Does moral realism deserve to be the default metaethical position? The issue is contested. While many realists have maintained that theirs is the view to beat, others contend that realists have offered no satisfactory argument for this position. In this essay and its companion, ‘Reidian Metaethics, Part II’, I maintain that Thomas Reid’s moral epistemology can help us make headway on the issue. Reid, I claim, offers an interesting line of argument, that when conjoined with some other assumptions, supports the claim that realism deserves to be the default metaethical position.