There was a recent public flap, brought to my attention by a post on my favorite blog, over a speech by Phil Robertson, the patriarch of Duck Dynasty. Its claim was that an atheist had no basis for moral judgement, no ground on which to describe horrific acts (described in some detail in the talk) as bad.
It occurred to me that the same claim had played a central role in a somewhat earlier argument, the dispute between two schools of philosophy, Ash'ari and Mu'tazila, in medieval Islam. One of the major points of disagreement between them was the question of whether it was possible to know good and evil, to at least some degree, by human reason or only by revelation. The Mu'tazili position was that it was knowable by reason, the Ash'ari position that it was not.
I see a logical problem with both Robertson's position and the position of his Ash'arite predecessors. You encounter a powerful supernatural being. If you have no ability to distinguish good from evil on your own, how can you tell whether he is God, the Devil, or, like the Greek and Norse gods, a morally ambiguous being, no more consistently good than the rest of us?
I doubt that Robertson has published much on moral philosophy, but does anyone know if this is an issue that has been explored in the literature, ancient or modern, and if so whether anyone has come up with a good rebuttal to my argument, which seems to imply that both believers and nonbelievers need some source of moral knowledge outside of religion?