In Book XII, Augustine seeks to quell the diversity of opinions about the interpretation of the book of Genesis. In this Book he concentrates on the most literary and intense kind of exegesis - the use of language. He admits that there is more than one true interpretation of language (for he has accepted figurative interpretation, which, by its very nature, must have more than one interpretation), but he wants to set the boundaries of exegesis. It appears that he wants to put a limit on exactly how much questioning and extrapolating can be done on each word of scripture. This he does later in the book with the Ten Axioms.
Augustine begins this Book with a discussion about the "formless matter" referred to in Genesis (the waters, a formless fluid), and God's heaven, which is not the created stars in the sky, but rather God's house. His house, or heaven of heaven, is filled with angels, the highest order of being after God. Being, also, is considered by Augustine to be the property of godliness, something in which all created things participate to some degree (see Books I, X, and XI). Therefore, being is not the temporal notion of being, such as the being and non-being of things on earth; being for God is eternal and unchanging.
Augustine finds it necessary to discuss the order in which things were created, which would seem to negate his idea that God created things not in a temporal sense, but eternally and all at once. The heaven of heavens was the created first, with the formless matter of earth coming after. The "first" and "after" here are a basis of non-temporal priority, rather than an expression of when. Augustine even admits that he is somewhat confused in this matter, and cites his time spent engrossed in Manichaean philosophy as the cause of his uncertainty. He tries to explain that matter (that is, everything on earth that has substance) is formless rather than having a form. The form of God(and this appears contradictory) is that he has no form and never changes. The formlessness of matter is that it is always changing and never constant, and therefore has no form (like time, which is always passing away, and can therefore have no existence).
After this Augustine discusses the nature of heaven and earth and explains that while they are very different, they are both in existence outside of time. The heaven of heaven is atemporal because it is the plain of existence closest to God (and has a nearly unchanging form), and the formless matter is atemporal because, though it is ever-changing, it never passes into non-existence. He cites the fact that Genesis does not begin numbering the days until God creates heaven and earth; this, he says, proves the atemporality of both heaven and earth.
Augustine ends Book XII with arguments aimed at his Christian critics, who might take issue with his figurative interpretation of Genesis. The critics believe that Moses meant literally what he said. In rebuttal, Augustine explains that figurative interpretation is necessary to remain orthodox, and that no one knows what Moses was thinking. The notion of divine inspiration of scripture is not discussed, but it is put as "those to whom you have granted insight to see them with their inward eye." This atemporality and unchangeability of God, which Augustine finds necessary, is at odds with the literal words of Genesis, so Augustine asserts that they must have a deeper meaning.
For his reading of the Bible, Augustine now embarks on his "Ten Axioms": God made heaven and earth; the "beginning" in Genesis refers to God's wisdom; "heaven and earth" is a label for all creation (the heaven of heavens and formless matter); mutability implies a kind of formlessness; something completely mutable (formless) cannot be temporal; formless matter can not suffer temporal successiveness; the origin or source of something sometimes is labeled as its product (formless matter became the earth); "earth and the abyss" refers to formed objects that have almost total formlessness; God made everything with a form and everything formable; and everything with a form is first formless. Based on these axioms several different - yet similar - readings of Genesis can take place (being different in only order of creation). Augustine refutes the idea that formless matter existed before God. God made formless matter, as he made all things, even though it isn't explicitly stated in Genesis.
Augustine continues to note that the many "figurative" readings of the Bible are present so that many different people can read and understand the truth. The truth of a reading can be found not necessarily in the exact philosophical meaning, but rather in the inspiration it gives the reader to achieve a deeper faith. The intention of the author is not as important as the truth of God, which lies in the Bible for all to see.
In this Book, Augustine mires himself several times in what appears to be a wholly negative philosophy. Time is continuously passing and cannot be pinpointed or defined, and therefore has no existence. Matter is continuously changing and is therefore formless. He appears to be "too clever by half," and in danger of philosophizing his way into nihilism. Augustine, however, has carefully parsed his seemingly muddled arguments so as not to contradict scripture (and Genesis, especially) in any way. Through negation of accepted notions of things like being, time, and matter, and through careful "figurative" interpretations of the individual words that make up scripture, he can make his worldview fit the Catholic teachings in an orthodox way. Whether you believe his arguments or reject them, they are carefully created, and not without precedent. A large number of his arguments stem from Neoplatonist and Platonist thought, formulated and refined over hundreds of years since Plato's day (427-347 B. C.).
The sometimes tortured way that Augustine crams Biblical exegesis into Neoplatonic and orthodox Catholic thought can be unintelligible to modern readers. Again, many of these notions (which Catholics have accepted for centuries) were new and still a matter of discussion in Augustine's day. In addition, there were not many theological geniuses writing about Christian dogma at this time; it was necessary for a brain like Augustine's to be put to the task of explicating, once and for all, many knotty points of Catholic scripture. Today, even the most conservative Catholic would not insist on such minute points of doctrine as Augustine did. But this is not a time when heresies over the nature of God could result in expulsion from the Church, persecution, or even death. Clarity and uniformity of doctrine was extremely important to the early Church.
This Book remains one of the least accessible to the modern reader, for some of Augustine's explanations seem facile. Though carefully explicated and discussed at length, sometimes Augustine's stretching of language and interpretation are more than logic can bear. But for Augustine this was a prayerful exercise - the use of logic was important, but it was not the only thing to be considered. Augustine accepted Catholic doctrine, and was struggling with understanding the scripture within that doctrine with all the intellectual tools he possessed.
The section on the heaven of heaven may seem particularly pointless to the modern Christian. If God has a heaven, why is it necessary for a Christian to understand its exact nature in relation to God? Augustine's answer would have been that the nature of all God's creation was absolutely necessary to the complete understanding and faith in God - for him. He admits (as he does in Book VII to Alypius) that many faithful people may not need this kind of philosophical rigor to come to God completely and fully.