THE REPORT OF DECEMBER 9TH, 1904 Together with Dr. E. von Hornbostel and Mr. O. Pfungst, I have tried during the past few weeks to find an explanation of the accomplishments of the horse 'Hans' by the experimental method. We had access to the horse in the absence of the master and groom. The results are as follows: The horse failed in his responses whenever the solution of the problem that was given him was unknown to any of those present. For instance, when a written number or the objects to be counted were placed before the horse, but were invisible to everyone else, and especially to the questioner, he failed to respond properly. Therefore he can neither count, nor read, nor solve problems in arithmetic. The horse failed again whenever he was prevented by means of sufficiently large blinders from seeing the persons, and especially the questioner, to whom the solution was known. He therefore required some sort of visual aid. These aids need not, however,--and this is the peculiarly interesting feature in the case,--be given intentionally. The proof for this is found in the fact that in the absence of Mr. von Osten the horse gave correct replies to a large number of persons; and to be more specific, Mr. Schillings and later Mr. Pfungst, after working with the horse for a short time, regularly received correct answers, without their being in any way conscious of having given any kind of signal. So far as I can see, the following explanation is the only one that will comport with these facts. The horse must have learned, in the course of the long period of problem-solving, to attend ever more closely, while tapping, to the slight changes in bodily posture with which the master unconsciously accompanied the steps in his own thought-processes, and to use these as closing signals. The motive for this direction and straining of attention was the regular reward in the form of carrots and bread, which attended it. This unexpected kind of independent activity and the certainty and precision of the perception of minimal movements thus attained, are astounding in the highest degree.