This keynote address investigates the aesthetic, cultural, and scientific discourses of synaesthesia that inspired the flourishing exchanges among the modern arts. It offers twenty theses on synaesthesia to trace the controversies surrounding the phenomenon: from the cooperation of the nineteenth-century arts and sciences in attempting to define synaesthesia to the present rift between them. The presentation first reconstructs the intellectual history of synaesthesia by exploring conflicting views on it as a lost primordial perception (Baudelaire), symptom of degeneration (Nordau), or future utopian mingling of the senses (Wagner). It next discusses the synaesthetic art and thought of Wassily Kandinsky, František Kupka, Aleksandr Scriabin, and Rainer Maria Rilke to offer an alternative genealogy of abstract art and visual music. The presentation ultimately argues that the modernist fascination with multisensory experiences stimulated and shaped experiments across the modern arts and advocates for a sensuous reading practice that may repair the divide between the humanities and sciences.