Chapter 31 — Part III: The Decalogue — The Eighth Commandment
Section 7. Moreover, as every man is bound to love himself, and is thus, in some sense, his own neighbour, it is unlawful for anyone to bear false witness against himself. He who does so brands himself with infamy and disgrace, and injures both himself and the Church of which he is a member, much as the suicide, by his act, does a wrong to the state. This is the doctrine of St. Augustine, who says: To those who do not understand (the precept) properly, it might seem lawful to give false testimony against one's self, because the words "against thy neighbour" are subjoined in the Commandment. But let no one who bears false testimony against himself think that he has not violated this Commandment, for the standard of loving our neighbour is the love which we cherish towards ourselves.