CHAPTER VII. Disputed Questions about the Limits of Knowledge and Certainty in Various Matters
20. I do not rightly know whether errors of this sort should be called sins-when one thinks well of a wicked man, not knowing what his character really is, or when, instead of our physical perception, similar perceptions occur which we experience in the spirit (such as the illusion of the apostle Peter when he thought he was seeing a vision but was actually being liberated from fetters and chains by the angel  Or in perceptual illusions when we think something is smooth which is actually rough, or something sweet which is bitter, something fragrant which is putrid, that a noise is thunder when it is actually a wagon passing by, when one takes this man for that, or when two men look alike, as happens in the case of twins-whence our poet speaks of "a pleasant error for parents"  -say I do not know whether these and other such errors should be called sins.
Nor am I at the moment trying to deal with that knottiest of questions which baffled the most acute men of the Academy, whether a wise man ought ever to affirm anything positively lest he be involved in the error of affirming as true what may be false, since all questions, as they assert, are either mysterious [occulta] or uncertain. On these points I wrote three books in the early stages of my conversion because my further progress was being blocked by objections like this which stood at the very threshold of my understanding.  It was necessary to overcome the despair of being unable to attain to truth, which is what their arguments seemed to lead one to. Among them every error is deemed a sin, and this can be warded off only by a systematic suspension of positive assent. Indeed they say it is an error if someone believes in what is uncertain. For them, however, nothing is certain in human experience, because of the deceitful likeness of falsehood to the truth, so that even if what appears to be true turns out to be true indeed, they will still dispute it with the most acute and even shameless arguments.
Among us, on the other hand, "the righteous man lives by faith."  Now, if you take away positive affirmation,  you take away faith, for without positive affirmation nothing is believed. And there are truths about things unseen, and unless they are believed, we cannot attain to the happy life, which is nothing less than life eternal. It is a question whether we ought to argue with those who profess themselves ignorant not only about the eternity yet to come but also about their present existence, for they [the Academics] even argue that they do not know what they cannot help knowing. For no one can "not know" that he himself is alive. If he is not alive, he cannot "not know" about it or anything else at all, because either to know or to "not know" implies a living subject. But, in such a case, by not positively affirming that they are alive, the skeptics ward off the appearance of error in themselves, yet they do make errors simply by showing themselves alive; one cannot err who is not alive. That we live is therefore not only true, but it is altogether certain as well. And there are many things that are thus true and certain concerning which, if we withhold positive assent, this ought not to be regarded as a higher wisdom but actually a sort of dementia.
21. In those things which do not concern our attainment of the Kingdom of God, it does not matter whether they are believed in or not, or whether they are true or are supposed to be true or false. To err in such questions, to mistake one thing for another, is not to be judged as a sin or, if it is, as a small and light one. In sum, whatever kind or how much of an error these miscues may be, it does not involve the way that leads to God, which is the faith of Christ which works through love. This way of life was not abandoned in that error so dear to parents concerning the twins.  Nor did the apostle Peter deviate from this way when he thought he saw a vision and so mistook one thing for something else. In his case, he did not discover the actual situation until after the angel, by whom he was freed, had departed from him. Nor did the patriarch Jacob deviate from this way when he believed that his son, who was in fact alive, had been devoured by a wild beast. We may err through false impressions of this kind, with our faith in God still safe, nor do we thus leave the way that leads us to him. Nevertheless, such mistakes, even if they are not sins, must still be listed among the evils of this life, which is so readily subject to vanity that we judge the false for true, reject the true for the false, and hold as uncertain what is actually certain. For even if these mistakes do not affect that faith by which we move forward to affirm truth and eternal beatitude, yet they are not unrelated to the misery in which we still exist. Actually, of course, we would be deceived in nothing at all, either in our souls or our physical senses, if we were already enjoying that true and perfected happiness.
22. Every lie, then, must be called a sin, because every man ought to speak what is in his heart-not only when he himself knows the truth, but even when he errs and is deceived, as a man may be. This is so whether it be true or is only supposed to be true when it is not. But a man who lies says the opposite of what is in his heart, with the deliberate intent to deceive. Now clearly, language, in its proper function, was developed not as a means whereby men could deceive one another, but as a medium through which a man could communicate his thought to others. Wherefore to use language in order to deceive, and not as it was designed to be used, is a sin.
Nor should we suppose that there is any such thing as a lie that is not a sin, just because we suppose that we can sometimes help somebody by lying. For we could also do this by stealing, as when a secret theft from a rich man who does not feel the loss is openly given to a pauper who greatly appreciates the gain. Yet no one would say that such a theft was not a sin. Or again, we could also "help" by committing adultery, if someone appeared to be dying for love if we would not consent to her desire and who, if she lived, might be purified by repentance. But it cannot be denied that such an adultery would be a sin. If, then, we hold chastity in such high regard, wherein has truth offended us so that although chastity must not be violated by adultery, even for the sake of some other good, yet truth may be violated by lying? That men have made progress toward the good, when they will not lie save for the sake of human values, is not to be denied. But what is rightly praised in such a forward step, and perhaps even rewarded, is their good will and not their deceit. The deceit may be pardoned, but certainly ought not to be praised, especially among the heirs of the New Covenant to whom it has been said, "Let your speech be yes, yes; no, no: for what is more than this comes from evil."  Yet because of what this evil does, never ceasing to subvert this mortality of ours, even the joint heirs of Christ themselves pray, "Forgive us our debts." 
 Cf. Acts 12:9.
 Virgil, Aeneid, X, 392.
 This refers to one of the first of the Cassiciacum dialogues, Contra Academicos. The gist of Augustine's refutation of skepticism is in III, 23ff. Throughout his whole career he continued to maintain this position: that certain knowledge begins with self-knowledge. Cf. Confessions, Bk. V, Ch. X, 19; see also City of God, XI, xxvii.
 Hab. 2:4; Rom. 1:17.
 A direct contrast between suspensus assenso-the watchword of the Academics-and assensio, the badge of Christian certitude.