Augustine in His Own Words
Augustine's Rebuttal: Truths of Logic and Mathematics
Given the global nature of Academic skepticism, Augustine argues that all he needs to prove is that there are at least a few certainties. Here he points to certain indisputable principles of logic and certain indisputable truths of mathematics. He also touches on the Academics' claim that we can never know with certainty whether we may be living in a dream world or trapped in some insane self-delusion. From Against the Skeptics, Book 3:
(9.21) Let us discuss what Zeno defined . . . I shall now use a very sound dilemma: that definition of his is either true or false. If true, then I really grasp something. If false, then there is something which can be perceived, even if it has signs in common with something that is false. . . . In any case, even if we are uncertain about that definition, we are not entirely without knowledge regarding it: we do know that the proposition is either true or false. Therefore, we do know something. . . .
(10.23) I myself am still far from being wise. Nevertheless, I know something about physics, for I am certain that either there is one world or not one; if there is not one world, then the number is either finite or infinite. . . . Furthermore, I know for certain that this world of ours has its present arrangement either from the nature of bodies or from providence; either that it always was and always will be, or that, having had a beginning it will never end, or that it did not have a beginning but will have an end, or that it began to exist but will not exist always. And I have the same kind of knowledge with regard to countless physics problems, for all these disjunctives are true—and no one can confuse them with "likeness" or falsity. "Now," says the Academic, "assume the truth of either member of the disjunction." No. I will not. That is the same as saying: "Abandon what you know; assert what you don't know." "But," he says, "your assertion is left hanging." Very well, better to hang in suspense than crash to the ground. While it is hanging, it is at least in plain view, and it can be pronounced either true or false. I can say that I know it as a proposition. . . .
(11.25) But you will ask me: "Is it the very same world that you are seeing, even if you were asleep?" . . . If there were one world and six worlds, then it would be clear that there would be seven worlds, no matter how I may be affected [by sleep or insanity]. And with all due modesty, I maintain that I know this. Show me that either this dilemma or the earlier disjunctives can be false by reason or sleep or mental derangement or the unreality of sense perception . . . It would still be necessarily true that 3 × 3 = 9, and that this is the square of rational numbers even if the whole human race were snoring!30
30. C. Acad. 3.9.21–11.25 (CCL 29:46–49); my trans., based on Kavanagh, FOTC 5:190–97.
The Philosopher 49
Augustine's Rebuttal: Skepticism Self-Contradictory
Augustine goes on to argue that if the Academics are truly wise as they claim, then they must at least know, however obliquely, what wisdom itself is, and that, therefore, they know something. Radical skepticism is thus inherently self-contradictory. From Against the Skeptics, Book 3:
(14.30) First of all, Alypius, let us carefully explore what impresses you with such acuity and makes you so cautious. You have said that the Academics' theory—the theory that a wise man knows nothing—is strengthened by many cogent arguments. . . . We are discussing things here not for the sake of gaining glory, but for the purpose of finding the truth. I am satisfied if I can cross over that towering ridgeline that confronts those who would enter philosophy—it towers up darkly from some dark hidden reservoirs and threatens to cast the entirety of philosophy into obscurity, precluding any hope of finding any light within its borders. But I have nothing further to desire now if it is already probable that the wise know something. Our only reason for believing that it was "truth-like" for the wise to withhold assent was the fact that it seemed "truth-like" that nothing could be understood. That assumption must now be abandoned, for it must be conceded that the wise at least understand wisdom. Therefore, there is no reason why the wise should not give assent at least to wisdom. For, without a doubt, it is more monstrous for the wise not to approve of wisdom than not to know wisdom.
(14.31) Now if you will, let us try and visualize for a moment this great spectacle of a quarrel of sorts between a wise man and Wisdom. The only claim Wisdom makes is that she is wisdom. But her opponent says: "I do not believe it." Now who is it that says to Wisdom: "I do not believe there to be wisdom"? Who but one with whom Wisdom could converse and with whom she has deigned to live—in a word, a wise man? Now go and seek me out as someone who is battling against the Academics. Now you have a new squabble here: a wise man and Wisdom battling one another. The wise man does not want to agree with Wisdom. And I, like yourself, am calmly awaiting the outcome, for who would not believe that Wisdom is invincible? Nevertheless, let us strengthen our own position with a dilemma of sorts: In this debate, either the Academic will conquer Wisdom, or he will be conquered by her. In the first case, he will then be conquered by me because he will not be a wise man. In the other case, we shall conclude that a wise man assents to wisdom. Therefore, either the Academic is not a wise man, or a wise man will assent to something—unless, of course, the one who was ashamed to say that a wise man did not know wisdom will not be ashamed to say that a wise man does