  Late in his life Hume deemed the Enquiry concerning the Principles of Morals his best work, and in style it is a model of elegance and subtlety. His method in that work differs from that of the Treatise: instead of explicating the nature of virtue and vice and our knowledge of them in terms of underlying features of the human mind, he proposes to collect all the traits we know from common sense to be virtues and vices, observe what those in each group have in common, and from that observation discover the “foundation of ethics” (EPM 1.10). The conclusions largely coincide with those of the Treatise. Some topics in the Treatise are handled more fully in the moral Enquiry; for example Hume’s account of the motive to just action is enriched by his discussion of a challenge from a “sensible knave.” However, without the detailed background theories of the mind, the passions, motivation to action, and social convention presented in the Treatise, and without any substitute for them, some of the conclusions of the moral Enquiry stand unsupported.