he by flight. Now it is lawful seemingly to escape death by flight, according to Si 9,18, "Keep thee far from the man that hath power to kill [and not to quicken]" [*The words in the brackets are not in the Vulgate]. Therefore it is also lawful for the accused to resist.
3. Further, it is written (Pr 24,11): "Deliver them that are led to death: and those that are drawn to death forbear not to deliver." Now a man is under greater obligation to himself than to another. Therefore it is lawful for a condemned man to defend himself from being put to death.
On the contrary The Apostle says (Rm 13,2): "He that resisteth the power, resisteth the ordinance of God: and they that resist, purchase to themselves damnation." Now a condemned man, by defending himself, resists the power in the point of its being ordained by God "for the punishment of evil-doers, and for the praise of the good" [*1P 2,14]. Therefore he sins in defending himself.
I answer that A man may be condemned to death in two ways. First justly, and then it is not lawful for the condemned to defend himself, because it is lawful for the judge to combat his resistance by force, so that on his part the fight is unjust, and consequently without any doubt he sins.Secondly a man is condemned unjustly: and such a sentence is like the violence of robbers, according to Ez 22,21, "Her princes in the midst of her are like wolves ravening the prey to shed blood." Wherefore even as it is lawful to resist robbers, so is it lawful, in a like case, to resist wicked princes; except perhaps in order to avoid scandal, whence some grave disturbance might be feared to arise.
Reply to Objection: 1. Reason was given to man that he might ensue those things to which his nature inclines, not in all cases, but in accordance with the order of reason. Hence not all self-defense is lawful, but only such as is accomplished with due moderation.
2. When a man is condemned to death, he has not to kill himself, but to suffer death: wherefore he is not bound to do anything from which death would result, such as to stay in the place whence he would be led to execution. But he may not resist those who lead him to death, in order that he may not suffer what is just for him to suffer. Even so, if a man were condemned to die of hunger, he does not sin if he partakes of food brought to him secretly, because to refrain from taking it would be to kill himself.
3. This saying of the wise man does not direct that one should deliver a man from death in opposition to the order of justice: wherefore neither should a man deliver himself from death by resisting against justice.
We must now consider injustice with regard to the person of the witness. Under this head there are four points of inquiry:
(1) Whether a man is bound to give evidence?
(2) Whether the evidence of two or three witnesses suffices?
(3) Whether a man's evidence may be rejected without any fault on his part?
(4) Whether it is a mortal sin to bear false witness?
Objection: 1. It would seem that a man is not bound to give evidence. Augustine say (Questions. Gn 1,26) [*Cf. Contra Faust. xxii, 33,34], that when Abraham said of his wife (Gn 20,2), "She is my sister," he wished the truth to be concealed and not a lie be told. Now, by hiding the truth a man abstains from giving evidence. Therefore a man is not bound to give evidence.
2. Further, no man is bound to act deceitfully. Now it is written (Pr 11,13): "He that walketh deceitfully revealeth secrets, but he that is faithful concealeth the thing committed to him by his friend." Therefore a man is not always bound to give evidence, especially on matters committed to him as a secret by a friend.
3. Further, clerics and priests, more than others, are bound to those things that are necessary for salvation. Yet clerics and priests are forbidden to give evidence when a man is on trial for his life. Therefore it is not necessary for salvation to give evidence.
On the contrary Augustine [*Can. Quisquis, caus. xi, qu. 3, cap. Falsidicus; cf. Isidore, Sentent. iii, 55] says: "Both he who conceals the truth and he who tells a lie are guilty, the former because he is unwilling to do good, the latter because he desires to hurt."
