A passage from David Velleman's Against the Right to Die struck me:
I don't pretend to understand fully the ethics of gifts and favors. It's one of those subjects that gets neglected in philosophical ethics, perhaps because it has more to do with the supererogatory than the obligatory.
Sure that's true, so long as you're not talking about human beings. Detailed norms of gift giving are a cultural universal, and if I can further overstep the bounds of my competence, I'll assert that they're never primarily supererogatory. I'm painfully aware of that fact, since I have a neurotic inability to navigate the social practices of gift exchange. Statements like this feed into my hunch that moral philosophers would benefit from more of an engagement with the study of culture, especially anthropology. To be clear, I don't know that Velleman is making any mistake here, since the opinion he's mentioning is one he suggests might characterize other moral philosophers.