Review of: Paul van den Akker, Looking for Lines. Theories on the Essence of Art and the Problem of Mannerism, Amsterdam: Amsterdam University Press 2010. The problem of the categorical definition of the nature of art was often regarded as the central challenge of art history. Paul van den Akker examines the history of this problem from the eighteenth century to the 1960s. Thus his book addresses ‘the essence of art history’. In order to do so, he uses the extremely contradictory discussions of ‘Mannerism’ and the use of the line in mannerist art as a relevant case study, and this is supplemented with extensive excursuses on historic concepts of ‘classical art’, of Renaissance painting, of medieval art, or connoisseurship. Johann Joachim Winckelmann, Heinrich Wölfflin, Alois Riegl and John Shearman play key roles in van den Akker’s book. The review mainly asks for the specificity of the analytical categories and the changing role of the visual material used by the antiquarians and early art historians from the later seventeenth to the early twentieth centuries to illustrate and accompany their texts and arguments.