Moral psychologists have recently turned their attention to a phenomenon they call 'moral dumbfounding'. Moral dumbfounding occurs when someone confidently pronounces a moral judgment, then finds that he or she has little or nothing to say in defense of it. This paper addresses recent attempts by Jonathan Haidt and Marc Hauser to make sense of moral dumbfounding in terms of their respective theories of moral judgment; Haidt in terms of a 'social intuitionist' model of moral judgment, and Hauser in terms of his 'Rawlsian creature' model of moral judgment. I show that Haidt and Hauser assume that moral dumbfounding is to be explained in terms of features of a.) moral judgment that are b.) construed in terms of the intrinsic features of individuals. By contrast, I hypothesize that moral dumbfounding is to be explained in terms of moral reasoning, more specifically in terms of the social dynamics of such reasoning. Finally, I argue that this hypothesis points towards an externalistic account of both moral reasoning and moral judgment.