David Hume’s Treatise of Human Nature is divided into three books: ‘‘Of the Understanding’’, ‘‘Of the Passions’’, and ‘‘Of Morals’’. One might wonder how these disparate topics are related, other than by virtue of the fact that they have something to do with the mind. But the links become clear on reading the text. Hume develops a theory of concepts (or ‘‘ideas’’) in the first book and a theory of emotions in the second book, and then he integrates these in the third by arguing that our moral concepts have an emotional foundation. The project is also unified by Hume’s allegiance to empiricism. His theory of concepts is based on the premise that ideas are stored copies of sensory impressions, and his theory of emotions is designed to be compatible with this empiricist view (he defines emotions as impressions of impressions).