At first, Memory seemed to have to do solely with mnemonics (memory aids) that would assist a budding orator in retaining his speech. However, it clearly had to do with more than simply learning how to memorize an already composed speech for re-presentation. The Ad Herennium author calls memory the "treasury of things invented," thus linking Memory with the first canon of rhetoric, Invention. This alludes to the practice of storing up commonplaces or other material arrived at through the topics of invention for use as called for in a given occasion. See copia.
Thus, Memory is as much tied to the improvisational necessities of a speaker as to the need to memorize a complete speech for delivery. In this sense Memory is related to kairos (sensitivity to the context in which one may communicate) as well as to the concepts of copia and amplification.
Memory, it can be seen has had to do with much more than just memorization. It was a requisite for becoming peritus dicendi, well-versed in speaking, something only possible if one had a vast deal of information on hand to be brought forth appropriately and effectively given the circumstances and the audience.
The canon of Memory also suggests that one consider the psychological aspects of preparing to communicate and the performance of communicating itself, especially in an oral or impromptu setting. Typically Memory has to do only with the orator, but invites consideration of how the audience will retain things in mind. To this end, certain figures of speech are available to help the memory, including the use of vivid description (ecphrasis) and enumeration. Along with Delivery, Memory has often been excluded from rhetoric. However, it was a vital component in the training of orators in antiquity.
Example
Orators were encouraged to envision where they would be speaking as a
preparation for memorizing their speech. Then, having completed the speech's
composition, they were to divide it into manageable portions, each of
which they would assign, in turn, to a different part of the room where
the speech was to occur. Thus, by casting their eyes about during their
speech, they would be reminded of the next part of their speech to give.