I answer that We must make a distinction in the matter of giving evidence: because sometimes a certain man's evidence is necessary, and sometimes not. If the necessary evidence is that of a man subject to a superior whom, in matters pertaining to justice, he is bound to obey, without doubt he is bound to give evidence on those points which are required of him in accordance with the order of justice, for instance on manifest things or when ill-report has preceded. If however he is required to give evidence on other points, for instance secret matters, and those of which no ill-report has preceded, he is not bound to give evidence. On the other hand, if his evidence be required by authority of a superior whom he is bound to obey, we must make a distinction: because if his evidence is required in order to deliver a man from an unjust death or any other penalty, or from false defamation, or some loss, in such cases he is bound to give evidence. Even if his evidence is not demanded, he is bound to do what he can to declare the truth to someone who may profit thereby. For it is written (Ps 81,4): "Rescue the poor, and deliver the needy from the hand of the sinner"; and (Pr 24,11): "Deliver them that are led to death"; and (Rm 1,32): "They are worthy of death, not only they that do them, but they also that consent to them that do them," on which words a gloss says: "To be silent when one can disprove is to consent." In matters pertaining to a man's condemnation, one is not bound to give evidence, except when one is constrained by a superior in accordance with the order of justice; since if the truth of such a matter be concealed, no particular injury is inflicted on anyone. Or, if some danger threatens the accuser, it matters not since he risked the danger of his own accord: whereas it is different with the accused, who incurs the danger against his will.
Reply to Objection: 1. Augustine is speaking of concealment of the truth in a case when a man is not compelled by his superior's authority to declare the truth, and when such concealment is not specially injurious to any person.
2. A man should by no means give evidence on matters secretly committed to him in confession, because he knows such things, not as man but as God's minister: and the sacrament is more binding than any human precept. But as regards matters committed to man in some other way under secrecy, we must make a distinction. Sometimes they are of such a nature that one is bound to make them known as soon as they come to our knowledge, for instance if they conduce to the spiritual or corporal corruption of the community, or to some grave personal injury, in short any like matter that a man is bound to make known either by giving evidence or by denouncing it. Against such a duty a man cannot be obliged to act on the plea that the matter is committed to him under secrecy, for he would break the faith he owes to another. On the other hand sometimes they are such as one is not bound to make known, so that one may be under obligation not to do so on account of their being committed to one under secrecy. In such a case one is by no means bound to make them known, even if the superior should command; because to keep faith is of natural right, and a man cannot be commanded to do what is contrary to natural right.
3. It is unbecoming for ministers of the altar to slay a man or to cooperate in his slaying, as stated above (Question , Article ); hence according to the order of justice they cannot be compelled to give evidence when a man is on trial for his life.
Objection: 1. It would seem that the evidence of two or three persons is not sufficient. For judgment requires certitude. Now certitude of the truth is not obtained by the assertions of two or three witnesses, for we read that Naboth was unjustly condemned on the evidence of two witnesses (1R 21). Therefore the evidence of two or three witnesses does not suffice.
2. Further, in order for evidence to be credible it must agree. But frequently the evidence of two or three disagrees in some point. Therefore it is of no use for proving the truth in court.
3. Further, it is laid down (Decret. II, qu. iv, can. Praesul.): "A bishop shall not be condemned save on the evidence of seventy-two witnesses; nor a cardinal priest of the Roman Church, unless there be sixty-four witnesses. Nor a cardinal deacon of the Roman Church, unless there be twenty-seven witnesses; nor a subdeacon, an acolyte, an exorcist, a reader or a doorkeeper without seven witnesses." Now the sin of one who is of higher dignity is more grievous, and consequently should be treated more severely. Therefore neither is the evidence of two or three witnesses sufficient for the condemnation of other persons.
On the contrary It is written (Dt 17,6): "By the mouth of two or three witnesses shall he die that is to be slain," and further on (Dt 19,15): "In the mouth of two or three witnesses every word shall stand."
