Reply Obj. 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.
Reply Obj. 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 1 John 4:1: "Believe not every spirit."
Reply Obj. 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. _______________________
FOURTH ARTICLE [II-II, Q. 70, Art. 4]
Whether It Is Always a Mortal Sin to Give False Evidence?
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.
Obj. 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.
Obj. 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 (Prov. 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.