Scholarly discussion of the music of John Cage.

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Hi,
I have used a quote from 'Begin Again' in my dissertation, however, it's a
quote within a quote and therefore I'm looking to see if anyone knows where 
the
original quote came from? The book is very misguiding as to where the quotes 
he
has used came from.
The quote is: "And composing a score by superimposing a transparent template 
on
an astronomical atlas, Cage wrote, was 'an attempt to introduce into music the
‘ready-made’ of Marcel Duchamp'"
If anyone has any idea where this is from it would be much appreciated.
Thanks,
Claire Harris