(c) There is an obligation of charity to testify (but not at the cost of serious inconvenience), if the testimony is necessary for averting a serious evil that threatens a private person. A person who can prove that the evidence which is about to hang an innocent man is false should testify for the accused, unless the testimony will bring an equal evil upon himself.
1986. Obligation of Appearing under Lawful Citation to Give Testimony.—(a) He who avoids citation (e.g., by flight into another jurisdiction, by concealment of his person when the subpoena is being served), more probably does not violate legal justice by this act, since a precept that has not been received cannot be violated. (b) He who disregards citation offends legal justice, since the summons to appear has a claim on his obedience. But it does not seem that he violates commutative justice, unless the party for whom he could testify has a strict right to the testimony.
1987. Obligation of Witness to Answer Truthfully.—A witness who is questioned legitimately (i.e., by one who has the authority to question him) and juridically (i.e., according to the form and order prescribed in law) is obliged _per se_ to answer according to the truth as he knows it, for one is bound to obey a superior when he gives a lawful command. But there are exceptions to this rule _per accidens_, that is, when a higher law exempts one from the necessity of divulging a certain matter, or when the question asked refers only to what one knows juridically. In all these cases the witness may answer that he does not know, for he has no knowledge that he may, or should, or must use.
(a) Thus, the natural law permits reticence when a revelation would work notable damage to the witness or those closely related to him, for the command of a superior does not oblige under such great inconvenience. This supposes, of course, that the revelation is not required in order to prevent a great harm to the commonwealth or a far greater harm to a private person than that which threatens the witness. A person who knows that he will be assassinated if he testifies against a powerful criminal is not ordinarily bound to make the sacrifice. Canon and civil law excuse witnesses from making disclosures that would expose them to prosecution or penalties (see Canon 1755, Sec. 2, n. 2).
(b) The natural law commands reticence when a revelation would be injurious to divine, public or private rights.
1988. Matters Regarding Which a Witness Should Not Testify.—There are certain cases in which natural law forbids a witness to make known a fact about which he is questioned.
(a) A witness may never testify to matters known to him only from Sacramental Confession, for to break the seal of confession is an injury to the rights of God. In an ecclesiastical process a priest may not testify from Sacramental knowledge, even though he has the penitent’s permission (Canon 1757, Sec. 3, n. 2).
(b) A witness may not testify as a rule to matters that are known to him only in a confidential way, such as the communications between lawyer and client, physician and patient; for the public interest as well as the interest of individuals requires that generally there be security against defamation for those who give their confidence to others, especially if they are in great need of professional assistance. Privileged communications are recognized both in Canon Law (Canon 1755, Sec. 2) and in civil law. But knowledge obtained as a secret may be used when this is necessary in order to avert a great evil that threatens the public welfare or the welfare of an innocent person, whether this person be the giver of the secret, or a third party, or oneself; for to oblige to secrecy in these cases would be to throw protection around crime. Thus, a lawyer may make disclosures of confidential knowledge, if this is necessary in order to defend himself against the false accusations of a client, or to prevent a crime which the client intends to commit.
(c) A witness may not testify to matters about which he has unjust knowledge (e.g., by wire-tapping, by unjust coercion, by intoxicating another person, by reading private papers without permission), for, as the knowledge was unjustly acquired, it cannot be justly used (see 2420).
1989. There are also certain cases in which a question refers only to what the witness knows juridically, or in which he is called upon to answer according to the mind of the questioner.