But why is tranquility the pleasure that confers a blessed life? Epicurus, like many Greek philosophers, thinks that a good life is a perfect life (pantelēs; PD 20, 21; Pyth. 116). It's not clear to me exactly why he holds this assumption (very curious for your thoughts; I think this is one of the big puzzles of his ethics, for which we don't have extant sources). "Perfect" in this sense means "not able to be made better, in the respect in which it's good." But pleasure fits a bit awkwardly with this assumption.
Someone feeling perpetual joy or delight is not in any way inferior or superior to feeling constant Serenity or Tranquility. Both are equally Pleaseant. Kinetic and Katastematic pleasures are variations of pleasure. Pretty sure this is Austin's view and Gosling and Taylor's as well. They are variations of the same condition, Epicurus recognized this variation (Dissgreeing with Kolosky) but didn't consider it a huge deal.
This is why Tranquility is not the absolute goal, but any continuous Pleaseant state, whethe it be either constant Tranquility/Serenity or Joy/Delight.
I think this is because of the dominance of Plato, he wants to respond to the Platonic objection that the good life can't be one filled with pleasure because pleasure doesn't have a perfectly attainable limit for humans to reach. So the good must be perfect was the accepted belief.
Epicurus responds that Pleasure can be perfect (in removing all pain, often thought the neutral state) and afterwards admits only variation. Just as the Stoic sage may vary in different qualities (rich/poor, young/old, Greek/Non-Greek), but not be more wise, pleasure varies in state to state (Joy/Tranquility), but not be more perfect
Regarding pantelēs: I'm quite ignorant of the nuances of Greek (not just the nuances, most of it) but I see from Nate Bartman's compilation of PD translations that translations of PD21 are fairly evenly divided by translating it as perfect and as complete. In my mind there's a significant difference between these two words, at least in English. "Perfect" seems to have platonic connotations of an ideal state, a state that doesn't empirically exist. "Complete," on the other hand, implies something that one can judge for oneself. I'm curious what the thoughts of those with knowledge of the Greek make of this....
I agree with the middle quote from wbernys . This is in fact a perfect (pun intended) example of why it's important to acknowledge the parsing of pleasure into intensity, location and duration, at least in my way of thinking. The only definitive difference between kinetic and katastematic pleasures is duration: katastematic duration is extended, kinetic duration is short. Both kinetic and katastematic can vary in intensity and location. Having said that, I consider katastematic pleasures to be mental, but there are also kinetic mental pleasures. And both kinetic and katastematic pleasures can cover the full range of intensities. This aids in personal choice of pleasures based on desires in particular circumstances, without concerns over an idealized ranking.
Quite an enjoyable and thought provoking thread!