In the first part of this paper, I argue that the sentiment of compassion is a factor of the first importance in moral theory. This sentiment, which causes us to act well toward persons in need, is an essential element in the psychology of the morally well-developed person. Moral rationalists such as Epictetus and Kant, who contend that the source of moral value is reason rather than compassion, produce a distorted picture of our moral lives. Hume’s moral psychology gives compassion the place it deserves as a motivating factor in moral action.In the second part of the paper, I study the impact of moral rationalism, as manifested in the work of Peter Singer, on the question of the treatment of mentally handicapped humans. Singer’s advocacy o...