Full Idea
Supererogatory actions are admirable and valuable, and we praise people for doing them, but they do not generate obligations to perform them, which casts doubt on obligation as the basic notion in ethics.
Gist of Idea
If excessively good actions are admirable but not required, then duty isn't basic
Source
Julia Annas (The Morality of Happiness [1993], 2.6)
Book Reference
Annas,Julia: 'The Morality of Happiness' [OUP 1995], p.115