1. About the Three Kinds of Lies

22. Second we must note what sort of sin a lie is.

And although lies are distinguished in many ways, yet for the present purpose a triple distinction suffices, namely into pernicious, useful, and jocose lies.

23. A pernicious lie is one that harms or is harmful of itself to the one I lie to or lie about. And if it harms him as to the Christian religion, namely as to faith or morals etc., then it is a mortal sin. But if it harms him as to his bodily life, or not preserving his conjugal fidelity, or taking away his children or persons in any way connected with him, or as to other temporal goods, then as the lie causes him more or less harm (which is to be weighed by the good it takes away), then it is counted as more or less serious. And generally every such lie, whereby one deliberately asserts what one does not know or what one knows the opposite of, is a mortal sin. For this is simply prohibited by the commandment, “Thou shalt not bear false witness against your neighbor” [Exodus 20.16]. And ‘witness’ is not precisely what is given in court, but is when anyone deliberately asserts something he does not know or whose opposite he does know. Whoever therefore has the intention to deceive someone to whom he is speaking or about whom he is speaking and says the opposite of what he knows to be true, and by thus speaking harms him to whom or about whom he is speaking, then he gives false testimony against his neighbor. But whether not doing so deliberately counts as an excuse will be touched in d.39 nn.13-21.

24. A useful lie is when it is useful for someone and harmful to no one.

25. A jocose lie is to tell stories about what everyone who hears knows is not true and not told as true. For the hearers are not deceived and the speaker does not intend to deceive, nor is his speech of itself deceptive because it is not such as to be naturally believed by the hearers. Rather it is known to be said without making any opinion as to its truth. The like holds if a jocose lie is when someone does intend to deceive by joking, so that the one deceived really is deceived, but not in anything that inflicts any great harm on him, and those too are joking who know he is deceived. And lastly he can be himself joking in the way that Augustine says of Joseph’s lie, who did truly want to deceive his brothers in the words ‘you are spies etc.’ [n.4], and yet he himself, who knew the truth, could be joking about their deception and about the fear that the deceived were running into. And the others, if any knew, could themselves also at last be joking when they perceived that the thing was not seriously said.