Donaldson IML (2005). Mesmer’s 1780 proposal for a controlled trial to test his method of treatment using ‘Animal Magnetism’.

© IML Donaldson, University of Edinburgh & Royal College of Physicians of Edinburgh. 9 Queen Street, Edinburgh EH2 1JQ, UK. E-mail:

Cite as: Donaldson IML (2005). Mesmer’s 1780 proposal for a controlled trial to test his method of treatment using ‘Animal Magnetism’. JLL Bulletin: Commentaries on the history of treatment evaluation (

Anton Mesmer’s move from Vienna to Paris

Anton Mesmer was effectively thrown out of Vienna where his supposed cures by ‘Animal Magnetism’ had aroused both the enthusiasm of some patients and the enmity of most of the medical establishment. He went to Switzerland and then to France in 1778, where his reputation had preceded him. In Paris, crowds came to see him – some to be treated and some from curiosity.

Mesmer wanted both approbation of his ‘discovery’ of Animal Magnetism and profit from its exercise and he seems to have believed that the best way to do this was to try to persuade the learned societies of Paris to approve his theory and methods. After a number of unsuccessful approaches to the Académie des Sciences and the Société Royale de Médecine, Mesmer paused in his attempt to persuade the learned of the virtues of his theory and concentrated, with great success, on building up a practice treating patients (see Bertrand 1826 and commentary on Commission Royale 1784).

However, Mesmer also wanted the government’s approval and hence benefit and profit to him; without this he was adamant that he would not divulge his secrets.

Deslon and the attack by the Faculté de Médecine

Some time passed before he approached the Faculté de Médecine but, in the meantime, he had made the acquaintance of one of its Regent Doctors, d’Eslon or Deslon (1750-1786). Deslon was a physician to the Comte d’Artois, one of Louis XVI’s brothers. Deslon became persuaded of the great utility of Mesmer’s method, but said that he did not yet know the nature of the agent involved, though he knew the procedure for treatment. In 1780 Deslon published his observations on what he had seen after using the treatment (Eslon 1780). Mesmer and Deslon became friends and the latter agreed, as a professor in the Faculté de Médecine, to propose to the Faculté that they examine Mesmer’s ‘discovery’.

The Faculté was far from happy, since its members had decided they wanted nothing to do with Mesmer; but they could not refuse the request made by Deslon, one of their own. They retaliated by agreeing to a M. de Vauzèmes bringing a charge against Deslon of having damaged the reputation of the Faculté by his espousal of the views of Mesmer. This charge was to be heard on the same day that Deslon was to present Mesmer’s proposals. The meeting to hear De Vauzèmes charge took place on 18 September 1780. The short description of events on that day given here is taken from Mesmer’s own account, which is unlikely to be unprejudiced. I have included only material that Mesmer quotes verbatim (and which was apparently deposited in writing at the meeting). I have ignored Mesmer’s numerous glosses on it.

De Vauzèmes spoke first and launched a vituperative attack against both Mesmer and Deslon. Reading this now one is struck not only by its virulence, but also by the complete absence of any consideration of whether it might be possible to test Mesmer’s claims independently rather than relying on the unfavourable reactions of other learned bodies. De Vauzèmes proposed that Mesmer’s methods should be rejected out of hand without dignifying them by examination since Mesmer was a charlatan – a charlatan who had not acted in good faith in his dealings with the learned societies of Paris and whose conduct would be no better towards the Faculté. De Vauzèmes was particularly vehement in his assertions that Deslon had demeaned the profession of medicine and damaged the Faculté by associating with, and supporting, Mesmer, and by his approval of Mesmer’s claims in his recently-published book (Eslon 1780). De Vauzèmes went further and proposed that the Faculté should reject out of hand all such claims for new cures that were contrary to the practice of sound medicine.

Deslon then spoke. He severely criticised the behaviour of the recently founded Société Royale de Médecine towards Mesmer, and flattered the Faculté by insisting that they, on the contrary, were interested only in finding the truth. His discourse was reasoned and the proposals he presented on Mesmer’s behalf for a trial of Animal Magnetism were not unreasonable. The trial was to be confined to observations on whether Mesmer’s patients improved more or less than did those treated conventionally.

Deslon’s presentation of a randomized trial proposed by Mesmer

Deslon proposed a trial in which twenty-four patients were to be divided into two groups of twelve, one group to be treated by ‘ordinary methods’ and the other to be treated by Mesmer (Mesmer 1781). To avoid any later arguments about ‘age, temperament, disease or symptoms’ the patients were to be assigned to the groups by drawing lots‘… la répartition se feroit par la voie du sort’.

