We present evidence from a pre-registered experiment indicating that a philosophical argument - a type of rational appeal - can persuade people to make charitable donations. The rational appeal we used follows Singers "shallow pond" argument (1972), while incorporating an evolutionary debunking argument (Paxton, Ungar and Greene, 2012) against favoring nearby victims over distant ones. The effectiveness of this rational appeal did not differ significantly from that of a well-tested emotional appeal involving an image of a single child in need (Small, Loewenstein and Slovic, 2007). This is a surprising result, given evidence that emotions are the primary drivers of moral action, a view that has been very influential in the work of development organizations. We found no support for our hypothesis that combining our rational and emotional appeals would have a stronger effect than either appeal in isolation. However, our finding that both kinds of appeal can increase charitable donations is cause for optimism, especially concerning the potential efficacy of well-designed rational appeals. We consider the significance of these findings for moral psychology, ethics, and the work of organizations aiming to alleviate severe poverty.