The above document has recently been making the rounds on tumblr, Facebook, Pinterist, and other social media sites. My students have brought it up in class, and artist Paul Walde, who teaches at the University of Victoria, wrote that his students have posted a copy in the school's grad lounge.
The authorship is always credited to John Cage, which doesn't ring true to me. Not the approach, the writing style, or the typesetting. And it doesn't appear in any of the numerous books I have by or about Cage.
I did a fast search and discovered the below copies, which would certainly lend credence to Cage's authorship, as they are on Merce Cunningham's letterhead. The dancer and choreographer met Cage in 1938 and they spent most of their lives as artistic and romantic partners.
So I sent the question to my favourite online art detective, Greg Allen at greg.org. Allen has brilliantly tracked down answers to other obscure questions surrounding contemporary art, and had previously written about Cage on several occasions, including a post on Cage's studio table, here.
Allen replied with an email less than an hour later, identifying the author as Sister Corita Kent, though with a few remaining unanswered questions. The following day he published this post, here, with more details.
If readers have any further information about the origins of this text, please leave it the comments, or send an email.