Perloff’s description of Cage’s work helps clearly define modernism and post-modernism:

U B U W E B :: Nancy Perloff “Postmodernism and the Music of John Cage”
In the early 1970s, as we see in the comments quoted above, Cage endorsed collaborative musical performances in which the audience worked together with performers, fusing art with its environment, and in which the composer’s will did not constrain participants’ activities. This de-centered, collaborative, and heterogeneous principle for musical performance seems very postmodern. Yet the decisive presence of Cage’s ego (“I like”, “I try to bring about”), as well as the value he attached to historical musical practice, steered a modernist course. He designed and determined the performance situation, no matter how many participants were involved, and relied on his invention of chosen traditions from the past.