Joshua, Kalosyni, and anyone else who is willing to get into the details of this discussion in Philebus, I hope you will weigh in because I think it is hard to overstate the significance of this issue.
"Philebus" is reputed to be one of Plato's most mature and important dialogs. It is his "tour-de-force" against Pleasure as the good. And the tip of the spear of his argument by which he defeats the pro-pleasure side is this very argument -- by convincing the pro-pleasure side that pleasure has no limit (it can always be increased or decreased) he persuades Philebus (the pro-pleasure side) to abandon the argument that pleasure is the highest good.
In Athens in the age of Epicurus it therefore seems to me that Epicurus would have viewed the necessity of defeating this argument as almost as important as defeating the argument in favor of supernatural gods, and the argument in favor of reward or punishment after death. Epicurus was a teacher right in the heart of Athenian logic and philosophy, and this logical argument against pleasure had been enshrined as the gold standard by the most important teacher in Greek philosophy.
I also feel sure there are statements of this argument beyond this one in Philebus, and the examples cited above in Seneca. I just haven't had time to find more at this point.
We need more and better examples to illustrate what is in issue, because it is hard to follow given the shades of meaning of the word "limit."
One way is to go back to reneliza 's pink circle model:
In this diagram, the "limit of pleasure" is the edge of each circle. Each circle can contain only so much color, and no more, and the total quantity - the total magnitude - of "pink / pleasure" is the "area" of the circle contained within its edge.
By our definition of pink as containing ALL shades of pink (just like we define pleasure as containing ALL kinds of pleasure) we state that circles 2, 3, and 4 have all reached the LIMIT OF PINK (Pleasure). We may want to superficially quibble that "all shades of pink are not pink!" but we have DEFINED pink as including all shades, so circles 2, 3, and 4 have all reached their limit because they are completely filled with pink.
Circle 1, alone, has not reached the limit of pleasure, because it contains a lot of "white" space (our stand-in here for pain). Circle 1 cannot reach the "limit of pink" until all the white space is filled in with some shade of pink.
And this is one of the huge points: We don't make circle one reach the limit of pleasure simply by getting rid of all the white!!!! We can't replace the white with black or gold or green or any other random color, and we also can't simply make the white "disappear!"
We have to replace all the white (pain) with pink (pleasure) in order to reach the limit of pink/pleasure!