“If slaughterhouses had glass walls, everyone would be a vegetarian” is a saying that has been printed on many images. It is usually credited to English singer-songwriter (and vegetarian since 1975) Paul McCartney, who narrated a 2009 “Glass Walls” for the animal-rights organisation People for the Ethical Treatment of Animals. McCartney has been associated with the quote since 1994, but it had been used in print ten years before, and it’s unlikely that McCartney’s words wouldn’t have reached print during that time.
 
“Though it is said that if slaughterhouses had glass walls, the whole world would be vegetarian” was cited in “The Rights of Animals” by William Ecenbarger, published in the Philadelphia (PA) Inquirer in May 1984. The article mentions Tom Regan, an American philosopher and animal rights author. “How would we fare psychologically if the walls of slaughterhouses were made of glass?” Regan asked in The Struggle for Animal Rights (1987). It is likely that Tom Regan—not Paul McCartney—is the original source of the quotation.
 
 
Wikipedia: Tom Regan
Tom Regan (born November 28, 1938) is an American philosopher who specializes in animal rights theory. He is professor emeritus of philosophy at North Carolina State University, where he taught from 1967 until his retirement in 2001.
 
Regan is the author of numerous books on the philosophy of animal rights, including The Case for Animal Rights (1983), one of a handful of studies that have significantly influenced the modern animal rights movement. In these, he argues that non-human animals are what he calls the “subjects-of-a-life”, just as humans are, and that, if we want to ascribe value to all human beings regardless of their ability to be rational agents, then to be consistent, we must similarly ascribe it to non-humans.
 
TomRegan.com
But for the Sake of Some Little Mouthful of Flesh…
By Tom Regan
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How would we fare psychologically if the walls of slaughterhouses were made of glass? What would we feel and do if we SAW the death of so-called “food animals”? Might not the psychological shield break if people peered through these glass walls and saw the meat on their plate for what it really is, not for what they pretend it to be?
   
Wkipedia: Paul McCartney
Sir James Paul McCartney, MBE (born 18 June 1942) is an English singer-songwriter, multi-instrumentalist, and composer. With John Lennon, George Harrison, and Ringo Starr, he gained worldwide fame with the rock band the Beatles, one of the most popular and influential groups in the history of pop music. His songwriting partnership with Lennon is one of the most celebrated of the 20th century.
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McCartney is a supporter of the animal-rights organisation People for the Ethical Treatment of Animals. He has appeared in the group’s campaigns and, in 2009, he narrated a short factory farm exposé titled “Glass Walls”.
 
But slaughterhouses do not have glass walls. And few people ever venture inside. And why should they? Whatever the details, everybody understands without looking that they can’t be pretty. So why go in? Who wants or needs to see all the blood and gore?
     
13 May 1984, Philadelphia (PA) Inquirer, “The Rights of Animals” by William Ecenbarger, pg. 20:
But the most important event in the rise of the animal rights movement to respectability was the publication and widespread intellectual acceptance last year of The Case for Animal Rights by Tom Regan, a respected philosophy professor at North Carolina State University. With the publication of the book, animal rights became an idea whose thinker had come.
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THOUGH IT IS SAID THAT IF slaughterhouses had glass walls, the whole world would be vegetarian, most minds would rather not linger on the messy, scarlet process by which man processes animals for his own nourishment. Meat for most is a wrapped, antiseptic fantasy in which the accompanying blood is called juice or drippings. And though it is the penultimate act in the metamorphosis of animal to meat, and therefore anathema to animal rightists, slaughtering has become considerably more humane than it was even 20 years ago.
 
Google Books
The Struggle for Animal Rights
By Tom Regan
Clarks Summit, PA: International Society for Animal Rights
1987
Pg. 67:
How would we fare psychologically if the walls of slaughterhouses were made of glass? What would we feel and do if we SAW the death of so-called “food animals”? Might not the psychological shield break if people peered through these glass walls and saw the meat on their plate for what it really is, not for what they pretend it to be?
     
13 August 1991, Des Moines (IA) Register, “PETA hints at fair action and new ad” by Mary Ann Lickteig, pg. 2A, cols. 3-4:
“Meat is murder. Struggling cows, lambs, pigs, and chickens do not voluntarily go to the knife. If slaughterhouses had glass walls, who but a Jeffrey Dahmer could bear to watch their meals being prepared.”
(PETA advertisement.—ed.)
 
Google Groups: alt.fan.jai-maharaj
Meatless - Keeping healthy
Jai Maharaj
12/13/94
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“If slaughterhouses had glass walls, everyone would be vegetarian. We feel better about ourselves and better about the animals, knowing we’re not contributing to their pain.”
Paul and Linda McCartney (musicians)
     
Google Groups: rec.arts.bodyart
HI
Keith Alexander
8/14/97
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“If slaughterhouses had glass walls, everyone would be vegetarian”
—Paul McCartney
 
3 October 1997, The Age (Melbourne, Australia), “Ethicist cautions human carnivores on animal rites” by Tim Winkler, pg. A4, cols. 3-4:
Environmentalists should avoid eating meat if they want to preserve nature, Professor Tom Regan, of North Carolina University, told Melbourne University’s Environmental Justice Conference.
Cols. 6-7:
“If slaughterhouses were made with glass walls, I think everyone would be a vegetarian. I think if everyone was confronted with the terror, the blood, the squeals, they would have a different perspective.”
 
Google Groups: talk.politics.animals
A day of peace
christinekochmann
4/7/98
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Oh that’s rich…a redneck, beer bellied, all american good ol boy with not much edumacation, slamming Paul McCartney….one of the richest, most successful entertainers in the world…Ohhh, when I tell him what Randy the hunting fool said…he’s going to be so depressed. :-(  Didn’t you get what the quote said Randy? Did a bullet take out some brain cells?  He’s not telling you what to do…he said, “IF slaughterhouses had glass walls…” What part didn’t you understand?  He said IF they had glass walls, you would become a vegetarian.
 
Google Books
The Paul McCartney Encyclopedia
By Bill Harry
London: Virgin
2002
Pg. 880:
Paul said, ‘If slaughterhouses had glass walls, everyone would be a vegetarian. We feel better ourselves about the animals, knowing we’re not contributing to their pain.’ Paul and Linda became patrons of the Vegetarian Society in 1995.
 
YouTube
Paul McCartney - If Slaughterhouses had Glass Walls
SgtPepperChannel
Published on Nov 4, 2013
“If slaughterhouses had glass walls, everyone would be vegetarian.”