between the spoken and the silent question, the vivid after-effect (the so-called "primary memory image") soon disappeared and the test was wholly unsuccessful. Practice, however, soon helped me to overcome this last difficulty also. I believe that my inability to evoke this specific reaction on the part of the horse, lay in the unfavorable position which I assumed, for it did not allow the horse to perceive my movements easily. For the same reason, Hans would at first indicate "no" and "zero" by turning to the right, seldom to the left. As in the case of counting, a high degree of concentration was also necessary here, but with this difference, that here attention was directed to ideas present to the mind, ("yes", "no", etc.), whereas in the counting process attention was directed toward expected sensory impressions (i. e., the taps of the horse). All that has been said thus far is readily understood psychologically. The following curious fact, however, is noteworthy. Hans used the head-movement to indicate two such different concepts as "zero" and "no"; it appeared therefore that in both cases he was receiving the same kind of directive. Observation proved that such was the case and the directive in question was none other than an imitation in miniature, or rather a movement anticipatory of the expected head-movement of the horse. Now, whereas the signs for "up", "down", "right", and "left" were natural expressive movements which are normally associated with the corresponding concepts, this cannot be said to be true of "no" and "zero". My laboratory observations (see page 107) lead me to conclude that the movements, by means of which the concepts "no" and "zero" are naturally expressed, are quite different; and neither of these corresponds to the signs for "zero" and "no" which the questioner involuntarily gave to Hans. What was the genesis of these unnatural forms of expression? If we might assume that the questioner always had in mind the movement he awaited on the part of the horse, and never thought of "zero" or "no", then the contradiction would solve itself. But I must deny decidedly that I ever thought of the movements of the horse's head, and Mr. Schillings, whom I questioned on this point, agreed with me in this, in so far as his own mental processes were concerned. I can see nothing for it but that in this instance the expressive movements normally connected with the concepts "zero" and "no" have been replaced by other forms, without the questioner becoming aware of it. That such displacements may occur, has been shown by the tests described on pages 107 to 112. That they did occur in this instance may be concluded from the following observation. In responding to me, as well as to Mr. Schillings, Hans always moved his head first to the left, then to the right, never in the opposite order. That this was _not_ a peculiarity of the horse, but must be ascribed to the signs