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 [55], Articles [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.