At the moment when the perceived sketch calls them forth, it is as if they were then grouped in families according to their relationship and resemblances. There are experiments of Muensterberg, earlier than those of Goldscheider and Mueller, which appear to me to confirm this hypothesis, although they were made for a very different purpose. Muensterberg wrote the words correctly; they were, besides, not common phrases; they were isolated words taken by chance. Here again the word was exposed during the time too short for it to be entirely perceived. Now, while the observer was looking at the written word, some one spoke in his ear another word of a very different significance. This is what happened: the observer declared that he had seen a word which was not the written word, but which resembled it in its general form, and which besides recalled, by its meaning, the word which was spoken in his ear. For example, the word written was "tumult" and the word spoken was "railroad." The observer read "tunnel." The written word was "Trieste" and the spoken word was the German "Verzweiflung" (despair). The observer read "Trost," which signifies "consolation." It is as if the word "railroad," pronounced in the ear, wakened, without our knowing it, hopes of conscious realization in a crowd of memories which have some relationship with the idea of "railroad" (car, rail, trip, etc.). But this is only a hope, and the memory which succeeds in coming into consciousness is that which the actually present sensation had already begun to realize. Such is the mechanism of true perception, and such is that of the dream. In both cases there are, on one hand, real impressions made upon the organs of sense, and upon the other memories which encase themselves in the impression and profit by its vitality to return again to life. But, then, what is the essential difference between perceiving and dreaming? What is sleep? I do not ask, of course, how sleep can be explained physiologically. That is a special question, and besides is far from being settled. I ask what is sleep psychologically; for our mind continues to exercise itself when we are asleep, and it exercises itself as we have just seen on elements analogous to those of waking, on sensations and memories; and also in an analogous manner combines them. Nevertheless we have on the one hand normal perception, and on the other the dream. What is the difference, I repeat? What are the psychological characteristics of the sleeping state?