The Augustine Catechism: The Enchiridon on Faith, Hope, and Love by Augustine of Hippo

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 not 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.

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