I answer that According to the Philosopher (Ethic. i, 3), "we must not expect to find certitude equally in every matter." For in human acts, on which judgments are passed and evidence required, it is impossible to have demonstrative certitude, because they a about things contingent and variable. Hence the certitude of probability suffices, such as may reach the truth in the greater number, cases, although it fail in the minority. No it is probable that the assertion of sever witnesses contains the truth rather than the assertion of one: and since the accused is the only one who denies, while several witness affirm the same as the prosecutor, it is reasonably established both by Divine and by human law, that the assertion of several witnesses should be upheld. Now all multitude is comprised of three elements, the beginning, the middle and the end. Wherefore, according to the Philosopher (De Coelo i, 1), "we reckon 'all' and 'whole' to consist of three parts." Now we have a triple voucher when two agree with the prosecutor: hence two witnesses are required; or for the sake of greater certitude three, which is the perfect number. Wherefore it is written (Qo 4,12): "A threefold cord is not easily broken": and Augustine, commenting on Jn 8,17, "The testimony of two men is true," says (Tract. xxxvi) that "there is here a mystery by which we are given to understand that Trinity wherein is perpetual stability of truth."
Reply to Objection: 1. No matter how great a number of witnesses may be determined, the evidence might sometimes be unjust, since is written (Ex 23,2): "Thou shalt not follow the multitude to do evil." And yet the fact that in so many it is not possible to have certitude without fear of error, is no reason why we should reject the certitude which can probably be had through two or three witnesses, as stated above.
2. If the witnesses disagree certain principal circumstances which change the substance of the fact, for instance in time, place, or persons, which are chiefly in question, their evidence is of no weight, because if they disagree in such things, each one would seem to be giving distinct evidence and to be speaking of different facts. For instance, one say that a certain thing happened at such and such a time or place, while another says it happened at another time or place, they seem not to be speaking of the same event. The evidence is not weakened if one witness says that he does not remember, while the other attests to a determinate time or place And if on such points as these the witness for prosecution and defense disagree altogether, and if they be equal in number on either side, and of equal standing, the accused should have the benefit of the doubt, because the judge ought to be more inclined to acquit than to condemn, except perhaps in favorable suits, such as a pleading for liberty and the like. If, however, the witnesses for the same side disagree, the judge ought to use his own discretion in discerning which side to favor, by considering either the number of witnesses, or their standing, or the favorableness of the suit, or the nature of the business and of the evidenceMuch more ought the evidence of one witness to be rejected if he contradict himself when questioned about what he has seen and about what he knows; not, however, if he contradict himself when questioned about matters of opinion and report, since he may be moved to answer differently according to the different things he has seen and heard.On the other hand if there be discrepancy of evidence in circumstances not touching the substance of the fact, for instance, whether the weather were cloudy or fine, whether the house were painted or not, or such like matters, such discrepancy does not weaken the evidence, because men are not wont to take much notice of such things, wherefore they easily forget them. Indeed, a discrepancy of this kind renders the evidence more credible, as Chrysostom states (Hom. i in Matth.), because if the witnesses agreed in every point, even in the minutest of details, they would seem to have conspired together to say the same thing: but this must be left to the prudent discernment of the judge.
3. This passage refers specially to the bishops, priests, deacons and clerics of the Roman Church, on account of its dignity: and this for three reasons. First because in that Church those men ought to be promoted whose sanctity makes their evidence of more weight than that of many witnesses. Secondly, because those who have to judge other men, often have many opponents on account of their justice, wherefore those who give evidence against them should not be believed indiscriminately, unless they be very numerous. Thirdly, because the condemnation of any one of them would detract in public opinion from the dignity and authority of that Church, a result which would be more fraught with danger than if one were to tolerate a sinner in that same Church, unless he were very notorious and manifest, so that a grave scandal would arise if he were tolerated.
Objection: 1. It would seem that a man's evidence ought not to be rejected except on account of some fault. For it a penalty on some that their evidence is inadmissible, as in the case of those who are branded with infamy. Now a penalty must not be inflicted save for a fault. Therefore it would seem that no man's evidence ought to be rejected save on account of a fault.
2. Further, "Good is to be presumed of every one, unless the contrary appear" [*Cap. Dudum, de Praesumpt.]. Now it pertains to a man's goodness that he should give true evidence. Since therefore there can be no proof of the contrary, unless there be some fault of his, it would seem that no man's evidence should be rejected save for some fault.
