The Price of Silence

This made me laugh for a long time. I remember studying John Cage's work in High School and my music teaching telling us that they sold 4 minutes 33 seconds cheaply as blank tapes. Isn't there prior art? Does Billy Joel's "River of Dreams" credit him too (does he have to pay everytime)?

"They say they are claiming copyright on a piece of mine called 'One Minute's Silence' on the Planets' album, which I credit Batt/Cage just for a laugh. But my silence is original silence, not a quotation from his silence.""

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