We had some interesting discussion in the Zoom tonight, with most of the comments being relatively consistent with what has been discussed already.
My own thoughts are not likely to be satisfied without more clarity (citations) on what exactly the non-Epicureans were arguing about about the nature of the "limits" argument (which I perceive to be likely another way of stating Plato's "class of the infinite" argument).
For me, the most clear statement at the moment of what I perceive to be the "limits" argument remains that of Seneca. I wonder that the next-to-last sentence in this translation could be made more clear, and I would like to see this more precisely stated using the word "limit" in the sense of edge or "definition" (in the sense of high-definition photo) but i think this helps a lot:
Seneca’s Letters – To Lucilius – 66.45: “What can be added to that which is perfect? Nothing otherwise that was not perfect to which something has been added. Nor can anything be added to virtue, either, for if anything can be added thereto, it must have contained a defect. Honour, also, permits of no addition; for it is honourable because of the very qualities which I have mentioned.[5] What then? Do you think that propriety, justice, lawfulness, do not also belong to the same type, and that they are kept within fixed limits? The ability to increase is proof that a thing is still imperfect.”
It's the last sentence that seems to me to be most revealing. I wonder if the meaning of "perfect" here is not "perfect in the sense of best" but rather "perfect in the sense of perfected / finished / sharp."
In other words, an incomplete / imperfect jar is not really a jar at all - it is something else that is in the process of becoming a jar, but it is not a jar at all.
If that is the direction of the argument, then incomplete or unfinished pleasure is not really pleasure at all, and the drift of the argument from Seneca's / Plato's / Cicero's perspective is that "pleasure" cannot really be experienced at all, because it is always accompanied by pain of some sort, and that the experience is mixed and not the same as pleasure.
From Epicurus' perspective, this argument (that it is impossible to experience pleasure at all because it is always mixed with pain) is solved by showing that what we are experiencing anytime we feel pleasure in some part of our experience. In other words, there is no such thing as "incomplete" pleasure, because if you feel pleasure you know it is pleasure because it does not feel painful.
This interpretation would place the emphasis on understanding from a conceptual perspective that pleasure is pleasure and pain is pain and they can exist in *different* parts of the body and in different amounts of duration and different intensities, but that these differences do not mean that what is being experienced at any moment of pleasure is not pleasure.
This interpretation would also mean that what is being discussed is not the "best" or the "highest" pleasure at all, but whether "pleasure" itself can be said to exist as a certain thing that can be experienced, or whether it is always (like a gas or liquid) something that can never be grasped and is always "slipping through our fingers."
Were it not for the understanding that every experience of pleasure is "complete in itself," then one would never be able to experience pleasure at all.
It seems to me that that might be an interpretation of PD03 that would explain how it fits in parallel along with PD01 and PD02 as an antidote (ok, a "remedy" if you like) for a major error that has to be refuted. The three are:
(1) The error that a "god" would concern himself with us is refuted by the position that a god is complete in itself,
(2) the error that the state of being dead is a concern for us is refuted by showing that where life is, death is not (life is complete without needing any aspect of death) and
(3) the error of thinking that pleasure can never be attained is refuted by establishing that pleasure too is complete in itself. All pleasure is complete in itself because given that there are only two feelings, whenever what we are feeling is not painful it is pleasurable - pleasure is complete wherever pleasure exists. The life that contains some pain does not fail to contain pleasure, it is just the life of a human being, in contrast to the life of a "god," which has attained the ability to never expeience pain.
PD03. The limit of quantity in pleasures is the removal of all that is painful. Wherever pleasure is present, as long as it is there, there is neither pain of body, nor of mind, nor of both at once.
Nothing here would then be intended to imply that "absence of pain" should be understood to describe is the most intense pleasure, the longest pleasure, or the pleasure covering the most numerous parts of the body. Nor would it imply at all that you need not concern yourself with continuing to stay alive, because you do in fact want to experience more pleasure by staying alive. The main point would be that since pleasure is complete whereever pleasure exists, the consideration of "pleasure" to be a proper goal of life does make sense, because it is in fact possible to experience pleasure. You're not doomed to be always drowning an inch below the surface never able to breathe - you do in fact figuratively "break through to the surface of the water and escape drowning" every time you experience any pleasure at all.
It would be very desirable to see if there are other surviving texts (such as the Seneca quote, or the statement in Hermotimus) that will make clear that this aspect of completeness was what was being argued by the non-Epicurean philosophers. Finding statements even more clear than Seneca's would help nail down this perspective and keep us from going around and around being frustrated that Plato's Philebus argument was not as clear as we would like it to be asking: SOCRATES: Have pleasure and pain a limit, or do they belong to the class which admits of more and less?
Understanding and being able to explain clearly to an ordinary person why asked that question, and why Epicurus' answer would differ from that of Plato, is key.