The letter starts from what "men say" (ut aiunt), namely that we should imitate the bees in reading. As they produce honey from the flowers they visit and then "assort in their cells all that they have brought in" (277), so we should, Seneca himself says "sift (separare) whatever we have gathered from a varied course of reading" because things keep better in isolation from one another.
He goes on to say that this is in his view only the first step in what is essentially a two-step process. The second step consists in compounding the separate "flavors" into a new delicious blend (279). "We must digest it, otherwise it will merely enter the memory and not the reasoning power" (281). In fact, what "our mind" should do is to "hide away all the materials by which it has been aided, and bring to light only what it has made of them" (281). And: "I would have my mind of such a quality as this; it should be equipped with many arts, and many precepts and patterns of conduct, taken from many epochs of history; but all should be blended into one" (283). This can be brought about only by "constant effort" and "by doing nothing without the approval of reason" (283).
While the analogy between knowledge, food, and digestion, would have struck Plato as abominable, because he thought that it was only Sophists that should be compared with cooks and bakers in catering to the merely sensual appetites, it is not unusual. Francis Bacon, for instance, thought that "some books are to be tasted, others to be swallowed and some few to be chewed and digested; that is, some books are to be read only in parts; others to be read, but not curiously; and some few to be read wholly, and with diligence and attention." But be that as it may, what is more important for Seneca's idea of note-taking is that we should
- first collect and "separate, treat apart or separately" what we take note of - for, I think these would be better translations of "separare" than "sift" - and then store these bits in separate cells or compartments,
- and secondly reconstitute the information in a form that is, if not entirely, then to a large extent, our own.
Seneca's account of note-taking shows that he was an eclectic, i.e. someone who thought that one should not cling uncritically to a single school and its assumptions, but that one should instead draw upon as many theories, styles, and ideas as possible, with the hope that this leads to deeper insights into the problems that we all face. He was a Stoic, but he was the first to admit that he learned much from other schools, perhaps most importantly the Epicureans.
It is also clear that this eclecticism was for him not just a theoretical concern, but a way of life. His way of note-taking also characterizes his way of living. Reading and writing are not just two activities among many, they are central concerns. Thus he reads because it is "indispensable" for two reasons:
- "to keep [him] from being satisfied with himself alone," and
- "to enable [him] to pass judgment" on the discoveries of others" and to "reflect on discoveries that remain to be made" (277).
While I could do without the "flowery" details - no pun intended - I agree with Seneca for the most part.
 I have used the following text: Seneca (2006) Epistles 66-92. With an English translation by Richard G. Gummere. Cambridge, MA: Harvard University Press (Loeb Classical Library), 277-285.