Augustine asks to know God as well as God knows him. God knows Augustine's heart. Why then does Augustine confess before his readers? They cannot know Augustine's heart, but in Christian charity, they will know he tells the truth. By his example, his fellow believers will be moved to thank God and to change their own lives if they are in despair. Even Augustine cannot know himself fully, and he cannot know God fully. He inquires how he can know God. The physical world testifies to its creator, but the physical world is not God. Even animals have senses to perceive the world, but they cannot apply reason to lead them beyond physical things. The human soul orders the perceptions of the senses, but Augustine must ascend beyond this function, to memory.
Memory stores not only sense perceptions but also skills and ideas, which are not apprehended through the senses. Learning is the process of gathering and ordering all these notions in the memory. Mathematics is purely abstract, incapable of making sense impressions, but the memory can hold it. Emotions, too, can remain in the memory, although they have no existence outside the mind. People only remember those emotions, rather than feel them anew. The images of all these things are present in memory, but what about the idea of memory itself, or the idea of forgetfulness? How can I remember "forgetfulness" if forgetfulness is the very act of not remembering?
The power of human memory is vast and awesome, but how can one move beyond memory to knowledge of God? If you forget something, you can look for its image in your memory, if only partially. All people want to be happy, but how do people know of happiness? True happiness is only with God. Human beings mistake earthly happiness for the happiness found in God; people hide from this truth and so become miserable. God is not a sense perception, an emotion, or even the mind itself, but God remains in the memory.
Book 10 is a distinct departure from the first nine books of the Confessions. Only now, after the story of his conversion is finished, does Augustine address the question of why he is writing. This question leads Augustine into a far-ranging discussion of the nature of the human mind, memory, and sense perceptions.
The simple answer to Augustine's question of "why write this story?" (and its corollary, "why read it?") is disposed of early on: Augustine's story can be an example to his fellow Christians, the people he has devoted his life to serving. Seeing Augustine, the good can be moved to praise God and give thanks for God's mercy; those stuck in the grasp of their sins can take hope and find energy to put those sins behind them. Augustine even asks the question his readers inevitably ask: How can they know that anything Augustine says is actually true? Augustine candidly admits that even he cannot really know everything about himself; only God has such perfect knowledge. Augustine does not try to avoid this fact or explain it away. His only reply is that love, the virtue that "believes all things," will lead his readers to discern the truth of what he says.
The more complex answer to Augustine's question is implied behind his long argument about the human mind and perception. The assumption, which Augustine never directly states, is that by knowing the self, human beings can begin to understand God. This is an idea borrowed from Platonism: Because the human spirit shares in some of the divinity of God, from which that spirit comes, knowledge of the inner self is also is some measure knowledge of the divine. This is not self-knowledge in the sense of contemplating one's own belly-button. Egotism and self-absorption are directly contrary to real knowledge of the self. The kind of self-knowledge Augustine wants is an understanding of the inner workings of the human soul, because those actions are initiated and should return to the divine. Augustine signals the nature of his quest by beginning it, in 4.7, with another reference to the Platonic ascent from the physical world to through the soul to the immaterial realm, as he presented in 7.17.
He proceeds through his argument in careful stages, asking how human beings acquire knowledge. The simplest kind of knowledge is received from the physical world. The human memory stores impressions received from the senses and allows them to be organized and recalled. But the mind also stores other kinds of "images" that are not physical: purely abstract concepts like mathematics, or human emotions, which do not exist outside the individual human mind. Likewise, Augustine concludes, the human memory stores an impression of the true happiness that is found only in God. Exactly how this is possible he does not say, but he implies that the memory retains a kind of memory of its existence in the divine realm or the substance that it shares with God. This memory of happiness accounts for the fact that all human beings, regardless of their personal differences, want to be happy. Inwardly, dimly, they long to regain this perfect happiness that they can now grasp only imperfectly because of their limited physical existence. The error that human beings commit is to substitute the limited happiness and pleasure that the physical realm gives for the ultimate happiness found in God. Human sinfulness means that human beings find it hateful to see this truth revealed to them. Sluggish and earthbound, the human mind languishes in a kind of inertia that keeps it from rising toward the true reality, foolishly confident in its own knowledge. Augustine himself is testimony to the fact that human beings can break free from this inertia — but only if they are aided by God's grace and accept God's will.
Augustine's dense discussion of human memory also demonstrates how deeply he values rationality. Augustine reasons his way toward these conclusions; they are not beyond human grasp, nor do they require secret knowledge or privileged revelations, such as the Manichees claimed to offer. The entire physical world offers evidence of these truths, if only the human eye looks carefully enough to see them.
Augustine's interest in language also appears throughout this section. The signs and symbols of language become associated with images in the memory, so that the sound of word recalls from the memory the image or idea it represents. These signs are not absolute. They vary among speakers of Latin and Greek, for example. Augustine's appreciation of language as a set of humanly constructed conventions has an unusually modern flavor and has attracted the attention of many post-modern analysts of language and literature. He further explores this subject, as well as extending his analysis of the human mind, in his treatise De trinitate (On the Trinity), begun at about the same time Augustine was writing the Confessions.