Page:Freud - The interpretation of dreams.djvu/444

426 composed of Ψ-systems has a direction. All our psychic activities proceed from (inner or outer) stimuli and terminate in innervations. We thus ascribe to the apparatus a sensible and a motor end; at the sensible end we find a system which receives the perceptions, and at the motor end another which opens the locks of motility. The psychic process generally takes its course from the perception end to the motility end. The most common scheme of the psychic apparatus has therefore the following appearance:



But this is only in compliance with the demand long familiar to us, that the psychic apparatus must be constructed like a reflex apparatus. The reflex act remains the model for every psychic activity.

We have now reason to admit a first differentiation at the sensible end. The perceptions that come to us leave a trace in our psychic apparatus which we may call a "Memory trace." The function which relates to this memory trace we call the memory. If we hold seriously to our resolution to connect the psychic processes into systems, the memory trace can then consist only of lasting changes in the elements of the systems. But, as has already been shown in other places, obvious difficulties arise if one and the same system faithfully preserves changes in its elements and still remains fresh and capable of admitting new motives for change. Following the principle which directs our undertaking, we shall distribute these two activities among two different systems. We assume that a first system of the apparatus takes up the stimuli of perception, but retains nothing from them—that is, it has no memory; and that behind this there lies a second system which transforms the momentary excitement of the first into