Patients, mostly women, are sitting around a large wooden tub filled with magnetic water, powdered glass, and iron filings. From its lid emerge a number of bent iron rods against which the patients expectantly press their afflicted areas. A rope attached to the tub is loosely coiled about them, and they are holding hands to create a “circuit”. Through the low-lit room — adorned with mirrors to reflect invisible forces — there wafts incense and strange music, the other-worldly sounds of the glass harmonica (invented by a certain Benjamin Franklin). Meanwhile, a charming man in an elaborate lilac silk coat is circulating, touching various parts of the patients’ bodies where the magnetic fluid may be hindered or somehow stuck. It appears that these blockages, in the ladies in particular, are generally in the lower abdomen, thighs, and sometimes “the ovaria”. The typical session would last for hours and culminate in a curative “crisis” of nervous hiccups, hysterical sobs, cries, coughs, spitting, fainting, and convulsing, thus restoring the normal harmonious flow of the fluid.

The man in the lilac coat is Franz Friedrich Anton Mesmer and this scene could be describing any number of animal magnetism sessions he held in late eighteenth-century Paris. While Mesmer’s antics are perhaps familiar to many today, lesser known is the key role they played in the development of the modern clinical trial — particularly in connection with the 1784 Franklin commission, “charged by the King of France, with the examination of the animal magnetism, as now practised at Paris”.

Animal magnetism was all the rage in eighteenth-century Europe, and its star Franz Friedrich Anton Mesmer, a captivating German physician, had celebrity status. His theory claimed that a special kind of imperceptible magnetic fluid pervaded the universe and that most if not all diseases were caused by an abnormal flow of this fluid inside the body. At first, Mesmer’s therapeutic technique consisted of waiving magnets around his patients after making them swallow iron filings. He later would forego the swallowing metal stage and also the magnets: he discovered that simply using his hands or other objects was just as effective. After such so-called “passes” the patient would go into a kind of trance state, then faint, convulse, shake, and so on (what would be aptly named a “crisis”). Ideally, after all this evidently flow-inducing activity, they would be healed.

Mesmer, a charismatic and cultivated man, was married to a rich older widow, and his wife’s moral and material support did not hurt when it came to building a successful medical practice in Vienna. Here he gained notoriety treating mostly upper-class women in his elaborate and rather erotically charged therapeutic sessions. One of his patients was the young pianist Maria Theresa von Paradis, known as “The Blind Enchantress”. Though Mesmer at first seemed to cure her apparent blindness, her recovery was temporary, and the young lady was so emotionally affected by the treatment that for a while she lost her ability to play her instrument. To make matters worse, her protectress (and likely godmother) was also not exactly pleased about the fact that the teenager moved in with Mesmer. Unfortunately for the doctor, she happened to be the Austro-Hungarian empress Maria Theresa.

By 1778, having fallen into considerable royal disfavour, Mesmer moved to Paris in a shroud of fame and controversy. Despite the opposition of the medical profession, who denied him a medical license, he partnered up with the respectable and licensed Charles Deslon, personal physician to the brother of King Louis XVI. Mesmer again ended up establishing a stupendously popular clinical practice, winning the favour of many highly influential people. In fact, it only took a few years for animal magnetism to grow into something close to an obsession among the French.

Soon Mesmer and a few disciples started offering magnetic group séances. By the mid-1780s mesmerism had become such a craze that concerned Parisian physicians persuaded the king to establish a royal commission to investigate its claims. The degree to which said craze was lucrative and the rate at which regular medical clinics were losing traffic may, of course, have played a role here. Admittedly, we can sympathise with the patients who deemed that magnetic séances compared favourably with the more mainstream practices of bloodletting and leeching. Either way, it is plausible that the total set of motivations very much included concern for scientific truth.

