I think you are getting what I am driving at. But I should clarify. Definition is best reached through as broad a comparative process as possible. It would be just as well to define duty to self and duty to others as both within ethics. The only caution here is that, in reading or hearing any argument, one ought to be careful not to impose ones own definitions on the words used in that argument.
Crucial definitions ought to be openly agreed on.
I think these places of divergence might be found most easily in the use of certain terms which are critical to the discussion at hand. This thread is a debate over whether an objective basis for morality exists. I.e., an objective thing is the same whether we perceive it or not. Human needs exist objectively, and are the same for all people. These needs are simply objective things which exist in all people, whether people perceive them in themselves and others, or not.