Although we have long assumed that morality is a uniquely human phenomenon, recent studies have shown that nonhumans and humans share a great deal that is relevant to morality, including basic moral emotions and corresponding behaviors. Do these similarities imply that nonhuman moral responses and human morality represent two points on a continuum, or do the remaining differences represent a qualitative difference? On closer examination, this debate, even though it is occasioned by contemporary scientific research, continues a much older debate over place of reason in morality. This paper argues that Aquinas' analysis of the passions and their complex relations to perception, activity, and (among humans) judgement offers an illuminating framework for making sense of what we know about the moral emotions in both ourselves and other kinds of animals. On this account, the moral passions play a central and necessary role, both in shaping our overall appraisals of a situation, and moving us to act. Yet human passions cannot function at all, let alone function well, apart from processes of rational formation and integration, which do make a qualitative difference to human morality
