The first kind of memory to be treated is the rough category of sensory perceptions--the most familiar and obvious kind of memories. Augustine draws the initial metaphor of a storehouse of memory, in which images of things experienced are stored (sometimes inconveniently), retrieved, and re-stored (sometimes in new places).
This leads Augustine to consider what sort of things the images stored in the memory are. Profoundly strange entities, these "images" can be tasted, heard, seen, etc., all without the things of which they are images actually being present. Augustine professes to be flabbergasted at the sheer immensity of such a storehouse of images, which can seem almost real: memory is "a vast and infinite profundity."
The vastness of memory is thus more than Augustine can grasp, which means that "I myself cannot grasp the totality of what I am." This state of affairs, however, seems to be a paradox. How, asks Augustine, could the mind be external to itself such than it cannot know itself? Memory is seeming increasingly enigmatic.
Leaving this train of thought for a moment, Augustine notes that his memory also holds skills. This kind of memory seems to be another case altogether, since it is not images of the skills but the skills themselves that are retained.
From skills, Augustine moves quickly to consider ideas, which constitute yet another distinct kind of memory. By ideas, Augustine means the ideas themselves, not any sensory information by which they might be communicated. How is it, he wonders, that a new idea can be self-evidently true? There are many cases in which we believe something not on the authority of the source, but because the idea itself strikes us as true.
Augustine's answer is a deeply Platonic one: the memory of such ideas must have been "there before I learnt them," waiting to be recognized. Augustine suggests that, although we don't recognize them as memories when we recognize the truth of ideas, the pieces of these ideas are present somewhere far back in our memories. In coming across an idea (whether through our own thoughts or through an external source) whose truth we recognize, we are actually "assembling" the disordered pieces of an eternal "memory."