successful without an oral statement of the
problem,--in other words, the auditory stimulus was quite superfluous.
The tests with the blinders showed that Hans was lost as soon as his
questioner was out of his view, but responded adequately the moment the
questioner was in sight. Hans, therefore, had established no idea of any
sort in connection with the terms "up", "down", etc., but in these
cases, likewise, he reacted in response to certain visual stimuli. The
nature of these stimuli I discovered at first in my observations of Mr.
von Osten and also of myself, when working with the horse.

Above all things it was necessary that the questioner, during these
tests, should stand perfectly erect. If he stooped ever so slightly, the
test was unsuccessful. If he carefully refrained from any movement
whatsoever, and looking straight before him asked the horse, "Which
direction is right?" or "Which way is upward?", Hans would execute all
sorts of head movements without rhyme or reason. It was evident that he
noted that a head movement of some kind was expected of him, but did not
know the particular one that was wanted. But if the questioner now
raised his head, Hans would begin to nod and would continue doing so
until the questioner lowered his head. This reaction was interpreted as
signifying "yes". Mr. von Osten had always asked Hans before each of the
more difficult tests whether he had comprehended the meaning of the
problem, and was reassured only upon seeing the horse's affirmative
response. But contrary to Mr. von Osten's expectation, Hans also
responded in this manner after a pair of ear-caps had been drawn over
his ears. In the case of the tests described at the beginning of the
chapter, in which the method was that of "procedure without knowledge",
Mr. von Osten had always insisted that we await Hans's nod of
comprehension before proceeding. We complied; Hans nodded and--regularly
disgraced himself!

When the questioner raised his head somewhat higher than normal, Hans
would throw his own upward, which was supposed to signify "upward". A
lowering of the head on the part of the questioner was followed by a
lowering on the part of Hans, which was his form of response for "down".
For some time I was in a quandary as to the difference between the
questioner's signal for this latter response and the one which was the
signal for the horse to begin tapping, although I had often given both
kinds unwittingly. Further expe