Cogito Ergo Sum V

Augustine, Enchiridion, Chapter 20:

But I am not sure whether one ought to argue with men who not only do not know that there is an eternal life before them, but do not know whether they are living at the present moment; nay, say that they do not know what it is impossible they can be ignorant of. For it is impossible that any one should be ignorant that he is alive, seeing that if he be not alive it is impossible for him to be ignorant; for not knowledge merely, but ignorance too, can be an attribute only of the living. But, forsooth, they think that by not acknowledging that they are alive they avoid error, when even their very error proves that they are alive, since one who is not alive cannot err. As, then, it is not only true, but certain, that we are alive, so there are many other things both true and certain; and God forbid that it should ever be called wisdom, and not the height of folly, to refuse assent to these.