One can be quite certain that this means – and would have been understood in the 18th century as meaning – casting or drawing lots in order to achieve random allocation of patients to the comparison groups. Nicot, Thresor de la langue française (1606), makes explicit the derivation of sort from the Latin sors, ‘lot’ or ‘fortune’, and faire sort he defines as: sortire, sortem ducere, meaning ‘to cast or draw lots’, ‘to appoint by lot’. He also gives as a synonym for sort, the Latin clerus, meaning ‘assignment by lots’. Editions of the Dictionnaire de L’Académie française between 1694 and 1798 include in the definition of sort,Sort, signifie aussi, la manière de décider quelque chose par le hasard.‘Sort also means the method of deciding something by chance’.

The details of the duration of treatment and of the examination of the patients in the experiment were to be decided in advance. A report of each patient’s state was to be drawn up and signed by the Faculté’s Commissioners, by Mesmer, and by representatives to be appointed by the government; the latter were to be persons not involved in medical practice to avoid any public suspicion of partisanship. All in all, it seems a very fair proposition if one accepts that only changes in the state of the patient’s were be examined and that Mesmer’s method were to remain secret.

Mesmer’s proposal for a randomized trial rejected by the Faculté

The Faculté was determined not to have any truck with Mesmer or his practice, however. It not only rejected the proposals Deslon had presented on Mesmer’s behalf, it also admonished Deslon to behave more circumspectly. It threatened that, unless Deslon repudiated Mesmer’s teachings by the end of the year, his name would be deleted from membership of the Faculté. In the meantime, Deslon was forbidden to take part in deliberations at meetings of the Faculté.

Mesmer responded to the rejection of his proposals by publishing them in the Journal de Paris. In the following year (Mesmer 1781) he published his account of his doings in Vienna and Paris as the Précis historique…

A good deal of feeling was aroused in some quarters that the Faculté had acted very arrogantly in dismissing Mesmer and his methods without any examination of these. It must be remembered, however, that Mesmer steadfastly refused to allow his methods to be scrutinised at all, and insisted that only the purported effects of his treatment should be examined.

For an account of what followed the rejection, of Mesmer’s direct appeals to the French government and to the queen, Marie Antoinette, led to the setting up of two commissions of enquiry in 1784, and the total rejection of Mesmer’s theory by one commission and of the results of his treatments by the other (see my commentary on the Commission Royale, Donaldson 2014; 1784).

Mesmer’s 1780 proposals as seen from today

Mesmer’s proposals, which the Faculté rejected on 18 September 1780, largely speak for themselves. They are in many ways remarkably modern in the conditions they set out. The proposal that each patient for the trial should be randomly allocated ‘by the method of lots’ to one of the two groups, to take account of differences between them and their illnesses and to avoid later argument about the outcome, is one of the earliest instances so far identified of random allocation being proposed for a clinical trial. An earlier such suggestion was that proposed by Van Helmont, devised about 1643 and published posthumously in the Ortus Medicinae of 1648 (see Van Helmont 1648). However, neither Van Helmont’s trial nor Mesmer’s actually took place.

It may strike the modern reader as extremely odd that anyone could contemplate a trial in which patients with virtually any disease were to be included. Although entry criteria for some controlled trials are sometimes deliberately loose because the conditions being treated may include a spectrum of disorders with similar manifestations, the inclusion of people with the diverse mixture of conditions treated by Mesmer would not make sense today, particularly with a very small sample and no stratification. Whatever Mesmer said, allocation by casting lots among a relatively small sample could not control for having chance differences in the numbers of patients with the same diseases in the two groups.

The key to this apparent contradiction – between taking care to make the groups comparable by random allocation, and at the same time, allowing the groups to contain different mixtures of patients with any disease whatsoever (except venereal diseases) – may be found in Mesmer’s theory of disease. Deslon reported (Commission Royale 1784, p 3) that Mesmer claimed that there is only ‘one Nature, one illness, one remedy; and that this remedy is Animal Magnetism’ – in summary, that Animal Magnetism was a universal panacea (Mesmer 1779 Avis au Lecteur page vj) –  according to Mesmer, all disease was caused by disturbance of the flow within the body of a universal and all-pervading fluid and all was to be cured by correcting the flow of this fluid using his method of treatment – Animal Magnetism. There is a sense in which Galenic medicine also took the view that disease was unitary in nature and was always caused by disturbance of the balance of the humours. Maybe there would have been no qualms on either side about including any mixture of patients when Mesmer and the Faculté jointly chose the twenty-four patients for the trial, provided each patient was randomly allocated to a group. Since the Faculté rejected Mesmer’s proposals out of hand, however, further speculation on this is idle.

Ironically, in Van Helmont’s 1648 proposal for a controlled trial of the treatment of fevers with and without blood-letting, it was Van Helmont who believed that diseases were distinct and had distinct causes, and his opponents, the ‘Galenists’, who believed that disease was essentially unitary in its causation. Mesmer took the ‘unitary’ view. However, Van Helmont also believed in a universal remedy (the Alkahest), though it was very different in nature from Mesmer’s Animal Magnetism. There is also room for serious doubt about whether Van Helmont’s proposals were ever intended to result in a real trial or whether they were just a rhetorical device to support his arguments against the ‘Galenists’ (for Van Helmont’s proposals for a trial of the treatment of fevers see Van Helmont 1648).

