330. The eighth commandment is: "Thou shalt not bear false witness against thy neighbor". Its purpose is to protect a man's right to his good name, or to the good opinion others have of him. Like most of the other commandments, this one, for the sake of brevity and impressiveness, mentions only one particular prohibition, namely calumny, that is, injuring a neighbor's good name by false statements. Other sins against this commandment are lying, detraction, insult, violation of secrecy, rash judgments, and unjust suspicions.
331. Falsehood, or lying, is speaking against one's mind: it is denying what we think true, or affirming what we think false. It is always wrong, even when it does no injury whatever to any one. For it is inordinate that a person's speech should contradict his thoughts. Man thus morally disfigures himself in his intellectual gifts, in which he is the image of God. The turpitude of this vice is shown by the odium attached to it in the estimation of men, and by the severe condemnation pronounced upon it in time Holy Scriptures: "Lying lips are an abomination to the Lord" (Prov. XII, 22).
We are not obliged to make known to every one the truth on every subject; nay, it is often our duty to conceal facts the revelation of which would work private or public injury; such are family and State secrets. In such cases prudent evasions may be allowed, but lying never.
332. Detraction consists in unjustly lessening a neighbor's good name by making known his faults in his absence. We say unjustly; for it is no detraction to reveal another's fault when this is done for his own good or to protect the rights of others. Like calumny (n. 331), detraction is a violation of justice; for men have a right to their good name as long as they have not forfeited it by their public crimes. Both these sins, lying and detraction, are grievous if they do great injury to the reputation of persons; and they entail the duty of repairing the injury done. These two remarks apply also to contumely, or insult, which consists in words or acts of contempt by which another's honor is violated in his presence.
A violation of secrecy is committed when we betray a secret which we are in duty bound to conceal, either from the very nature of things, -- in the case of a natural secret -- , or because we have promised to keep it a secret, or because it has been entrusted to us on condition of secrecy. It is also wrong, and often grievously sinful, to try by unfair means to discover another's secrets, for instance, by stealthily reading his letters.
We are not only forbidden to lower a neighbor unjustly in the estimation of others, but also to do so in our own estimation. This is done by rash judgments and by unjust suspicions. As a rule, no man has a right to summon another before his judgment seat: "Judge not, that you may not be judged. . . Why seest thou the mote that is in thy brother's eye, and seest not the beam that is in thy own eye?" asks the gentle Saviour (Matt. VII, m, 3). Those, however, who have charge of others have a right to suspect and to judge them; but never rashly, that is, without good reasons. In self-defence we may be cautious, but we should not be suspicious.