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.