This chapter examines the lively intellectual and artistic exchange between Pierre Boulez and John Cage that took place from 1949 to 1952. The writings of the French poet, dramatist, actor and visual artist Antonin Artaud (1886–1948) inspired the ‘organised delirium’ in Boulez’s Second Sonata for piano (1946–8). Its continuous variation and reading ‘a great deal of Artaud’, contributed to Cage’s decision to compose the Music of Changes (1951) using chance operations. Both composers were interested in ‘non-tempered pitch space’ – Cage, in his Sonatas and Interludes (1946–8) and Boulez in his Quatour pour Ondes Martenot (1945–6) and Le Visage nuptial (1946, 1948/1951–3). In the early 1950s, Cage and Boulez explored different approaches to a dialectical relationship between choice and chance, which eventually led to the publication of ‘Alea’, Boulez’s scathing condemnation of ‘accidental chance’ in 1957 and the end of their friendship.