As a thinker, Augustine had a great affinity for Plato and his doctrines. While Plato thought of the philosopher's vision of the forms, Augustine referred to religious vision, a theory of illumination. “There is present in us the light of eternal reason, in which light the immutable truths are seen,” he said. For Augustine, this illumination comes from God just as light comes from the sun. It is the illumination of your judgment whereby you are able to discern that certain ideas contain necessary and eternal truths. But the skeptics of the Platonic school, especially Pyrrho of Elis (360-270 B.C.), expressed doubts that human beings could know anything at all with certainty. Augustine thought that skepticism must be laid to rest before any knowledge could be attained.

In his early work Against the Academics he addressed himself to the skeptics of the New Platonic Academy (a period in the Academy dominated by Carneades, 213-129 B.C.) The skeptics held two theses: (1) Nothing can be known, and (2) assent should not be given to anything. For Augustine, it wasn't just pursuit of the truth that made someone happy — it was the finding of the truth; therefore he believed a way out of skepticism must be found.

Skepticism and Logical Truths

Augustine sought to answer the skeptics. He did this by showing that they were mistaken in assuming that certain knowledge was impossible.

When the skeptics say, for example, that you cannot attain certainty and that only probable knowledge is possible, they fail to realize that probability is a step toward certainty. So Augustine's tactic is to use the experience of uncertainty and doubt against the skeptics.

Human reason does indeed have certainty about various concepts. You know, for example, that the principle of contradiction, as stated by Aristotle, is true. That is, a thing cannot both be and not be at the same time. Can it ever be true, for example, that it is raining and not raining at the same time and in the same respect? No, it can never be. So the skeptics are wrong to say that no knowledge is certain, since logical principles are certain.

I Doubt; Therefore, I Am: Si Fallor Sum

An extreme skeptic, Pyrrho (360-270 B.C.) had declared that sense experiences can never yield knowledge. All you can know are your private sensations. You can never know that your sensations agree with their objects. For instance, if you see a silver quarter, you can say, “The quarter appears silver to me.” But you can never know that it is.

That example refers to sight, but the same applies to taste. You can say, “The honey appears sweet to me or tastes sweet to me.” But you cannot say for sure “The honey is sweet.” To the Skeptic, therefore, the prudent approach is to suspend judgment and assume nothing at all. The same would hold for moral judgments. Since you cannot know anything for sure, you should stop striving for absolute moral truth — and probably be more tolerant of a variety of viewpoints.

As with his example of the principle of contradiction, Augustine replied by trying to show that there is at least one thing you can know with certainty. The skeptic may doubt that the coin is silver or the wine is bitter, but they cannot doubt the existence of perception itself. A person who doubts all truths is thus caught in a logical dilemma. For even the act of doubting is a form of certainty. The individual remembers what he is doing in the very act of self-doubt. If you doubt that it's now seven o'clock at night, at least you cannot doubt that you are doubting.

Surely, you must exist in order to doubt. In other words, you can prove the absolute reality of your own soul. Augustine said Si Fallor sum: if one can doubt then one surely is. You cannot doubt that you live and understand either. So Augustine shows skepticism can be defeated.