The reason is simple.  The professional medium, paid to display certain powers, which are in any case extremely uncertain in their response to the call for them, invariably begins to imitate them when they fail.  The mediums are invariably persons of an inferior order of intellect, avid of notoriety, and mostly mercenary, so that the results of the consultations with them were almost sufficient to deter serious-minded people from dealing with them a second time, while the people who formed the regular circles and had made a sect with a devotional character in it, rapidly degenerated into a credulity and materialism which were more discouraging than the most arid skepticism.  Physical phenomena which met every demand for absolute guarantee of their genuineness, were very rare, and to meet with them required great patience and persistence, while the scientific student, in the habit of dealing with experiments that had definite results, obeying known or conjectured laws, if entering into an investigation which met at the threshold a frivolous, and possibly fraudulent, “manifestation,” threw up the subject, the more readily that in general the student of physical science has no sympathy with psychical research.

Recognizing the correctness of this attitude and the unreliability as well as the utter want of essential importance in the physical manifestations and the invariable inconsequence and silliness of the intellectual results, I withdrew entirely from circles in which mediums took part or in which physical phenomena were sought for, and limited my investigations to the cases in which the good faith of all the company was unquestionable, and the investigation conducted in privacy and sincerity.  Here, of course, there was still great uncertainty, and often the most curious triviality and low intelligence, but we were able to check the possible tendency to the simulation of the supra-normal activity.  And even so the character of the “manifestation” was generally so trivial and opposed to all preconceived ideas of spiritual intelligence as to justify the conclusion that the departed had left their wits behind them, so that even in those “circles” which included only personal friends and individuals of unquestionable sincerity the results rarely had any intellectual importance.  And I came to the conclusion that that form of the phenomena which alone gave any intellectual result, i.e. which manifested ideas in any way transcending the commonplace capacities of commonplace minds, had nothing in common with the physical manifestations, but seemed rather to consist in an exaltation of the intellectual powers of the subject, so that the evidence of any supra-normal power was rather moral than scientific, and had value only according to the relation between the subject and the hearer, and therefore no determinable value to physical science.