16. This being the case, when that verse of Maro's gives us pleasure,
"Happy is he who can understand the causes of things," 
it still does not follow that our felicity depends upon our knowing the causes of the great physical processes in the world, which are hidden in the secret maze of nature,
"Whence earthquakes, whose force swells the sea to flood, so that they burst their bounds and then subside again," 
and other such things as this.
But we ought to know the causes of good and evil in things, at least as far as men may do so in this life, filled as it is with errors and distress, in order to avoid these errors and distresses. We must always aim at that true felicity wherein misery does not distract, nor error mislead. If it is a good thing to understand the causes of physical motion, there is nothing of greater concern in these matters which we ought to understand than our own health. But when we are in ignorance of such things, we seek out a physician, who has seen how the secrets of heaven and earth still remain hidden from us, and what patience there must be in unknowing.
17. Although we should beware of error wherever possible, not only in great matters but in small ones as well, it is impossible not to be ignorant of many things. Yet it does not follow that one falls into error out of ignorance alone. If someone thinks he knows what he does not know, if he approves as true what is actually false, this then is error, in the proper sense of the term. Obviously, much depends on the question involved in the error, for in one and the same question one naturally prefers the instructed to the ignorant, the expert to the blunderer, and this with good reason. In a complex issue, however, as when one man knows one thing and another man knows something else, if the former knowledge is more useful and the latter is less useful or even harmful, who in this latter case would not prefer ignorance? There are some things, after all, that it is better not to know than to know. Likewise, there is sometimes profit in error-but on a journey, not in morals.  This sort of thing happened to us once, when we mistook the way at a crossroads and did not go by the place where an armed gang of Donatists lay in wait to ambush us. We finally arrived at the place where we were going, but only by a roundabout way, and upon learning of the ambush, we were glad to have erred and gave thanks to God for our error. Who would doubt, in such a situation, that the erring traveler is better off than the unerring brigand? This perhaps explains the meaning of our finest poet, when he speaks for an unhappy lover:
"When I saw [her] I was undone, and fatal error swept me away," 
for there is such a thing as a fortunate mistake which not only does no harm but actually does some good.
But now for a more careful consideration of the truth in this business. To err means nothing more than to judge as true what is in fact false, and as false what is true. It means to be certain about the uncertain, uncertain about the certain, whether it be certainly true or certainly false. This sort of error in the mind is deforming and improper, since the fitting and proper thing would be to be able to say, in speech or judgment: "Yes, yes. No, no."  Actually, the wretched lives we lead come partly from this: that sometimes if they are not to be entirely lost, error is unavoidable. It is different in that higher life where Truth itself is the life of our souls, where none deceives and none is deceived. In this life men deceive and are deceived, and are actually worse off when they deceive by lying than when they are deceived by believing lies. Yet our rational mind shrinks from falsehood, and naturally avoids error as much as it can, so that even a deceiver is unwilling to be deceived by somebody else.  For the liar thinks he does not deceive himself and that he deceives only those who believe him. Indeed, he does not err in his lying, if he himself knows what the truth is. But he is deceived in this, that he supposes that his lie does no harm to himself, when actually every sin harms the one who commits it more that it does the one who suffers it.
 Virgil, Georgios, II, 490.
 Ibid., 479.
 Sed in via pedum, non in via morum.
 Virgil, Eclogue, VIII, 42. The context of the passage is Damon's complaint over his faithless Nyssa; he is here remembering the first time he ever saw her-when he was twelve! Cf. Theocritus, II, 82.