(a) If the witness is asked to state what he knows about a case, he is not obliged to mention what he merely thinks or what he is uncertain about; and if he is asked what he has heard, he is not obliged to state what was told him by persons of poor authority.
(b) If he is asked whether the accused was to his knowledge guilty of a crime, he is not obliged to mention an act of the accused that was unlawful but done in good faith. But in a civil case, in which inquiry is made about juridical faults, the witness should testify even to the existence of delinquencies in which there was no element of theological fault.
(c) If he is the only one who has knowledge of a delinquency and it will be certainly useless for him to testify about the matter without corroboratory evidence, it seems that he may keep silence about what he knows. But if the testimony of one witness is sufficient according to law, then the witness should speak of the facts known to him.
1990. Sinfulness of False Testimony.—When we speak of false testimony, we mean testimony which the witness knows to be false.
(a) By reason of his false oath, the witness is guilty of perjury, which is a grave sin against the virtue of religion.
(b) By reason of the injury done by the testimony, the witness is guilty of injustice, which from its nature is a grave sin. In the Decalogue (Exod., xx. 16) false testimony is forbidden among the sins against justice: “Thou shalt not bear false testimony against thy neighbor.” Legal justice is offended, since false testimony is an act of disobedience to lawful authority, and usually commutative justice is also violated, since by false testimony one of the litigants as a rule suffers loss.
(c) By reason of the deliberate falsehood, the witness is guilty of lying, which, however, is not always a grave sin.
1991. It may happen then, though rarely, that false testimony is only a venial sin, for example, when the witness is not under oath and he gives false testimony in a matter of small importance, or without full deliberation on what he is saying, or when he forges or corrupts a document to supply for another that has been lost and from which his certain right could be proved.
1992. Obligation of Witness to Make Restitution.—The obligations of restitution by a witness on account of failure to perform his duties properly are as follows:
(a) if the witness has not sinned against commutative justice, there is no obligation of restitution (see 1753). Hence, if he has evaded testimony to which he was bound in legal justice or charity alone, he is guilty of sin, but he is not held to restitution. Similarly, if he has given false testimony and thereby deprived the State of a fine under a penal law, or saved a guilty party from punishment, he has sinned against legal justice, but is not obliged to make good the fine or pay damages;