Reason Is And Ought Only To

From Hume's A Treatise of Human Nature (1739), Book II, Part III, Section III, "Of the Influencing Motives of the Will." Hume opens the section by dismantling the longstanding philosophical tradition — from Plato to Descartes — that elevated reason above passion in governing human action. He argues that reason alone is incapable of motivating the will: it can only inform us about facts and means, while the actual impulse to act must come from desire or emotion. Far from a lament, Hume presents this as a basic truth of human psychology, and it became the foundation of his empiricist moral philosophy — a direct challenge to the rationalism of his contemporaries.