Bust of Marcus Aurelius at the Metropolitan Museum of Art in New York.
[Image from the Wikipedia 'Marcus Aurelius' page, public domain, via Wikimedia Commons.]
Earlier in the analysis, an investigation was begun into the meaning of some clues that Lecter gives Starling, while the two of them are conversing in Memphis, Tennessee. It was observed that not only are these clues meant for Starling (ostensibly, to help the FBI apprehend Buffalo Bill), but for the Silence of the Lambs audience as well, to help interpret the movie itself. The part of the conversation quoted below which is in bold, is the material of interest for right now:
Lecter (to Starling): Well I’ve read the case files, have you...? Everything you need to find him is right there in those pages.
Starling: Then tell me how.
Lecter: First principles, Clarice. Simplicity. Read Marcus Aurelius. Of each particular thing, ask: What is it, in itself, what is its nature...? What does he do, this man you seek?
...
Found in the writings of 2nd century philosopher and Roman emperor, Marcus Aurelius, is what it is that is being referred to by the part of Lecter's statement that is in bold above. It is in Book 8.11 of Aurelius's major philosophical work, Meditations, and it states as follows (Long's translation):
"This thing, what is it in itself, in its own constitution? What is its substance and material? And what its causal nature [or form]? And what is it doing in the world? And how long does it subsist?" (material inside square brackets in original).
Here, Aurelius is using two of the methods of analysis that he developed: an 'ask yourself this question' method, and an analysis into material, cause, and reference. He is distinguishing the material from the causal: to each thing there is both inert content (substance, material) and an activating causal principle with a discernible reference.
We know from part 21 of this analysis that the act whereby Gumb is making a whole suit of skin from various pieces obtained from his victims' bodies, represents Lecter's/Satan's (evil hermaphroditic Jews') pupil (evil Freemasons) attempting to 'usurp' God’s power of creation. So given Lecter's reference to Aurelius, it's logical to believe that there must be some relationship between Gumb’s (attempted) act of creation, and the nature of each member of some 'collection' of individual things, i.e., each physical piece that Gumb is using to assemble the suit of skin.
As an aside, a thorough check of Meditations shows that neither 'B5160' nor 'B1329' are references to it (there is a Book 5.16 in Meditations, but its contents do not have applicability to our movie).
Marcus Aurelius's Meditations (Long)