That was the question I was asked: how would you prove to a blind man, that photography exists?
I knew what he was getting at. We had been discussing psychics. He was a firm believer in psychic powers, had had psychic experiences, and regularly visited a psychic. His point was, since I had not experienced psychic powers, I would never be able to believe in what he “knew” to be true. You could never prove to a blind man that photography exists, and likewise no one would ever be able to demonstrate to me that psychic powers were real.
It took me about ten seconds to think of a way to show he was wrong. This is what I said. Give the blind man a camera, a tripod and a remote shutter release. (Ideally the camera is a Polaroid, or a digital with an instant picture facility.) Everyone leaves the room but the blind man. He takes a picture of himself, and holds up a number of fingers (1 to 5) at random. The sighted person comes back into the room, looks at the picture and says “you were holding up X fingers”. If he gets the right number, and continues to do so every time this experiment is performed, the blind man will eventually conclude that photography is real. Technically, he will conclude the hypothesis that “a camera can record a visual image”, might be true.
He will want to repeat the experiment with different rooms and different sighted people. He will want to tighten his controls to make sure no one can see through the window or the keyhole. He will want other blind friends of his to do the same experiment successfully. But essentially, he will be convinced by this method.
The believer went quiet. (It must be annoying when your analogy is turned against you.) But I decided to push it further. I wanted to ask him some questions.
My first question was, if you did this 1,000 times, and the sighted person got the correct number of fingers (say) 225 times out of 1,000 (where pure chance would be 200 times), would the blind man believe that this “anomaly” was proof of photography? Wouldn’t he expect nearly 1,000 correct out of 1,000? What if when the controls were tightened, the result was reduced to close to 200 correct – pure chance? What if the sighted person was found to have cheated?
What if the blind man had to do a drawing and hold it up in front of the camera, instead of his fingers? The sighted person had to write down what he thought the drawing was of, and then a judge got to grade the description based on the photograph of the drawing? Say the blind man drew a circle and the sighted person thought it was a tree, and the judge rated that 7 out of 10 because a tree is roughly circular? Would the blind man be convinced?
What if the blind man had to select one drawing from four “targets” and hold it up in front of the camera, instead of his fingers? The sighted person is shown the four targets and asked to rate the degree to which each matches the one in the photograph. If the sighted person assigns the highest rating to the correct target, it is scored as a "hit." If the sighted person gets a hit, say 35% of the time (when chance would predict 25%), would the blind man be convinced? What if the person running the experiment was in the room when the photo was taken, and prompted the sighted person during the judging process - would the blind man be convinced then? What if numerous other experimental errors were noted?
What if a scientific body spent 25 years researching whether sighted people could guess how many fingers blind people were holding up in front of a camera, but concluded that there is ultimately very little, if any data that support the hypothesis that they can?
What if a conjuror offered one million dollars for any sighted person who could successfully perform the five finger test, but no one was able to do it?
Wouldn’t the blind man say to all this, “why can’t you just tell me how many fingers I’m holding up?”
The guy didn’t want to answer. He conceded his analogy was about me not having had a psychic experience. But apparently the analogy didn’t apply if I turned it around to his beliefs.
And they say skeptics are closed minded.
See my post Pretty Soon for a lighthearted look at the history of parapsychology.