Chapter 23 sets Goethe’s Farbenlehre (Theory of Colours) in context. Colour had been the subject of intensive study, both aesthetic and scientific, in the eighteenth century, and the chapter reconstructs the many influences on Goethe and his contemporaries, from the recent discoveries of Herschel and Ritter, to earlier figures, above all Newton, but even Aristotle and Hippocrates. The chapter also presents the central tenets of Goethe’s Farbenlehre, with a particular focus on the theoretical first part, which offers a physiological theory of colours and deals with the physical nature of light.