3 For which purpose we must see what a lie is. For not every one who says a false thing lies, if he believes or opines that to be true which he says. Now between believing and opining there is this difference, that sometimes he who believes feels that he does not know that which he believes, (although he may know himself to be ignorant of a thing, and yet have no doubt at all concerning it, if he most firmly believes it:) whereas he who opines, thinks he knows that which he does not know.
Now whoever utters that which he holds in his mind either as belief or as opinion, even though it be false, he lies not. For this he owes to the faith of his utterance, that he thereby produce that which he holds in his mind, and has in that way in which he produces it. Not that he is without fault, although he lie not, if either he believes what he ought not to believe, or thinks he knows what he knows not, even though it should be true: for he accounts an unknown thing for a known.
Wherefore, that man lies, who has one thing in his mind and utters another in words, or by signs of whatever kind. Whence also the heart of him who lies is said to be double; that is, there is a double thought: the one, of that thing which he either knows or thinks to be true and does not produce; the other, of that thing which he produces instead thereof, knowing or thinking it to be false. Whence it comes to pass, that he may say a false thing and yet not lie, if he thinks it to be so as he says although it be not so; and, that he may say a true thing, and yet lie, if he thinks it to be false and utters it for true, although in reality it be so as he utters it.
For from the sense of his own mind, not from the verity or falsity of the things themselves, is he to be judged to lie or not to lie. Therefore he who utters a false thing for a true, which however he opines to be true, may be called erring and rash: but he is not rightly said to lie; because he has not a double heart when he utters it, neither does he wish to deceive, but is deceived. But the fault of him who lies, is, the desire of deceiving in the uttering of his mind; whether he do deceive, in that he is believed when uttering the false thing; or whether he do not deceive, either in that he is not believed, or in that he utters a true thing with will to deceive, which he does not think to be true: wherein being believed, he does not deceive though it was his will to deceive: except that he deceives in so far as he is thought to know or think as he utters.
Source: On Lying (New Advent)