Animal Magnetism: addressing its effects vs. assessing its existence

Had the Faculté agreed to the trial proposed in 1780, perhaps Mesmer would have agreed to be involved in it, provided it was organized on the lines he proposed. However, his proposals to the Faculté ran counter to the views that he held after the Faculté’s rejection, as he explained vehemently in the Précis. His position in 1781 was that the usefulness of his ‘discovery’ should only be judged on the basis of attested reports of the results of his treatment of patients, not on the results of experiments carried out for the purpose of testing the theory. For example, speaking of arrangements for the examination that he wished the government to make of his ‘discovery’ Mesmer says:

That the government could name Commissioners, not to examine my methods, not to negotiate with me, but to take note of attested facts and report on them. (Mesmer 1781, p 195).

And, more extravagantly, describing negotiations on the same subject with the principal Royal doctor, M. de Lassonne :

Here is what I said to M. de Lassonne; however bizarre [it may seem] at first sight it is nevertheless entirely serious and very much applicable to the question.

When a thief is convicted of theft he is hanged: when a murderer is convicted of murder he is executed on the wheel. But to exact these terrible penalties the thief is not required to thieve again to prove that he is a thief, and the murderer is not required to murder a second time to prove that he is a murderer. One is content to establish by testimony and by material evidence that the theft or the murder was committed and then one hangs or executes on the wheel in good conscience.

Very well! It is the same with me. I ask, kindly, to be treated like a man to be executed on the wheel or hanged and that an effort should be made to establish that I have cured [patients] without asking me to perform new cures to prove that I am to be regarded as someone who cures. (Mesmer 1781, p 196).

Thus, by 1781, Mesmer was insisting that any trial of the effectiveness of his treatment should be retrospective, whereas in 1780 he was clearly proposing a prospective trial. No doubt these views, as well as his complete unwillingness to allow his methods to be examined, explain Mesmer’s refusal to take part in the later trials. Indeed, it was not Mesmer’s application of Animal Magnetism but Deslon’s that was examined by the commissions of enquiry of 1784 (Commission Royale 1784, p 3).

The Royal Commissioners of 1784, however, took a different view. They were well aware that many patients reported dramatic effects from ‘Animal Magnetism’ and that many claimed that their symptoms were relieved by it. They deliberately avoided trying to determine by experiment whether the treatment was effective because they were well aware of the difficulties of ascribing ‘cure’ to a particular treatment. They pointed out that many diseases resolve with no treatment and many different treatments may apparently produce ‘cures’ (Lavoisier (1784)Commission Royale 1784, p 12, 14). In short, they felt that this question was unanswerable.

The Commissioners chose a different question to which they believed their experiments could provide an unequivocal answer. The aim of these was to discover whether there was any evidence for the mysterious properties that Mesmer called ‘Animal Magnetism’. A negative answer to this question would, in their minds, settle the question of its efficacy in treatment also, since, if the ‘magnetic’ properties did not exist, the question of whether they were effective in treatment simply could not arise (Commission Royale 1784, p 8). The Commissioners had themselves been ‘magnetised’ by Mesmer’s associate Deslon, but felt no effects. Next, they tested subjects who had reported dramatic effects from ‘magnetisation’ and were thus considered susceptible to it. They found that these subjects reported dramatic effects when presented either with ‘magnetised’ objects or with non-magnetised ones that they believed to have been magnetised. They also found that a typical crise, held to be induced by magnetism, and often culminating in convulsions, could be induced in such susceptible individuals merely by suggesting that ‘magnetisation’ had taken place. On the other hand, the same subjects were unaware that they had been ‘magnetised’ when this was done without their knowledge. These and other experiments led the Commission to the conclusion that Animal Magnetism did not exist and that its purported effects must be ascribed to ‘the imagination’ (Commission Royale 1784, p 64 and passim).

The Royal Commissioners anticipated that Mesmer would deny that his Animal Magnetism was the same as Deslon’s and therefore were prudent enough to state explicitly at the end of their report that they expected him to make just such a denial; and, in advance, they dismissed it as baseless. Their expectations were fulfilled: when the Royal Commission found no basis for belief in the existence of Animal Magnetism, Mesmer made exactly this denial, saying it was Deslon’s practice and not his that had been examined and been found to have no basis. One might summarise the difference between the Commissioners’ outlook and that of Mesmer by saying that Mesmer was interested in a trial of whether his method had worked for patients, as evidenced only by reports of the outcome of treatment, whereas the Commissioners were concerned primarily with whether there was a physical basis on which the method could work (see commentary on Commission Royale 1784).

This James Lind Library commentary has been republished in the Journal of the Royal Society of Medicine 2005;98:572-575. Print PDF


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