"In the first place, Herodotus, you must understand what it is that words denote, in order that by reference to this we may be in a position to test opinions, inquiries, or problems, so that our proofs may not run on untested ad infinitum, nor the terms we use be empty of meaning. [38] For the primary signification of every term employed must be clearly seen, and ought to need no proving; this being necessary, if we are to have something to which the point at issue or the problem or the opinion before us can be referred.
"Next, we must by all means stick to our sensations, that is, simply to the present impressions (ἐπιβολὰς) whether of the mind *or* of any criterion* whatever (εἴτε διανοίας εἴθ᾽ ὅτου δήποτε τῶν κριτηρίων), and similarly to our actual feelings (παθη), in order that we may have the means of determining that which needs confirmation and that which is obscure.
I read this as Epicurus conveying that words can be - should be - referenced back to and denoting impressions of the senses. He urges Herodotus to test "opinions, inquiries, or problems" in reference to real sensations impressed upon the senses (including the mind) from the real, true external-to-ourselves world.
I see "or of any criterion"* as referring to the other senses - tasting, hearing, etc. - and he includes the mind (διανοίας "thinking faculty, intelligence, understanding" LSJ) specifically in that list of "sensations" as all members in his list of criteria.
The "every term... ought to need no proving" appears to also say that words need to refer back to a mental/physical sensation of some kind, an impression from the real world.
The criteria of truth then, to me, are the sensations, the prolepseis, and the feelings, precisely because they all interact directly with the "real external world." They are the impressions set upon us from the world outside ourselves. These criteria are our first line of contact, unmitigated by "opinions, inquiries, or problems", with the real, true, existing world in which we live.
Now, are opinions almost instantaneous sometimes, following directly on the heels of sensations and prolepseis? Sure! I have no problem with that. Consider you're walking through the woods, your sensations register a long skinny shape on the ground, your prolepseis have identified this shape as a danger in the past (the grooves are well worn in your eye's and mind's apprehension of the shape... so the "prolepseis" slips right into the groove, metaphorically).. your reason jumps in with "Snake!" and you jump back. It is only seconds later that you realize it was a discarded rope. Your opinion was in error, even though there was indeed a long slender shape that registered in your sensations from our external environment. Your sensations were true. Your prolepsis faculty registered the pattern seen and reinforced. Your feelings registered pain. It was your opinion layered on top that got it "wrong."
That's a VERY rudimentary scenario illustrating where my mind is at right now on this topic. To get a "modern Epicurean" take on things, I still think it is fruitful to dig into the work of Dr. Lisa Feldman Barrett and her colleagues. I think that line of modern research has a lot of ideas worth exploring when it comes to really understanding how the mind actually works, and I still find a number of very interesting Epicurean echoes if not parallels in it.
Here's my take on this topic... as of this writing. Views subject to change in the time it takes me to type this!
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