By a lucky coincidence, Benjamin Franklin was in France as the first US ambassador with a mission to ensure an official alliance against its arch nemesis, the British. On account of his fame as a great man of science in general and his experiments on one such invisible force — electricity — in particular, Franklin was appointed as head of the royal commission. The investigating team also included the chemist Antoine-Laurent Lavoisier, the astronomer Jean-Sylvain Bailly, and the doctor Joseph-Ignace Guillotin. It is a curious fact of history that both Lavoisier and Bailly were later executed by the guillotine — the device attributed to their fellow commissioner. The revolution also, of course, brought the same fate to King Louis XVI and his Mesmer-supporting wife Marie Antoinette.

In a stroke of insight, the commissioners figured that the cures might be affected by one of two possible mechanisms: psychological suggestion (what they refer to as “imagination”) or some actual physical magnetic action. Mesmer and his followers claimed it was the magnetic fluid, so that served as the experimental condition if you like. Continuing with the modern analogies, suggestion would then represent a rudimentary placebo control condition. So to test animal magnetism, they came up with two kinds of trials to try and separate the two possibilities: either the research subject is being magnetised but does not know it (magnetism without imagination) or the subject is not being magnetised but thinks that they are (imagination without magnetism). The fact that the trials were blind, or in other words, the patients did not know when the magnetic operation was being performed, marks the commission’s most innovative contribution to science.

The report also shows an acute awareness that illness is very likely to go away by itself over time. In fact, in the era of leeching, bloodletting, induced vomiting, laxatives, and so on, it was common for patients to improve without treatment. So for the purposes of rigour, the commissioners rejected the requests from mesmerists to make the study longitudinal and see if their patients get better over an extended period of time. This led Mesmer to refuse to cooperate with the commission. They instead turned to his prime disciple and staunch supporter Charles Deslon (whom an irked Mesmer would subsequently denounce).

A few of the most striking experiments should give you a flavour of the methodology used. The inspiration for the most famous trial likely came from the following incident involving Mesmer, here described in the commission’s report.

One evening M. Mesmer walked with six persons in the gardens of the prince de Soubise. He performed the magnetical operation upon a tree, and a little time after three ladies of the company fainted away. The duchess of C—, the only remaining lady, supported herself upon the tree, without being able to quit it. The count de Mons—, unable to stand, was obliged to throw himself upon a bench. The effects upon M. Ang—, a gentleman of a very muscular frame, were more terrible. M. Mesmer’s servant, who was summoned to remove the bodies, and who was inured to these scenes, found himself unable to move. The whole company were obliged to remain in this situation for a considerable time.

Due to Franklin’s old age, it was a particular apricot tree in his orchard at Passy that was selected for the purpose. Deslon magnetised the tree while a twelve-year-old boy of his choosing stayed in the house. The boy was then made to hug several trees for two minutes each, none of which were magnetised; he collapsed in a full-fledged crisis at the fourth.

Similarly, the commissioners persuaded one of Deslon’s patients, dame P—, who was blindfolded, that Deslon was performing animal magnetism on her, whereas in fact, he was not even around. In a different room, mademoiselle B— was told that Deslon was behind a door. Both women fell into a crisis within the space of three minutes. On the reaction of mademoiselle B— the report said the following:

Her respiration was quick, she stretched out both her arms behind her back, twisting them extremely, and bending her body forward; her whole body trembled; the chattering of her teeth became so loud that it might be heard in the open air; she bit her hand, and that with so much force, that the marks of the teeth remained perfectly visible.

On another occasion dame P— was given several cups of water to drink which she believed to be magnetised. She had a crisis at the fourth cup and, ingeniously, was given some actually magnetised water to help her recover her senses. This she drank gratefully and felt much better afterwards. Finally, mademoiselle B— was magnetised for a full half hour from behind a paper partition while happily chatting away with one of the commissioners. However, when the magnetiser emerged from behind the partition and repeated the same moves in full view, mademoiselle collapsed in a convulsive crisis within minutes.

