Cogito Ergo Sum VII

Augustine, On the Trinity, Book XV, Chapter 12:

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 chooses 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.