Hume's Enquiry Concerning the Principles of Morals › Reason: The Slave of the Passions

Hume's most famous claim appears in the earlier *Treatise*, but the *Enquiry* develops its implications for ethics. > Reason is, and ought only to be the slave of the passions. What does this mean? Reason can tell you that a certain action will produce certain consequences. It can inform you that lying will damage your reputation, that exercise will improve your health, that studying will help you pass the exam. But reason alone cannot make you care about your reputation, your health, or your exam results. That caring comes from somewhere else: from the passions, desires, and sentiments that constitute your motivational structure. ::: {.callout-warning title="Common Misconception"} Hume is not saying reason is unimportant to ethics. Reason plays a crucial role in discovering facts, predicting consequences, and correcting errors. What reason cannot do is provide the ultimate motivation to act. Without sentiment, we would be perfectly informed but utterly inert. ::: Consider someone with antisocial personality disorder who understands perfectly well that his actions cause suffering. He can reason about consequences as well as anyone. What he lacks is the sentiment of sympathy that would make others' suffering matter to him. His reason is intact; his affective responses are impaired. This shows that moral motivation cannot be purely rational. The implications extend beyond individual psychology. Hume is making a point about the logical structure of moral reasoning. You cannot derive an "ought" from an "is." No amount of factual information about the world can, by itself, generate a conclusion about what you should do. The facts must be combined with some sentiment, some caring, some valuation, before they can guide action.