Orientierung aneinander

In September of last year (1924) Leningrad suffered a great flood. The dogs were saved only with considerable difficulties, and under extraordinary circumstances. Within five or ten days, when everything was put in order again, our dog, to all appearances perfectly healthy, greatly perplexed us in the experimental room. All the positive conditioned reflexes had completely disappeared: the dog did not salivate at all and did not take the food offered to it in the usual manner. For a long time we could not guess what was wrong. None of our initial suppositions about the reason for this phenomenon could be substantiated.
Finally it dawned on us that the strong effect produced on the dog by the flood persisted. Then we did the following. We now usually conduct our experiments with conditioned reflexes so that the dog is alone in the experimental room, while the experimenter is in another room from where he acts on the dog, gives it food and records the results of the experiments. For our dog we now made certain changes. Dr. Speransky sat quietly in the room together with the dog and did nothing, while I performed the experiment for him from the other room. To our great satisfaction the conditioned reflexes reappeared and the dog began to take the food. By repeating this method for a considerable period of time, at first infrequently and then more often, gradually weakening it, i.e., sometimes leaving the dog alone in the room, we finally restored the dog in a certain measure to its normal condition.

The other method was only a variation of the one just described. We placed in the dog’s room only part of the experimenter’s clothing, rather than the experimenter himself, and this was enough markedly to increase the reflexes. The dog did not see the clothing and, consequently, it was the odour that acted.“
(Pavlov, I. P.: Psychopathology and Psychiatry. Selected Works)

… aber er versteht die Bedeutung seiner Beobachtung in keiner Weise.