The belief in such phantasms may be partially based on experience, whether hallucinatory or otherwise and inexplicably produced.
"The Fairy-Faith in Celtic Countries"
W. Y. Evans Wentz
It is necessary, then, to understand that he was not a conscious charlatan; he loathed mechanical tricks such as he occasionally came across; he was perfectly and serenely convinced that the powers which he possessed were genuine, and that the personages he seemed to come across in his mediumistic efforts were what they professed to be; that they were not hallucinatory, that they were not the products of fraud, that they were not necessarily evil.
"The Necromancers"
Robert Hugh Benson
Throughout her entire sojourn here she was oriented to time, place and person; except for having stated at one time in a sort of careless and apparently indifferent way that she had heard someone calling her by name, and upon looking for the person could find no one, she manifested no hallucinatory disturbances.
"Studies in Forensic Psychiatry"
Bernard Glueck