The September commission had answered the question of tricks and referred the question of thinking to what Stumpf called “a purely scientific court”. The court was Stumpf himself, with Erich von Hornbostel keeping the record and Oskar Pfungst running the experiments, in a tent in von Osten’s courtyard from 13 October to 29 November 1904. Stumpf wrote the result up alone, signed it alone, and gave it to the press on 9 December. The New York Times had it by cable the next day. It is printed as Supplement IV of Pfungst’s book, in Carl Rahn’s English of 1911, and this is that text entire; the only change is the division into paragraphs, which follows the book.

The report

“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.

“The movements which call forth the horse’s reaction, are so extremely slight in the case of Mr. von Osten, that it is easily comprehensible how it was possible that they should escape the notice even of practised observers. Mr. Pfungst, however, whose previous laboratory experience had made him keen in the perception of visual stimuli of slightest duration and extent, succeeded in recognizing in Mr. von Osten the different kinds of movements which were the basis of the various accomplishments of the horse. Furthermore, he succeeded in controlling his own movements, (of which he had hitherto been unconscious), in the presence of the horse, and finally became so proficient that he could replace these unintentional movements by intentional ones: He can now call forth at will all the various reactions of the horse by making the proper kind of voluntary movements, without asking the relevant question or giving any sort of command. But Mr. Pfungst meets with the same success when he does not attend to the movements to be made, but rather focuses, as intently as possible, upon the number desired, since in that case the necessary movement occurs whether he wills it or not. In the near future he will give a special detailed report of his observations, which gives promise of becoming a valuable contribution to the study of involuntary movements. Also he will give an account of our tests and of the mechanism of the various accomplishments of the horse. We must also defer, till then, the disproof of certain seemingly relevant arguments in favor of the horse’s power of independent thought.

“Some defenders of the view which maintains the horse’s rationality may urge that it was only through our experiments that the animal became trained and spoiled in so far as the ability to think is concerned. They are refuted in this, however, by the fact that the horse still continues to solve problems involving decimal fractions and to determine calendar dates for Mr. von Osten, as brilliantly as ever, as is shown by his recent demonstration before a large group of spectators. That these results are now being achieved in a manner essentially different from formerly is nothing but a bare assertion.

“On the other hand, now that the possibility has been established that these wonderful results may be obtained in all their complexity by means of intentional signs, many will question whether Mr. von Osten did not himself train the horse from the very beginning to respond to these signs. No one has the right, however, to charge an old man, who has never had a blemish on his reputation, with having invented a most refined network of lies, if the facts can be explained in a satisfactory manner in some other rational way. And this can be done in this case. For we have seen that there is another alternative, other than the theory that the horse can think or the assumption that tricks have been employed.

“And now, aside from the specific results obtained, what is the scientific and philosophic import of the whole affair?—For one thing, the revolution in our conception of the animal mind, which had been hoped for by some, and feared by others, has not taken place. But a conclusion of an opposite character is justified. If such unexampled patience and high pedagogical excellence as was daily brought to bear by Mr. von Osten during the course of four long years, could not bring to light the slightest trace of conceptual thinking, then the old assertion of the philosophers that the lower forms are incapable of such thinking, finds corroboration in the results of these experiments so far as the animal scale up to and including the ungulates is concerned. For this reason the tremendous effort put forth by Mr. von Osten, is not, in spite of the self-deception under which he labors, lost to science. If anyone has the courage to try the experiment with the dog or the ape, the insight which we have now gained will enable him to beware of one source of error which hitherto has not been noticed.