3. Further, no man is rendered unfit for things necessary for salvation except by some sin. But it is necessary for salvation to give true evidence, as stated above (Article ). Therefore no man should be excluded from giving evidence save for some fault.
On the contrary Gregory says (Regist. xiii, 44): "As to the bishop who is said to have been accused by his servants, you are to know that they should by no means have been heard": which words are embodied in the Decretals II, qu. 1, can. Imprimis.
I answer that As stated above (Article ), the authority of evidence is not infallible but probable; and consequently the evidence for one side is weakened by whatever strengthens the probability of the other. Now the reliability of a person's evidence is weakened, sometimes indeed on account of some fault of his, as in the case of unbelievers and persons of evil repute, as well as those who are guilty of a public crime and who are not allowed even to accuse; sometimes, without any fault on his part, and this owing either to a defect in the reason, as in the case of children, imbeciles and women, or to personal feeling, as in the case of enemies, or persons united by family or household ties, or again owing to some external condition, as in the case of poor people, slaves, and those who are under authority, concerning whom it is to be presumed that they might easily be induced to give evidence against the truth.Thus it is manifest that a person's evidence may be rejected either with or without some fault of his.
Reply to Objection: 1. If a person is disqualified from giving evidence this is done as a precaution against false evidence rather than as a punishment. Hence the argument does not prove.
2. Good is to be presumed of everyone unless the contrary appear, provided this does not threaten injury to another: because, in that case, one ought to be careful not to believe everyone readily, according to 1Jn 4,1: "Believe not every spirit."
3. To give evidence is necessary for salvation, provided the witness be competent, and the order of justice observed. Hence nothing hinders certain persons being excused from giving evidence, if they be considered unfit according to law.
Objection: 1. It would seem that it is not always a mortal sin to give false evidence. For a person may happen to give false evidence, through ignorance of fact. Now such ignorance excuses from mortal sin. Therefore the giving of false evidence is not always a mortal sin.
2. Further, a lie that benefits someone and hurts no man is officious, and this is not a mortal sin. Now sometimes a lie of this kind occurs in false evidence, as when a person gives false evidence in order to save a man from death, or from an unjust sentence which threatens him through other false witnesses or a perverse judge. Therefore in such cases it is not a mortal sin to give false evidence.
3. Further, a witness is required to take an oath in order that he may fear to commit a mortal sin of perjury. But this would not be necessary, if it were already a mortal sin to give false evidence. Therefore the giving of false evidence is not always mortal sin.
On the contrary It is written (Pr 19,5): "A false witness shall not be unpunished."
I answer that False evidence has a threefold deformity. The first is owing to perjury, since witnesses are admitted only on oath and on this count it is always a mortal sin. Secondly, owing to the violation of justice, and on this account it is a mortal sin generically, even as any kind of injustice. Hence the prohibition of false evidence by the precept of the decalogue is expressed in this form when it is said (Ex 20,16), "Thou shalt not bear false witness against thy neighbor." For one does nothing against a man by preventing him from doing someone an injury, but only by taking away his justice. Thirdly, owing to the falsehood itself, by reason of which every lie is a sin: on this account, the giving of false evidence is not always a mortal sin.
Reply to Objection: 1. In giving evidence a man ought not to affirm as certain, as though he knew it, that about which he is not certain and he should confess his doubt in doubtful terms, and that which he is certain about, in terms of certainty. Owing however to the frailty of the human memory, a man sometimes thinks he is certain about something that is not true; and then if after thinking over the matter with due care he deems himself certain about that false thing, he does not sin mortally if he asserts it, because the evidence which he gives is not directly an intentionally, but accidentally contrary to what he intends.
2. An unjust judgment is not a judgment, wherefore the false evidence given in an unjust judgment, in order to prevent injustice is not a mortal sin by virtue of the judgment, but only by reason of the oath violated.
3. Men abhor chiefly those sin that are against God, as being most grievous and among them is perjury: whereas they do not abhor so much sins against their neighbor. Consequently, for the greater certitude of evidence, the witness is required to take a oath.
Summa Th. II-II EN Qu.69