First, of what sort and how great is the very knowledge itself that a man can attain, be he ever so skillful and learned, by which our thought is formed with truth, when we speak what we know? For to pass by those things that come into the mind from the bodily senses, among which so many are otherwise than they seem to be, that he who is overmuch pressed down by their resemblance to truth, seems sane to himself, but really is not sane; whence it is that the Academic philosophy has so prevailed as to be still more wretchedly insane by doubting all things; passing by, then, those things that come into the mind by the bodily senses, how large a proportion is left of things which we know in such manner as we know that we live? In regard to this, indeed, we are absolutely without any fear lest perchance we are being deceived by some resemblance of the truth; since it is certain, that he who is deceived, yet lives. And this again is not reckoned among those objects of sight that are presented from without, so that the eye may be deceived in it; in such way as it is when an oar in the water looks bent, and towers seem to move as you sail past them, and a thousand other things that are otherwise than they seem to be: for this is not a thing that is discerned by the eye of the flesh. The knowledge by which we know that we live is the most inward of all knowledge, of which even the Academic cannot insinuate: Perhaps you are asleep, and do not know it, and you see things in your sleep. For who does not know that what people see in dreams is precisely like what they see when awake? But he who is certain of the knowledge of his own life, does not therein say, I know I am awake, but, I know I am alive; therefore, whether he be asleep or awake, he is alive. Nor can he be deceived in that knowledge by dreams; since it belongs to a living man both to sleep and to see in sleep. Nor can the Academic again say, in confutation of this knowledge: Perhaps you are mad, and do not know it: for what madmen see is precisely like what they also see who are sane; but he who is mad is alive. Nor does he answer the Academic by saying, I know I am not mad, but, I know I am alive. Therefore he who says he knows he is alive, can neither be deceived nor lie. Let a thousand kinds, then, of deceitful objects of sight be presented to him who says, I know I am alive; yet he will fear none of them, for he who is deceived yet is alive. But if such things alone pertain to human knowledge, they are very few indeed; unless that they can be so multiplied in each kind, as not only not to be few, but to reach in the result to infinity. For he who says, I know I am alive, says that he knows one single thing. Further, if he says, I know that I know I am alive, now there are two; but that he knows these two is a third thing to know. And so he can add a fourth and a fifth, and innumerable others, if he holds out. But since he cannot either comprehend an innumerable number by additions of units, or say a thing innumerable times, he comprehends this at least, and with perfect certainty, viz. that this is both true and so innumerable that he cannot truly comprehend and say its infinite number. This same thing may be noticed also in the case of a will that is certain. For it would be an impudent answer to make to any one who should say, I will to be happy, that perhaps you are deceived. And if he should say, I know that I will this, and I know that I know it, he can add yet a third to these two, viz. that he knows these two; and a fourth, that he knows that he knows these two; and so on ad infinitum. Likewise, if any one were to say, I will not to be mistaken; will it not be true, whether he is mistaken or whether he is not, that nevertheless he does will not to be mistaken? Would it not be most impudent to say to him, Perhaps you are deceived? when beyond doubt, whereinsoever he may be deceived, he is nevertheless not deceived in thinking that he wills not to be deceived. And if he says he knows this, he adds any number he choses of things known, and perceives that number to be infinite. For he who says, I will not to be deceived, and I know that I will not to be so, and I know that I know it, is able now to set forth an infinite number here also, however awkward may be the expression of it. And other things too are to be found capable of refuting the Academics, who contend that man can know nothing. But we must restrict ourselves, especially as this is not the subject we have undertaken in the present work. There are three books of ours on that subject, written in the early time of our conversion, which he who can and will read, and who understands them, will doubtless not be much moved by any of the many arguments which they have found out against the discovery of truth. For whereas there are two kinds of knowable things, one, of those things which the mind perceives by the bodily senses; the other, of those which it perceives by itself, these philosophers have babbled much against the bodily senses, but have never been able to throw doubt upon those most certain perceptions of things true, which the mind knows by itself, such as is that which I have mentioned, I know that I am alive. But far be it from us to doubt the truth of what we have learned by the bodily senses; since by them we have learned to know the heaven and the earth, and those things in them which are known to us, so far as He who created both us and them has willed them to be within our knowledge. Far be it from us too to deny, that we know what we have learned by the testimony of others: otherwise we know not that there is an ocean; we know not that the lands and cities exist which most copious report commends to us; we know not that those men were, and their works, which we have learned by reading history; we know not the news that is daily brought us from this quarter or that, and confirmed by consistent and conspiring evidence; lastly, we know not at what place or from whom we have been born: since in all these things we have believed the testimony of others. And if it is most absurd to say this, then we must confess, that not only our own senses, but those of other persons also, have added very much indeed to our knowledge.

22. All these things, then, both those which the human mind knows by itself, and those which it knows by the bodily senses, and those which it has received and knows by the testimony of others, are laid up and retained in the storehouse of the memory; and from these is begotten a word that is true when we speak what we know, but a word that is before all sound, before all thought of a sound. For the word is then most like to the thing known, from which also its image is begotten, since the sight of thinking arises from the sight of knowledge; when it is a word belonging to no tongue, but is a true word concerning a true thing, having nothing of its own, but wholly derived from that knowledge from which it is born. Nor does it signify when he learned it, who speaks what he knows; for sometimes he says it immediately upon learning it; provided only that the word is true, i.e. sprung from things that are known.