Book X is the beginning of the philosophical portion of Confessions. Augustine sets out to fully vindicate his faith and explain as much of the tenets of Christianity in the context of philosophy as possible. After having told us of his life and conversion, he now mimics the state of his mind after conversion by showing us as much of his faith as he can. The remaining Books are longer in length than the preceding one.
The primary purpose of Book X is to show that the search for God is an inward search of one's own mind. The function of memory, which is shared somewhat by both animals and humans, produces a bewildering array of conundrums for Augustine. He goes through each type of memory (senses, skills, ideas, and emotions) and wonders at the difficulty in understanding how they work. With no knowledge of physiology, Augustine tackles the problem of human memory and its importance to faith. In the Platonic dialogue Meno, Plato argues that the soul does not truly learn anything that is true - it merely remembers things from the time before it was born. This thought was taken up by the Neoplatonists, and serves as a starting point for Augustine's own idea.
Augustine expresses the belief that knowledge (that is, knowledge of true things) is unconsious - it can be found in all people but is not acknowledged or recognized unless touched by God. Augustine admits that he cannot find God using any of his senses, so he must turn his quest inward, into his mind. He likens memory to a storehouse: he begins with sensory memories, and wonders at how these memories can have such an effect on the person remembering them. He is confounded by the number and variety of images that are stored in a human mind. This leads him to praise God, and see this as further evidence that human beings cannot fully understand their total selves. This causes a philosophical paradox for Augustine for the rest of his days.
Skill-memories present new difficulties for Augustine, because they are not related (he says) to sensory memories. The pictures of the skills aren't stored; rather, the ability to perform the skill is stored in the memory.
Ideas are, to Augustine, a separate kind of memory, and he quickly moves on to his Platonic idea that things can be recognized as true when the mind lights upon them. This he takes as evidence for the truth already being there in the memory, and simply recognized by the mind when it is reminded of it.
Emotional memories are also problematic for Augustine. He marvels that an emotion can be remembered without being re-experienced, or elicit different emotions upon remembering (such as distaste at former joy in sin). He cannot understand the nature of an emotional memory, for it is certainly not the same as the other kinds of memories. This leads to a discussion of the paradox of remembering forgetfulness, which takes Augustine into a state of confusion. He is amazed at the vastness and mystery of the faculty of human memory. With this established, he glorifies God at the "multiplicity" of memory. The complexity and vastness of the human memory serves as a proof of the vastness of God. He implies here that God, even before he is consciously known by the mind, is already in the human memory.
Augustine now moves on to the universal human pursuit of joy. He claims that even if people search for it in error among the lower things of creation, they are all really seeking God. He presumes that the human memory contains the original joy (before the fall of Adam) and that is how we are born knowing that we seek joy. Free will, and the human ability to turn away from God, is what leads most people not to seek joy in God, but to seek joy in things of the earth. Similar to the desire for joy is desire for truth - for which he posits the egocentric notion that no one wants to be deceived. The misapprehension of true things, however, means that the true things in our memory are not always recognized. Also, human pride intervenes because we do not want to admit we have been deceived.
Augustine ends Book X bewailing the fact that he is unable to let go of all the desires of the flesh. Though celibate, he still has erotic dreams. He enjoys food and music too much. He is not negating the beauty of creation, but instead the excessive attachment to those beauties to the exclusion of God. He admonishes that art, which appeals to that most knowledgeable of senses, sight, should be particularly careful to be only of the most moral content. Augustine explains that he still has the vice of pride and enjoys being praised by others. He knows he should have a selfless reaction to this and should only enjoy praise to the effect of knowing that his actions have helped others. He tries to reconcile this pride by asserting that the goodness in him comes only from God.
Augustine's lengthy and impassioned discussion of memory presents many problems for the modern reader. Modern science has shed some light on the faculty of human memory, but the nature of it is still fundamentally a mystery. The largest omission that Augustine makes is about the unreliability of memory: memories can be shaped or even mis-remembered or changed by emotion and prejudices coming from the ego. False memories and inaccuracies are common, and memories tinged by nostalgia or prejudice against a person or time are also common. How would Augustine reconcile these with his storehouse image concept?
The storehouse image, however, is surprisingly accurate when one considers that there are regions of the brain that store certain types of memories (sensory, emotional, etc). Also, the idea that they can be brought out and re-experienced and then "filed" back in a different place is also prophetic of some of the latest discoveries in the field of neuroscience. Augustine's ommisions (and especially his almost fundamental misunderstanding of the mutability of human memory) make this philosophical treatise less useful for the modern reader. Also, he doesn't have access to any kind of physical knowledge of the nervous system, so ideas such as the fact that remembered skills are partially attributed to muscle memory would never occur to him. Nonetheless, his division of memory into types is useful, and his wonder at the vastness and complexity of memory is something we can share.
Understanding the function of the brain is not Augustine's goal in Book X. He is rather trying to understand how God can be experienced by a human being. The Platonists' (and Neoplatonists') idea about the memory of all knowledge leads him directly to conclude that the truth of God can only be accessed through the mind. The glory of the Lord is evident in the amazing faculty of memory, but he finds truth by looking inward and uncovering and reassembling the truth of God - something that, he says, was already there.
Augustine ends Book X with a critique of the visions of God about which the Neoplatonists wrote. Once again, Augustine reminds us that the pagan Neoplatonists might have had rigorous and inspired philosophy, but they were fundamentally lacking in the truth of Jesus Christ. Therefore, they were not perceiving God, but were in error.