Brilliant essay about John Cage and 4'33" here
Really like this bit:
During the 1940s, when Cage was writing percussion and prepared piano pieces, he became concerned with a new change. He noticed that although he had been taught that music was a matter of communication, when he wrote a sad piece people laughed, and when he wrote a funny one they started crying. From this he concluded that "music doesn't really communicate to people. Or if it does, it does it in very, very different ways from one person to the next." He said, " No one was understanding anybody else. It was clearly pointless to continue that way, so I determined to stop writing music until I found a better reason than 'self expression' for doing it."