“I don't know if there are men on the moon, but if there are they must be using the earth as their lunatic asylum”
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George Bernard Shaw 413
Irish playwright 1856–1950Related quotes

Source: god is Not Great: How Religion Poisons Everything

“Earth is the insane asylum of the universe.”

“When I used to speak of the lunatic fringe, I didn’t know I was going to be head of it.”
The Memoirs of an Amnesiac (1965)

This quotation is often found on the internet attributed to Magellan, but never with a source, and no English occurrence prior to its use by Robert Green Ingersoll in his essay "Individuality" http://www.infidels.org/library/historical/robert_ingersoll/individuality.html (1873) has been located. Thus, it it most likely spurious. In that essay Ingersoll states:
It is a blessed thing that in every age some one has had individuality enough and courage enough to stand by his own convictions, — some one who had the grandeur to say his say. I believe it was Magellan who said, "The church says the earth is flat; but I have seen its shadow on the moon, and I have more confidence even in a shadow than in the church." On the prow of his ship were disobedience, defiance, scorn, and success.
Disputed
Variant: The Church says that the Earth is Flat, but I know that it is Round. For I have seen its Shadow on the Moon and I have more Faith in a Shadow than in the Church.
Source: As quoted in Oxford Academic (25 July 2013) http://oupacademic.tumblr.com/post/56463634957/misquotation-i-have-seen-the-shadow-of-the-earth

Fiction, The Other Gods (1921)
Context: The moon is dark, and the gods dance in the night; there is terror in the sky, for upon the moon hath sunk an eclipse foretold in no books of men or of earth's gods...' There is unknown magic on Hatheg-Kla, for the screams of the frightened gods have turned to laughter, and the slopes of ice shoot up endlessly into the black heavens whither I am plunging... Hei! Hei! At last! In the dim light I behold the gods of earth!
Why this professor's climate-crisis solution is rankling Twitter: 'The worst thing you can do is have a child' https://www.yahoo.com/lifestyle/why-professor-climate-crisis-solution-rankling-twitter-155305526.html (13 February 2020) Yahoo!Life