I began this evaluative summary Monday. For the full text of Augustine's On Christian Doctrine, see here.
Augustine goes on to discuss how to resolve ambiguities of pronunciation or of syllabification (since the words were run together). Some can be resolved by the rule of faith, in other cases one can consult the original Greek.
In chapter 5 of Part 2, Augustine gets into the more difficult question of the metaphorical. He sets down this rule: "We must beware of taking a figurative expression literally." In retrospect, it seems fairly obvious that Augustine often blurred that which was actually intended literally in the biblical text with his figurative or figural interpretations of them. As it were, he adopts a God-like perspective to determine what was figurative and what was literal, rather than following the literary clues of the texts themselves inductively.
So he places much of the Jewish Law into the category of figurative, using the example of opposition to Jesus healing on the Sabbath in chapter 6. The Jewish leaders take the figurative Sabbath law literally. Perhaps he is correct about God's mind here (the problem is always how we can know this), but he is wrong about the texts themselves. Nothing in the Old Testament texts themselves would lead a person to take them figuratively, and even Jesus himself in Mark 2 only claims an exception to the Sabbath law, he does not make an allegory out of it.
The problem, Augustine asserts, is that people mistake the sign, the symbolic cue, for the thing itself to which it points. They mistake the sign for the thing signified. The thing signified is spiritual, and it must be our focus. Jesus and the apostles have handed down very few rites, baptism and communion for example. Augustine considered the spiritual meaning of these fairly obvious.
Chapter 10 then addresses the opposite situation where a person takes a literal form of speech as if it were figurative. Here Augustine sets: down a fundamental rule: "Whatever there is in the word of God that cannot, when taken literally, be referred either to purity of life or soundness of doctrine, you may set down as figurative." What you must take literally in Scripture, therefore, is any instruction relating to the love of God and one's neighbor (purity of life), and you must take literally any teaching relating to the catholic faith (soundness of doctrine). On the negative side, we must take literally any instruction in Scripture that tells us to avoid lust.
So any biblical teaching that seems to ascribe some sinful action to God must be taken figuratively, just as any teaching ascribing holiness to humanity. Augustine also shows some awareness of the importance of context and intention when assessing morality. What is appropriate for one time may not be at another, and the same action can be either virtuous or sinful depending on the intention of the person doing it. Blindness to our own context can also hinder us from seeing points where our own customs are out of sync with love of God and neighbor. Meanwhile, others can fall into a sort of relativism because they are aware of how culture affects what one considers right and wrong.
But the rule to "do to others as you would have them do to you" is universal. It cannot be altered by the customs of one's people. So here is the rule again: "carefully turn over in our minds and meditate upon what we read till an interpretation be found that tends to establish the reign of love. Now, if when taken literally it at once gives a meaning of this kind, the expression is not to be considered figurative" (chap. 15). Or again, "If the sentence is one of command, either forbidding a crime or vice, or enjoining an act of prudence or benevolence, it is not figurative. If, however, it seems to enjoin a crime or vice, or to forbid an act of prudence or benevolence, it is figurative" (chap. 16).
From our standpoint today, we can see that Augustine blurs together interpretation and appropriation. Whether a text is figurative or literal, in the first instance, depends on whether the author was using words in their normal senses for the time and place when he or she was writing, not on whether its content suits our theology. However, we as Christians, on the other hand, rightly make decisions on how to appropriate texts by their relation to the law of love or the rule of faith, both of which we believe are literally instructed and taught in various places in Scripture.
The symbolism of some passages can be clarified by those same symbols elsewhere. So a passage where the symbolic meaning of a shield is unclear can be clarified by another passage where it is. It is also not problematic if a person gives an interpretation to one Scripture that fits with a clear interpretation elsewhere. Once again, Augustine here is reading texts theologically rather than in terms of what they originally meant. The original meaning of a passage is a function of its individual context, not of meanings of other passages written in quite different contexts.
Knowledge of tropes or literary devices helps clarify meaning. These are numerous and learned in school, things like allegory, parable, metaphor, and irony. Augustine ends Book 3 of On Christian Doctrine with seven rules found in the works of someone called Tichonius relating to finding allegorical meanings in Scripture.