solution of the riddle: Why can the unconscious furnish the motive power for the wish-fulfillment only during sleep? The answer to this question must throw light on the psychic nature of wishes; and it will be given with the aid of the diagram of the psychic apparatus.

We do not doubt that even this apparatus attained its present perfection through a long course of development. Let us attempt to restore it as it existed in an early phase of its activity. From assumptions, to be confirmed elsewhere, we know that at first the apparatus strove to keep as free from excitement as possible, and in its first formation, therefore, the scheme took the form of a reflex apparatus, which enabled it promptly to discharge through the motor tracts any sensible stimulus reaching it from without.