Metaethical rationalism can be roughly characterized as the idea that the requirements of ethics are requirements of practical reason. The idea is attractive, in part because it can explain the plausibility of certain versions of motivational internalism about moral judgments. Since rationalism entails that right action is a species of rational action it appears rational people must be motivated to do what is right, something many internalists believe. But rationalism’s attractions are often not well enough appreciated because the very feature that makes it attractive also generates a prima facie objection. Rationalism seems to require that those who refuse to acknowledge correct moral demands therefore be irrational. Yet such people don’t always seem irrational to us. People sufficiently removed from ourselves in time, place, and culture often have a divergent conception of what morality requires. If we are right about what morality requires, then they are wrong. Yet it seems unfair to accuse them of irrationality as opposed to some other sort of mistake; nothing in their...