ring the process of tapping. But by means of which sense organ was it
received by the horse?

We began by examining the sense of vision, and in the following manner.
Blinders were applied, and it is worthy of mention that Hans made no
attempt to resist. The questioner stood to the right of the horse, so
that the animal knew him to be present and could hear, but not see him.
Hans was requested to tap a certain number. Then the experimenter would
step forward into the horse's field of vision and would put the same
problem again. Since, in the tests of the first kind, Hans would always
make the most strenuous efforts to get a view of the questioner, and
since he would rave and tear at the lines whenever the attempt was made
to tie him,--a thing which he had never done hitherto,--it was
impossible to determine in some cases whether or not he had seen the
questioner during the process of tapping. I am using, therefore, in the
following exposition, besides the two categories of "not seen" and
"seen", a third which I have called "undecided". A total of 102 tests
were made in which large blinders were used. In 35 of these, the
experimenter certainly was "not seen" in 56 cases he was "seen" and the
remaining 11 are "undecided". Under the first of these categories 6% of
Hans's answers were correct (i. e. only two), under the second head 89%
were correct and under the third 18% were right. In other words, the
horse was at a loss the moment he was prevented from seeing the
questioner; whereas his responses were nearly always correct when the
experimenter was in sight, certain proof that the horse's failures are
to be attributed to the elimination of visual stimuli and not to the
general inconvenience occasioned by the blinders. It is evident
therefore, that the horse required certain visual stimuli or signs in
order to make a correct response.[H]

[Footnote H: Throughout this treatise I am using the word "sign," or
"signal," whereas all other writers who have touched upon the
Hans-problem, have always spoken of "aids." Following von Sanden,[4]
however, I would distinguish clearly between the two. I would
designate as aids all immediate stimulations of the horse's body (i.
e. by means of contact), which have been designed with reference to
the animal's physiological movement-mechanism in such a way that
they truly 'aid' him in the production of the required movements. I
would regard as signs on t