“In the face of much misapprehension which has arisen, I wish once more to say emphatically that the committee of September 12th in no wise declared itself to be convinced that the horse had the power of rational thinking. The committee restricted itself entirely to the question whether or not tricks were involved, and, intentionally and rightly referred the positive investigation to a purely scientific court. I would also report that for some time Mr. Schillings has been convinced, by his own observations, of the horse’s lack of reason, and when he was apprised of our conclusion in the matter, he embraced it without wavering. I have no intention of taking part in any discussion which may arise in the press as a result of the present report. Unless they wish to confine themselves to mere guesswork, the defenders of other views will not shrink from the task of basing their criticism upon careful methodical experimentation, and they will keep a detailed record of their results day by day; for statements based solely upon memory, without specific report of experimental conditions, prove nothing.

“Prof. Carl Stumpf. December 9th, 1904.”

What was said back

The Associated Press, the same day, got the finding into four sentences for the American papers: the secret of the horse’s replies was “in his powers of observation, which enable him to perceive while he looks at his questioner the instant he has reached a correct answer”; the horse “was unable to tap out a correct answer to a question when the person putting it did not know the answer”; “nor was he able when wearing blinders to calculate or perform the simplest counting”; and “Dr. Stumpf does not doubt the good faith of Herr von Osten and his assistants.” The New York Times’s own correspondent, cabling under the heading “Horse’s trainer exonerated”, added that “if this theory is accepted the achievements of Hans remain quite as extraordinary and quite as inexplicable as if the theory that the horse actually thinks is entertained.”

In Berlin, the Berliner Tageblatt of 12 December reported that “Tests have been made upon Hans with blinders over his eyes and it is to be noted that, in spite of these, he still responds correctly”, and Pfungst, who had seen what passed for blinders in the courtyard before the fifteen-centimetre pair was made, put the report down to them. Stumpf’s introduction of 1907 records the rest: a hippologist who thought hippologists should have been consulted, a telepathist who thought telepathists should, talk of suggestion and thought-reading and the occult “without the slightest attempt to elucidate these vague terms”, some who stayed with “the old cry of ‘fraud,’ for a share of which Mr. Pfungst now fell heir”, a few who claimed priority and said Stumpf had “confessed” to their view, “as if there were anything like ‘confessions’ in science!”, and “disappointed Darwinists who expressed fear lest ecclesiastical and reactionary points of view should derive favorable material from the conclusions arrived at in my report. Needless fear.”

The claim of priority was Albert Moll’s. On 5 January 1905, by the Berliner Lokal-Anzeiger’s account, he told the Berlin Psychological Society that he welcomed the report as the adoption of the view he had held since his own tests of 1903, that the September statement had been an error “due to insufficient regard for the unconscious and involuntary signs”, and that he differed from Stumpf on one thing: the horse “learns to react to fine signs only when they are gradually developed out of gross ones”, and “that Herr von Osten should have made these gross movements unconsciously is not exactly probable.” Moll believed the training had been conscious. Pfungst answered that in the sixth chapter of his book, with the argument that a man who had trained a horse to signals would have had control of it, and von Osten had none: four tries before an audience to get the horse to 20, an answer of “left” and then “down” and then “up” to the question where the numerator of a fraction stands, and a rate of error that matched Pfungst’s own when he worked without controlling his movements and never approached the zero he reached when he did.

Von Osten’s answer was a letter forbidding further experiments. Karl Krall’s was to go to the courtyard the next summer and begin again.

Sources

• Oskar Pfungst, Clever Hans (The Horse of Mr. von Osten), translated by Carl L. Rahn, New York: Henry Holt, 1911: Supplement IV, pages 261 to 265, quoted entire; Stumpf’s introduction, pages 10 to 14; chapter II, page 45, on the Berliner Tageblatt of 12 December; chapter VI, pages 228 to 230, on von Osten’s want of control over the horse. archive.org
• The New York Times, 11 December 1904, the cable from Berlin of 10 December and the Associated Press dispatch. archive.org
• Karl Krall, Denkende Tiere, Leipzig, 1912, pages 319 to 324, for the German text of the report, and the Berliner Lokal-Anzeiger of 6 January 1905 on Moll’s remarks of the day before, translated here by the author of this entry. archive.org