In two passages of Book 2 of De doctrina christiana (“On Christian Teaching”), Augustine gives clear expression to the basic principle of the analogy of Scripture, summarized, for example, in Westminster Confession of Faith 1.9: “The infallible rule of interpretation of Scripture is the Scripture itself; and therefore, when there is a question about the true and full sense of any scripture (which is not manifold, but one), it must be searched and known by other places that speak more clearly” (and cf. this post on Bavinck).
First, in discussing Song of Solomon 4:2 and the use of “obscure” passages in Scripture:1
Exactly why this picture gives me greater pleasure than if no such imagery were presented by the divine books, since the topic is the same, and the lesson the same, it is difficult to say; this, however, is another question entirely. But no one disputes that it is much more pleasant to learn lessons presented through imagery, and much more rewarding to discover meanings that are won only with difficulty. Those who fail to discover what they are looking for suffer from hunger, whereas those who do not look, because they have it in front of them, often die of boredom. In both situations the danger is lethargy. It is a wonderful and beneficial thing that the Holy Spirit organized the holy scripture so as to satisfy the hunger by means of its plainer passages and remove boredom by means of its obscurer ones. Virtually nothing is unearthed from these obscurities which cannot be found quite plainly expressed somewhere else. (2.6.8).
Later in the book, after a discussion of the canon (he favors the Alexandrian canon, or the LXX, for the Old Testament canon, in contrast to Jerome’s urging of the Hebraica veritas),2 he returns to this point and makes it with more, shall we say, perspicuity:
These are all the books in which those who fear God and are made docile by their holiness seek God’s will. The first rule in this laborious task is, as I have said, to know these books; not necessarily to understand them but to read them so as to commit them to memory or at least make them not totally unfamiliar. Then the matters which are clearly stated in them, whether ethical precepts or articles of belief, should be examined carefully and intelligently. The greater a person’s intellectual capacity, the more of these he finds. In clearly expressed passages of scripture once can find all the things that concern faith and the moral life (namely hope and love, treated in my previous book). Then, after gaining a familiarity with the language of the divine scriptures, one should proceed to explore and analyse the obscure passages, by taking examples from the more obvious parts to illuminate obscure expressions and by using the evidence of indisputable passages to remove the uncertainty of ambiguous ones. Here memory is extremely valuable; and it cannot be supplied by these instructions if it is lacking. (2.9.14)