enter's intentions. This observation has also proved useful in
non-hypnotic experiments. We certainly knew before the days of hypnotism
that the signs by which A betrays his thoughts to B may gradually become
more delicate. We see this, for example, in the case of the schoolboy,
who gradually learns how to detect from the slightest movement made by
his master whether the answer he gave was right or not. We find the same
sort of thing in the training of animals--the horse, for instance, in
which the rough methods at first employed are gradually toned down until
in the end an extremely slight movement made by the trainer produces the
same effect that the rougher movements did originally. But even if this
lessening in the intensity of the signals exists independently of
hypnosis, it is the latter that has shown us how easily neglect of this
factor may lead to erroneous conclusions being drawn. The suggestibility
of the hypnotic makes these infinitesimal signals specially dangerous in
his case. But when once this danger was recognized, greater attention
was paid to this source of error in non-hypnotic cases than before. It
is certain that many psychological experiments are vitiated by the fact
that the subject knows what the experimenter wishes. Results are thus
brought about that can only be looked upon as the effects of suggestion;
they do not depend on the external conditions of the experiment but on
what is passing in the mind of the subject.

An event which at the time of its occurrence created a considerable
commotion (I refer to the case of Clever Hans), will show how far we may
be led by neglecting the above lesson taught us by hypnotism. If the
Berlin psychologist Stumpf, the scientific director of the committee of
investigation, had but taken into consideration the teachings of
hypnotism, he would never have made the fiasco of admitting that the
horse, Clever Hans, had been educated like a boy, not trained like an
animal.

Clever Hans answered questions by tapping his hoof on the stage; and the
observers, more particularly the committee presided over by Stumpf,
believed that answers tapped out were the result of due deliberation on
the part of the horse, exactly as spiritists believe that the spirits
hold intelligent intercourse with them by means of "raps." One tap
denoted a, two taps b, three taps c, etc.; or, where numbers were
concerned, one tap signified 1, two taps 2, etc. In this way the animal
answered th