Pierre-Marie Morel spoke yesterday in BACAP, discussing what Epicurus meant by the technical term prolepsis. His view? Prolepsis refers to both a mental image and a movement of thought, and it plays five different functions in an Epicurean "proleptic method".
I found this account unsatisfying, because fractured. Although Morel's lecture was comprehensive, it failed to give a coherent account, because it failed to give a unified account. (David Konstan attempted a more synthetic view in his commentary.)
Since the topic seems so unsettled, here are my thoughts.
To me it seemed that both Morel and Konstan failed to construe correctly the single most important text on prolepsis (Diogenes Laertius, X 33). I'll quote it here, in the Long and Sedley translation, with a few changes (largely highlighted), and indicate how I think the text should be parsed. I'll also leave the crucial term untranslated.
Probably the four examples are meant to correspond to the four items given in the list: (i) is memory; (ii) is ennoia; (iii) is katalêpsis; and (iv) is doxa orthê. The crucial phrase in the introductory remark is "of that which has frequently become evident externally", which is meant to apply to all four items, and which explains that what is essential to prolêpsis is that it makes available a breadth of prior experience. Each example then draws attention to the fact that what is drawn upon is prior ('preceded beforehand', 'prior', 'previously') and immediately available (enarges--which does not mean self-evident).
A prolêpsis they [the Epicureans] say, is the sort of thing as if (hoion ei) it is a perception (katalêpsis), correct opinion (doxa orthê), conception (ennoia), or universal “store notion” (katholikê noêsis enapokeimenê)--that is, a memory--of that which has frequently become evident externally. For example:
(i) “Such and such a kind of thing is a man.” For as soon as the word “man” is uttered, immediately by prolêpsis its delineation also comes to mind, the [relevant] ense perceptions having preceded beforehand (proêgoumenwn twn aisthesewn). Thus what is primarily associated with (hupotetagmenon) a name is something non-inferential (enarges).
(ii) And what we inquire about we would not have inquired about if we had not had prior knowledge of it. For example: “Is what’s standing over there a horse or a cow?” This presupposes (dei) by prolêpsis that one had at some time have come to know the form of a horse and that of a cow.
(iii) Nor would we even have named something if we had not previously by prolepsis learnt its delineation. Thus prolêpseis are non-inferential (enargeis).
(iv) And opinion depends on something prior and non-inferential (enarges), which is what we are implicitly relying upon when we say (eph' ho anapherontes), e.g., “How do we know if this is a man?”
The passage, I think, is not meant to give a definition of prolepsis, as many commentators suppose, so that we then have to build into it elements of, e.g. memory and belief. Neither is it enumerating exhaustively or scientifically its 'functions', as Morel interpreted it. Rather, it is giving examples of how, in a variety of things that we do, we rely on 'tacit' sense knowledge. By the nature of the case this tacit knowledge must be non-inferential (on pain of an infinite regress) and also appropropriately directed (because appropriate things are thus available to us and suggested to us).
To get a good handle on this outlook, I would recommend: read David Hume.