Perhaps unsurprisingly, the commissioners concluded that imagination alone could produce the same striking effects as mesmerism. Furthermore, animal magnetism by itself was completely toothless and impotent. It is admirable that Deslon himself seems to have acknowledged the power of the commissioners’ approach and accepted the main findings to do with the existence of this “magnetic fluid”. However, even if it proved to be nothing more than imagination, he insisted on its clinical utility. We will never know in what ways Deslon would have developed the practice, as he died just two years later, apparently during a magnetic séance.

In 1784 the reports were drawn: one public and one secret, only for the eyes of the king on account of its indecent elements. Here we encounter another possible reason for the infamous convulsions, which also made the treatment “dangerous to morality”. It is noted that the magnetiser is, as a rule, a man, and his patients are mostly women. Moreover, the setup is such that “the two faces almost touch, the breath is intermingled, all physical impressions are felt in common, and the reciprocal attraction of the sexes must consequently be excited in all its force.” He often positions himself such that the knees are touching, his hands are wandering closer to the lower parts of the body and “these the most sensitive”.

The discerning men of science observed that “[w]hen this kind of crisis is approaching, the countenance becomes gradually inflamed, the eye brightens, and this is the sign of natural desire”, which of course “may be wholly unperceived by the woman who experiences it, but it cannot escape the observant eye of the physician”. This situation is all the more worrying since such treatment could become addictive: “she continues in it, she returns to it, and discovers her peril when it is too late”. It is pointed out that strong women would inevitably flee from such corrupting influences, but “the morals and health of the weak may be impaired”. On these grounds, the report recommended that the king curb the practice of animal magnetism as soon as possible. Presumably, this was in case exposing the false premises of mesmerism alone was not sufficient to stop it being practised.

Whatever the moral case may be, the report paved the way for the modern empirical approach in more ways than one. Stephen Jay Gould called the work “a masterpiece of the genre, an enduring testimony to the power and beauty of reason” that “should be rescued from its current obscurity, translated into all languages”. Just to mention a few further insights, the commissioners were patently aware of psychological phenomena like the experimenter effect, concerned as they were that some patients might report certain sensations because they thought that is what the eminent men of science wanted to hear. That seems to be what propelled them to make the study placebo-controlled and single-blind. Other phenomena reminiscent of the modern-day notion of priming, and the role of expectations more generally, are pointed out throughout the document. The report also contains a detailed account of how self-directed attention can generate what are known today as psychosomatic symptoms. Relatedly, there is an incredibly lucid discussion of mass psychogenic illness, and mass hysteria more generally, including in cases of war and political upheaval. Just five years later, France would descend into the chaos of a violent revolution.

Although the details of animal magnetism may sound absurd (and even morally dubious) to modern ears, within its context Mesmerism was very much in line with the latest scientific developments. In some ways, exciting experiments with invisible forces — gravity, electricity, magnetism, wondrous gases like hydrogen — defined the era. Unlike the occultists of the previous ages, Mesmer was striving to give his practices a rational scientific as opposed to a religious flavour. Indeed, although the magnetic fluid part did not work out, in an important sense, animal magnetism marked the beginnings of hypnosis and psychological suggestion. These are very real and possibly still clinically useful phenomena, as a recent resurgence in research shows. ※

However we judge Mesmer, the ideas and practice of animal magnetism served to precipitate some important developments in how we think about scientific studies today. This episode left a legacy both of humility regarding our collective ability to reason soundly and of new methods for improving that ability. In the words of our learned commissioners:

Perhaps the history of the errors of mankind, all things considered, is more valuable and interesting than that of their discoveries. Truth is uniform and narrow; it constantly exists, and does not seem to require so much an active energy, as a passive aptitude of soul in order to encounter it. But error is endlessly diversified; it has no reality, but is the pure and simple creation of the mind that invents it. In this field the soul has room enough to expand herself, to display all her boundless faculties, and all her beautiful and interesting extravagancies and absurdities.