Abstract

In this paper, on the basis of psychological research concerning psychopathy, I argue against one claim a moral rationalist—such as Michael Smith —might make. First, I distinguish three rationalist claims the moral rationalist might make: the rationalists' conceptual claim, the rationalists' substantial claim, and the practicality requirement. Then, I go on to discuss some of the subtle relations between these claims. I argue that, if we have reason to reject the rationalists' substantial claim, this gives us prima facie reason to reject the rationalists' conceptual claim as well, or else to accept the view that there are no substantial truths of morality. Next, I present evidence from psychological research on psychopaths, and argue that these considerations undermine the rationalists' substantial claim. Lastly, I consider a few replies on behalf of Smith, and conclude that they are not successful in defeating my arguments against the rationalists' substantial claim.