The true nature of moral motives must be concealed from the agent’s point of view if those motives are to be efficacious. Suppose that people came to believe Mandeville’s theory or the evolutionary theory, and as a result they gave up their moral practices. Then those accounts would lack transparency.
That quote is from an essay by the philosopher Christine Korsgaard, and I have to ask: does anybody really believe that? Has anybody actually given up their moral practices – whatever that means – when they came to understand “the true nature of moral motives”? Wouldn’t we immediately and correctly judge somebody claiming to have done so as not really having those moral motives in the first place?