When PD09 refers to "intensity," location, and duration, are we talking about how pleasures differ from one another and how saying "absence of pain = 100% pleasure" does not tell us all we need to know about which pleasure to choose?
PD09. If every pleasure could be intensified so that it lasted, and influenced the whole organism or the most essential parts of our nature, pleasures would never differ from one another.
I think that the answer to this question must be "Yes!" If you want to describe an ethics of pleasure, you need to go into detail as to how to work with pleasure. You can't stop at "absence of pain = 100% pleasure", which is overarching and somewhat abstract.
Epicurus, as far as I can tell, developed at least three methods to approach a personal ethics of pleasure:
1) understand the difference between desires and pleasures, and work with the three categories of desires
2) examine the attributes of your various pleasures using intensity, location and duration
3) understand and work with katastematic and kinetic pleasures (which is difficult, given the dearth of texts on the matter)
These methods are not mutually exclusive, and in fact are mutually supportive. Or at least that's my current take.
For Epicurus, pleasure is simply that feeling which is not painful derived from actions or states which do not result in struggle, distress, anxiety, pain, etc. Additionally, Pleasure comes in two "flavors" - that which is felt in a state of rest; that which consists in motion and activity.
I would add to this as per my comments above. Also, I'm not sure that I agree with "simply" in the above quote, Don . Some pleasures do involve various pains, but the resultant pleasure outweighs the pain involved in obtaining the pleasure (I think we all agree on that). I think what you're describing is pleasure resulting from natural and necessary desires.
Now, I'm not sure what we are to glean from the translation referring to a "neutral state" ἀπαθείαις (apatheiais) because that's not one of the two feelings of pleasure: κινήσεσι and στάσεσιν. But that might have to wait for another thread.
Could it be that, early on, Epicurus had not yet settled on the idea of no neutral state? If so, could this give a rough idea of the approximate date the letter? (Just a thought for another thread.)