EXPERIMENTS AND OBSERVATIONS 43 



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 1 1 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 re- 

 sponses were nearly always correct when the experi- 

 menter 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.* 



* 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,* 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 con- 

 tact), which have been designed with reference to the animal's physio- 

 logical movement-mechanism in such a way that they truly ' aid ' him. 

 in the production of the required movements. I would regard as signs 

 on the other hand, all stimulations (whether mediate or immediate) 

 which are selected without especial regard to the anatomy or physiology 



