Eh, I don't know but the 'open question' argument has always made sense to me: 'For any non-moral description of an action or object it seemed that competent speakers could without confusion doubt that the action or object was appropriately characterized using a moral term such as ‘good’ or ‘right’. The question of whether the action or object so described was good or right was always open, even to competent speakers.'

http://plato.stanford.edu/entries/moral-cognitivism/