Here is an argument from David Sedley's "Epicurus On Dialectic" that is relevant to this dispute. It seems to me that this is related to the issue of whether the good has a limit, or whether it is among those things which have a "more or less" quality. Here, Epicurus is using the argument that just because honey can be "more or less" sweet, that doesn't mean that we should say honey is not sweet. The argument could be applied to pleasure -- just because a particular pleasure can be more or less pleasing, that does not mean that we should say that pleasure is not always pleasing.
Post from a reader at Facebook: Are you all familiar with this figure? I want to share it with those who haven't seen it. I also want to ask about the 3rd cup. What does it mean? Does it tip over sometimes or does extravagance create a pendulum effect? Photo source:
First, I want to apologize to Aapo for the delay in receiving and approving this post. It's now almost two weeks since he posted it but just a moment ago was the first time it popped up in its proper place on my page to review and approve it. A lot has changed with the way Facebook is tendering posts to administrators and I don't know what's going on. Last Sunday two of us Admins went looking for this (because Aapo asked about what happened) and we couldn't find it to approve it. Now here on Friday almost a week later it appears exactly as normal for approval. This is very frustrating. So the point here is to please understand that if you submit a post to the group and it doesn't come up shortly, please message one of the admins to ask about it and understand that there is a possibility that we may not have seen it.
Second, to answer the question generally, this is a graphic I prepared some years ago on the topic of the analogy of the leaky vessel used at the beginning of Lucretius Book Six. If I were redoing it today I would probably do it differently, but I do think that it does help serve as a point of discussion (as it is doing here) that the issue of "how much pleasure should be pursued" is one to think about carefully. The current version of my longer post discussing many of the cites to the texts that are relevant to this question is here: https://www.epicureanfriends.com/wcf/blog/index.php?entry/18-the-full-cup-fullness-of-pleasure-model/
And now to answer the question more precisely: The third cup example is intended to illustrate what happens when the vessel (an analogy for human life) is "overfilled" with pleasures. I believe the point of the "vessel" analogy in the first place is to serve as an illustration of the Epicurean doctrine that the "limit of quantity of pleasure" as described in PD03 can be thought of as "a human life completely full of pleasure." For purposes of the illustration it means roughly all the experiences from birth to death of any living thing. The optimum state for any living being would be to experience only pleasure for every moment of its life from birth to death. No one is saying that that goal is possible for a human being, and in Epicurean doctrine it's probably only possible for Epicurean gods in the intermundia. But for purposes of defining a goal - for purposes of responding to Plato or Aristotle as to what is the best life possible - we can define a the goal as a vessel which is absolutely full of pleasure.
Now, as stated in PD18 (and elaborated in 19 and 20):
PD18. The pleasure in the flesh is not increased when once the pain due to want is removed, but is only varied: and the limit as regards pleasure in the mind is begotten by the reasoned understanding of these very pleasures, and of the emotions akin to them, which used to cause the greatest fear to the mind.
PD19. Infinite time contains no greater pleasure than limited time, if one measures, by reason, the limits of pleasure.
PD20. The flesh perceives the limits of pleasure as unlimited, and unlimited time is required to supply it. But the mind, having attained a reasoned understanding of the ultimate good of the flesh and its limits, and having dissipated the fears concerning the time to come, supplies us with the complete life, and we have no further need of infinite time; but neither does the mind shun pleasure, nor, when circumstances begin to bring about the departure from life, does it approach its end as though it fell short, in any way, of the best life.
So the point of the third cup illustration would be to bring home to your mind an image that the cup of life can be filled only so far, and that any pleasure that one might think about adding to it after that point would be spilled out and would only at best amount to variation of what is already there.
The illustration has a number of uses, including the obvious one that we're discussing here. As explained further in the article linked here, the point of a "limit of quantity of pleasure" also has important uses in logical debate over whether it is correct to think of pleasure as the guide or end of life. Plato and others argue that pleasure cannot be the goal because the goal must be something that can be a superlative ("the best") and cannot be exceeded. Most people think that pleasure can be extended indefinitely, so if you take a rigorously logical perspective that would in some minds disqualify pleasure as meeting the definition of a "final goal." One point of a vessel analogy like this would be to respond to such an argument with the response: "You say pleasure must have a limit? Yes it does have a limit - the limit of pleasure in a human life is a life filled with pleasure for that lifetime (the vessel)."