18 This being from the very first and most firmly established, touching other lies the question proceeds more securely. But by consequence we must also see that all lies must be kept aloof which hurt any man unjustly: because no man is to have a wrong, albeit a lighter one is done to him, that another may have a heavier kept from him. Nor are those lies to be allowed, which, though they hurt not another, yet do nobody any good, and are hurtful to the persons themselves who gratuitously tell them.
Indeed, these are the persons who are properly to be called liars. For there is a difference between lying and being a liar. A man may tell a lie unwillingly; but a liar loves to lie, and inhabits in his mind in the delight of lying. Next to such are those to be placed who by a lie wish to please men, not that they may do wrong or bring reproach upon any man; for we have already before put away that kind; but that they may be pleasant in conversation. These, differ from the class in which we have placed liars in this respect, that liars delight in lying, rejoicing in deceit for its own sake: but these lust to please by agreeable talk, and yet would rather please by saying things that were true, but when they do not easily find true things to say that are pleasant to the hearers, they choose rather to tell lies than to hold their tongues.
Yet it is difficult for these sometimes to undertake a story which is the whole of it false; but most commonly they interweave falsehood with truth, where they are at a loss for something sweet. Now these two sorts of lies do no harm to those who believe them, because they are not deceived concerning any matter of religion and truth, or concerning any profit or advantage of their own. It suffices them, to judge the thing possible which is told, and to have faith in a man of whom they ought not rashly to think that he is telling a lie.
For where is the harm of believing that such an one's father or grandfather was a good man, when he was not? Or that he has served with the army even in Persia, though he never set foot out of Rome? But to the persons who tell these lies, they do much harm: to the former sort, because they so desert truth as to rejoice in deceit: to the latter, because they want to please people better than the truth.
Source: On Lying (New Advent)