The Blindfold: How Science Learned Not to Look

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Welcome to Origins. In this program we explore the technological and scientific roots of the modern world. Let's begin.

June 1784. A garden in Passy, on the edge of Paris, where Benjamin Franklin was living as the American envoy. A boy of about twelve is led out of the house with his eyes bound, walked to an apricot tree, and told to put his arms around it.

He is about to become one of the first subjects of a method science still depends on: the blind experiment.

To see why anyone would blindfold a child in an orchard, start with Franz Anton Mesmer, a physician who had practiced in Vienna and arrived in Paris in 1778 with a theory. An invisible fluid, he said, ran through all living things, and illness came from its blockages. He called it animal magnetism, and a trained magnetizer could redirect it.

His treatment rooms were theatrical. Patients sat in rings around a covered tub, holding iron rods that stuck out of its lid, tied to one another with a cord. A piano played. The magnetizer pointed a finger or an iron wand at their faces and pressed on their bellies, sometimes for hours. And people responded. They coughed, sweated, laughed, wept, and fell into convulsions known as the crisis. When one person went into a crisis, others tended to follow.

That was the difficulty. The effects were real. Anyone could watch them. So how do you test whether a force exists, when the people under it are visibly shaking?

In March 1784, King Louis the Sixteenth appointed a commission to find out. It included Franklin, the chemist Antoine Lavoisier, the astronomer Jean-Sylvain Bailly, and a physician named Joseph-Ignace Guillotin. Mesmer refused to cooperate. His former partner, Charles Deslon, agreed to demonstrate.

The commissioners' move was simple to state and new in practice. If the fluid acts on the body, it should act whether or not the patient knows it is being applied. So take away the knowing.

They had a blindfold made for the purpose: two cups of elastic gum, padded with eiderdown, sewn into cloth and tied behind the head, leaving the nose free and letting in no light at all. A piece of laboratory equipment, built to switch off one sense.

Blindfolded subjects were told they were being magnetized when nobody was doing anything, and they felt heat, prickling, drowsiness. Others were magnetized in earnest and felt nothing.

The orchard was Deslon's own test, on a subject of his choosing. He magnetized one apricot tree while the boy waited inside. Then the boy was led out, blindfolded, and made to embrace four other trees, two minutes each. None of them had been touched.

At the first, he sweated and coughed. At the second, his head ached. At the third, he said he must be getting close. At the fourth, about twenty-four feet from the magnetized tree, he fainted, his limbs went stiff, and he was carried to the grass.

Another test used a seamstress who had fallen into a crisis within three minutes at Passy. This time she was seated in a doorway covered by a paper screen, and on the other side a commissioner magnetized her for half an hour by Deslon's rules. She chatted happily and said she felt perfectly well. Then the same commissioner walked into her room and repeated the gestures where she could see them. Within three minutes she was in full convulsion.

The report, signed in August 1784, did not call the patients frauds. It said the crises came from what it called the imagination. The cause was inside the people, not in any fluid.

Fifteen years later the idea crossed the Channel. Metal rods called tractors, patented by the American Elisha Perkins, were selling in England for five guineas, said to draw pain out of the body. In January 1799, at the General Hospital in Bath, the physicians John Haygarth and William Falconer had two fake tractors carved from wood and painted to look like metal. They drew them lightly over five patients with chronic rheumatism. Four said their pain eased. The next day the real tractors did no better.

In 1835, in Nuremberg, the method went one step further. A group of citizens set out to test a homeopathic remedy: ordinary salt, diluted again and again until almost nothing was left. They met in a tavern. A hundred vials were numbered and shuffled, half with the dilution and half with plain snow water, and the key went into a sealed list. Now the people handing out the vials did not know what was in them either. Fifty volunteers reported back. Eight felt something unusual. Five of those had the salt, and three had taken water.

The historian Michael Stolberg has pointed out a catch. Most of the volunteers were skeptics, and a skeptic can report nothing whatever is in the bottle. Hiding the contents does not fix who you ask.

The last piece came from a horse. In Berlin, a stallion called Clever Hans appeared to do arithmetic and read numbers, tapping answers with a hoof. A panel of thirteen men found no trickery in 1904. Then the psychologist Oskar Pfungst asked questions nobody in the room could answer. In one series, when the questioner knew the answer, Hans was right ninety-eight percent of the time. When nobody knew, eight percent.

Hans was reading people. A questioner leaned forward slightly to start him tapping, and gave a small upward jerk of the head when the right count arrived. Pfungst later measured movements like it in the laboratory, often about a millimeter. The questioners had no idea they were doing it.

That is the case for blinding the experimenter as well as the subject, which is what double-blind means. A drug trial today hands out identical-looking pills, and neither patient nor doctor knows who got which until the code is broken. And the horse is still in use. In 2019, researchers found that an image-recognition system identified horses partly by a small source tag in the corner of the photographs. They called it a Clever Hans predictor.

Humans are a social species, finely tuned to expectation, to faces, to the tilt of a head. That sensitivity is a large part of what makes us capable, and it bends what we feel and what we see. So the blindfold earned its place among the instruments of science, next to the thermometer and the scale. It works by keeping something from the people involved.

Thanks for listening to Origins. The next time a pill comes in a plain bottle with a number on it, there's an apricot tree in Franklin's garden somewhere behind it.