I have been searching online for years for this story that's been half-submerged in my memory, with no luck; I finally found it in an old journal entry dated June 8, 1985. It was cut out of a newspaper, but I have no idea which one, and I can't find this exact version of the story anywhere:
One day Einstein was asked by a pesky reporter to describe his theory of relativity in a few simple words. He responded with the following story:
"A man was asked by a blind man to describe the color white. The man said, 'White is the color of a swan.' The blind man said, 'What is a swan?' The man said, 'A swan is a bird with a crooked neck.'
"The blind man asked, 'What is crooked?' The man was becoming impatient. He grabbed the blind man's arm, straightened it and said, 'This is straight.' Then he bent it and said, 'And this is crooked.'
"Whereupon the blind man quickly said, 'Yes, yes, thank you. Now I know what white is.'"
So there's the first clue why I could never find this story online -- I had thought it was one of John Cage's zen stories, but it's actually a story told by Einstein. This in itself is a good illustration of the fallibility of memory. Here is another version I found, and this fits the pattern of all the references I found to this story:
Einstein and his blindfriend. This story shows how complex Einstein could be. Not long after his arrival in Princeton he was invited, by the wife of one of the professors of mathematics at Princeton, to be guest of honor at a tea.-Reluctantly, Einstein consented. After the tea had progressed for a time, the excited hostess, thrilled to have such an eminent guest of honor, fluttered out into the center of activity and with raised arms silenced the group. Bubbling out some words expressing her thrill and pleasure, she turned to Einstein and said: "I wonder, Dr. Einstein, if you would be so kind as to explain to my guests in a few words, just what is relativity theory?"
Without any hesitation Einstein rose to his feet and told a story. He said he was reminded of a walk he one day had with his blind friend. The day was hot and he turned to the blind friend and said, "I wish I had a glass of milk."
"Glass," replied the blind friend, "I know what that is. But what do you mean by milk?"
"Why, milk is a white fluid," explained Einstein.
"Now fluid, I know what that is," said the blind man. "but what is white ?"
"Oh, white is the color of a swan's feathers."
"Feathers, now I know what they are, but what is a swan?"
"A swan is a bird with a crooked neck."
"Neck, I know what that is, but what do you mean by crooked?"
At this point Einstein said he lost his patience. He seized his blind friend's arm and pulled it straight. "There, now your arm is straight," he said. Then he bent the blind friend's arm at the elbow. "Now it is crooked."
"Ah," said the blind friend. "Now I know what milk is."
And Einstein, at the tea, sat down.
Now the plot thickens. Here is a similar version of the milk story, but with Einstein now completely out of the picture, as told by the Hungarian-British writer George Mikes, in one of his books, which I found quoted in a post to the Pakistan Gardening Forum, of all places:
A blind man asks a young girl to describe milk. The young girl is astonished that he doesn't even know what milk is. "Milk is white," she tells him. The old man tells her that he is blind and doesn't know what white means. The young girl tells him that this is very easy to explain and tells him that a swan is white. The old man tells her that a swan may be white, but he has never seen a swan. "It has a curved neck," she tells him. The blind man says that he has no idea what 'curved' is. She lifts her arm, bends her wrists forward like a swan's neck. "Feel it," she says, "that's curved." The old man feels the girl's arm, touches the girl's wrist and exclaims joyfully: "Thank God. Now at last I know what milk is."
What of this strange connection between the "white" things called "milk" and "swans"? Turns out that goes back to Hinduism and Sanskrit, according to the swan page on Wikipedia:
Swans are revered in Hinduism, and are compared to saintly persons whose chief characteristic is to be in the world without getting attached to it, just as a swan's feather does not get wet although it is in water. The Sanskrit word for swan is hamsa or hansa, and it is the vehicle of many deities like the goddess Saraswati. It is mentioned several times in the Vedic literature, and persons who have attained great spiritual capabilities are sometimes called Paramahamsa ("Great Swan") on account of their spiritual grace and ability to travel between various spiritual worlds. In the Vedas, swans are said to reside in the summer on Lake Manasarovar and migrate to Indian lakes for the winter. They're believed to possess some powers such as the ability to eat pearls. They are also believed to be able to drink up the milk and leave the water from a saucer of milk adulterated with water. This is taken as a great quality, as shown by this Sanskrit verse:
Hamsah shwetah, bakah shwetah, kah bhedah hamsa bakayo?
Neeraksheera viveketu, Hamsah hamsah, bakah bakah!
(The swan is white, the duck is white, so how to differentiate between both of them? With the milk-water test, the swan is proven swan, the duck is proven duck!)
I guess the ancients required empirical evidence to distinguish a swan from a duck, a task that many modern humans can perform with relative ease.
Of all the versions of this story that might be floating around the universe, I like the original one I clipped from an unknown newspaper all those years ago, because to me the idea of describing the color white to a blind person is much more abstract and interesting than describing what milk is, since milk, after all, is a substance that can be discerned by other senses. But how can you possibly describe "white" without referencing other things? Such is relativity.