Readers who wish to inform themselves of all the personal circumstances in the case may best read the text just as it stands. Those who desire to get at the pith of the matter without reference to its historical settings, may be advised to omit the Introduction by Professor Stumpf of the University of Berlin, together with supplements II, III and IV. CONTENTS PAGE PREFATORY NOTE (By JAMES R. ANGELL) v INTRODUCTION (By C. STUMPF) 1 CHAPTER I. THE PROBLEM OF ANIMAL CONSCIOUSNESS AND "CLEVER HANS" 15 II. EXPERIMENTS AND OBSERVATIONS 30 III. THE AUTHOR'S INTROSPECTIONS 88 IV. LABORATORY TESTS 102 V. EXPLANATION OF THE OBSERVATIONS 141 VI. GENESIS OF THE REACTION OF THE HORSE 212 CONCLUSION 240 SUPPLEMENTS: I. MR. VON OSTEN'S METHOD OF INSTRUCTION (By C. STUMPF) 245 II. THE REPORT OF SEPTEMBER 12th, 1904 253 III. AN ABSTRACT FROM THE RECORDS OF THE SEPTEMBER-COMMISSION 255 IV. THE REPORT OF DECEMBER 9th, 1904 261 TABLE OF REFERENCES 267 INTRODUCTION [BY C. STUMPF] A horse that solves correctly problems in multiplication and division by means of tapping. Persons of unimpeachable honor, who in the master's absence have received responses, and assure us that in the process they have not made even the slightest sign. Thousands of spectators, horse-fanciers, trick-trainers of first rank, and not one of them during the course of many months' observations are able to discover any kind of regular signal. That was the riddle. And its solution was found in the unintentional minimal movements of the horse's questioner. Simple though it may seem, the history of the solution is nevertheless quite complex, and one of the important incidents in it is the appearance of the zooelogist and African traveler, Schillings, upon the scene, and then there is the report of the so-called Hans-Commission of September 12, 1904. And finally there is the scientific investigation, the results of which were published in my report of December 9, 1904.