Yang Xiong (53 BC-AD 18), the Han philosophical master remarks at one point in his Exemplary Figures, "Books are as sexy as women." Modern readers may frown at a comparison they regard as less than apt. Yang was supremely aware, however, of longstanding traditions, ascribed both to the sages and to the Classics, contrasting the unusual strength of the basic drives for food and sex with the general weakness of the acquired inclinations toward moral...