Hume's Moral Philosophy and the Is-Ought Problem

Hume's moral philosophy, developed most fully in Book III of the Treatise (1740) and later reworked in the more polished An Enquiry Concerning the Principles of Morals (1751, a work Hume himself, in his brief autobiography My Own Life, considered "of all my writings, historical, philosophical, or literary, incomparably the best"), rests on the thesis that moral distinctions — our judgments that an action or character trait is virtuous or vicious — are not, and cannot be, derived from reason alone. Hume's central argument, sometimes called the "motivation argument," holds that reason, on its own, is entirely inert and can never by itself produce or prevent any action or affective response, since reason's proper business is confined to discovering relations between ideas (as in mathematics) or matters of fact (as in causal reasoning about the world), neither of which is capable, by itself, of generating the motivating pull that moral approval or disapproval evidently carries; as Hume puts it in a famous, deliberately provocative formulation (Treatise, II.iii.3), "reason is, and ought only to be, the slave of the passions." Since moral evaluation clearly does move us to act and to feel approval or disapproval, and since reason alone can never do this, Hume concludes that the ultimate source of moral distinctions must lie not in reason's cool assessment of relations or facts but in sentiment — a special kind of feeling of approbation or disapprobation that arises in us, under the right conditions, when we contemplate a character trait or action.