Lecture 2 of Pierre Rosenberg's Humanitas Visiting Professorship in the History of Art at the University of Cambridge, October 2013. Humanitas is a series of Visiting Professorships at Oxford and Cambridge designed to bring leading academics, practitioners and scholars to both universities to address major themes in the arts, social sciences and humanities. Created by Lord Weidenfeld, the programme is managed and funded by the Institute for Strategic Dialogue and co-ordinated in Cambridge by the Centre for Research in the Arts, Social Sciences and Humanities (CRASSH) and in Oxford by the Humanities Division.