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 (Q. 55, AA. 3, 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 Obj. 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.

Reply Obj. 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 (Rom. 1:32),
that "they . . . are worthy of death . . . that consent"