The proof to which I refer was found in the sphere of hypnotic phenomena. In the year 1889 I was present at the remarkably impressive demonstrations by Liébault and Bernheim, in Nancy, and there I witnessed the following experiment. A man was placed in a condition of somnambulism, and then made to go through all sorts of hallucinatory experiences. On being wakened, he seemed at first to know nothing at all of what had taken place during his hypnotic sleep. Bernheim then asked him in so many words to tell him what had happened while he was under hypnosis. The man declared that he could not remember anything. Bernheim, however, insisted upon it, pressed him, and assured him that he did know and that he must remember, and lo and behold! the man wavered, began to reflect, and remembered in a shadowy fashion first one of the occurrences which had been suggested to him, then something else, his recollection growing increasingly clear and complete until finally it was brought to light without a single gap. Now, since in the end he had the knowledge without having learnt anything from any other quarter in the meantime, we are justified in concluding that these recollections were in his mind from the outset. They were merely inaccessible to him; he did not know that he knew them but believed that he did not know. In fact, his case was exactly similar to what we assume the dreamer’s to be. I hope you are duly surprised that this fact is already established and that you will ask me: “Why did you not refer to this proof before, when we were considering errors and came to the point of ascribing to a man who had made a slip of the tongue intentions behind his speech, of which he knew nothing, and which he denied? If it is possible for a man to believe that he knows nothing of experiences of which nevertheless he does possess the recollection, it seems no longer improbable that there should be other mental processes going on within him about which also he knows nothing. We should certainly have been impressed by this argument and should have been in a better position to understand about errors.” Certainly, I might have brought forward this proof then, but I reserved it for a later occasion when there would be more need for it. Some of the errors explained themselves, others suggested to us that in order to understand the connection between the phenomena it would be advisable to postulate the existence of mental processes of which the person is entirely ignorant. With dreams we are compelled to seek our explanations elsewhere, and besides, I am counting on your being more ready to accept in this connection a proof from the field of hypnosis. The condition in which we perform errors must seem to you normal and, as such, to bear no similarity to that of hypnosis. On the other hand there exists a clear relationship between the hypnotic state and sleep, the essential condition of dreaming. Hypnosis is actually called artificial sleep; we