The psychologist G. Meyer was the first to notice that with extreme concentration of attention on any memory or fantastic image, one can achieve that they appear before us with a distinctness that is not inferior to ordinary sensory perception. Thus, he himself managed to evoke only visual images and tactile sensations.
Later, similar experiments were repeated by many others, and it turned out that auditory hallucinations can be caused arbitrarily; with less certainty, this relates to the taste and smell sensations, which, however, is quite natural, since the memories in this area do not have sufficient brightness.
Experiments have also shown that the degree of success depends largely on the individual. Some experiments succeed easily, others almost never succeed; some have only visual hallucinations, others only auditory hallucinations, etc. However, such hallucinations never mix with reality, since the person making the experience is always aware that the impression he has received is caused by him arbitrarily, with a certain effort.
The situation is different when, with an increase in susceptibility to suggestion, some memory takes hold of the attention for a long time. The reasons for this state can be varied: sometimes suggestions from the outside act, involuntarily focusing attention on well-known images and representations through oral or written stories about these subjects; in other circumstances, we are dealing with self-hypnosis, which arose out of expectation or fear.
In either case, concentration of attention can cause a transition of a representation into a hallucination, not only equal in brightness and distinctness to actual sensory perception, but even allowing their complete confusion, especially since the individual does not realize that the image he feels is caused by himself … Here are a few facts from the history of superstition that can most easily be explained as induced hallucinations.
According to old stories, a clairvoyant can transmit a vision to another clairvoyant by a simple touch. However, we do not have reliable information about such phenomena; although there are indications of such facts in the literature, there is no real guarantee that both clairvoyants did in fact have identical visions. If one communicated their content to the other, then the coincidence became natural.
Persons prone to hallucinations are also easily susceptible to suggested hallucinations; therefore, the story of the vision of one clairvoyant affected another as a suggestion, causing him to have the same vision. An example of this kind is found in the vision of Pastor Lysius, which was passed on by verbal suggestion to the sisters.
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However, we do not know whether the content of the vision was the same for both relatives; probably they also saw the corpse, as he definitely states this, but still it is doubtful that the visions coincided in all details. In short, the transfer of a hallucinatory image from one person to another can be explained as the action of suggestion.
Visions of spirits in most cases can also be explained by suggestions or self-hypnosis. If a person believes in spirits and expects to see them at a certain moment, then the susceptibility to suggestion, strengthened by faith, will have its effect and he will have a corresponding hallucination. Such a phenomenon, as eyewitnesses say, often occurs among Siberian peoples.
The shaman, having come to a state of ecstasy, constantly sees spirits in the form of people or animals. Those present, who are sure that he is possessed by spirits, often see a blue mist, as it were, emanating from him, and think that it is the spirits leaving.
Obviously, in spiritualistic seances, it happens in exactly the same way: all light phenomena and more or less materialized images of spirits are just suggested hallucinations; only in some special cases spirits have a different origin and a more material background.
One might argue that these are all hypotheses, the correctness of which has not been proven by anything. "How do you know," the spirit says, "that spirits do not really exist?" To this we can answer that it is completely unnecessary to allow the intervention of spirits where everything can be explained in the simplest way, through natural factors.
The burden of proof always falls on the one who puts forward a new hypothesis; therefore, spiritualists must prove that the manifestations of spirits in all those cases where there is no obvious deception and the medium is not in a state of trance, were not the effect of induced hallucinations. Furthermore, only the believer sees spirits, and faith and expectation almost inevitably entail hallucinations.
Ostyaks and Tungus see how the spirits fly away from the shaman, the spirit sees the spirits near the medium, in the Middle Ages numerous witnesses saw how demons left the possessed under the influence of spells. Ancient authors say that demons were seen by many, or they left visible traces of their presence.
The expulsion of demons is one of the favorite motifs of ancient icon painting, where they are depicted as winged creatures emerging from the mouth. Why does the European traveler not see the spirits visible to everyone else present in the yurt?
Why does the critical observer not see the spirits in seances, except when the medium himself takes on their role by wearing the appropriate costume? Finally. The possessed still exist today: they are unfortunate hysteroepileptics, whose treatment has now passed to psychiatrists.
Why, however, do the latter not see flying devils while curing their patients of seizures? The answer is very simple: you cannot see spirits, and only those who are waiting for their appearance and by means of suggestion bring themselves to hallucinations see them.
Alfred Lehman from the book "The World of Superstition and Magic"