By virtue of the principle of pain the first system is therefore altogether incapable of introducing anything unpleasant into the mental associations. The system cannot do anything but wish. If this remained so the mental activity of the second system, which should have at its disposal all the memories stored up by experiences, would be hindered. But two ways are now opened: the work of the second system either frees itself completely from the principle of pain and continues its course, paying no heed to the painful reminiscence, or it contrives to occupy the painful memory in such a manner as to preclude the liberation of pain. We may reject the first possibility, as the principle of pain also manifests itself as a regulator for the emotional discharge of the second system; we are, therefore, directed to the second possibility, namely, that this system occupies a reminiscence in such a manner as to inhibit its discharge and hence, also, to inhibit the discharge comparable to a motor innervation for the development of pain. Thus from two starting points we are led to the hypothesis that occupation through the second system is at the same time an inhibition for the emotional discharge, viz. from a consideration of the principle of pain and from the principle of the smallest expenditure of innervation. Let us, however, keep to the fact—this is the key to the theory of repression—that the second system is capable of occupying an idea only when it is in position to check the development of pain emanating from it. Whatever withdraws itself from this inhibition also remains inaccessible for the second system and would soon be abandoned by virtue of the principle of pain. The inhibition of pain, however, need not be complete; it must be permitted to begin, as it indicates to the second system the nature of the memory and possibly its defective adaptation for the purpose sought by the mind.
The psychic process which is admitted by the first system only I shall now call the primary process; and the one resulting from the inhibition of the second system I shall call the secondary process. I show by another point for what purpose the second system is obliged to correct the primary process. The primary process strives for a discharge of the excitement in order to establish a perception identity with the sum of excitement thus gathered; the secondary process has abandoned this intention and undertaken instead the task of bringing about a thought identity. All thinking is only a circuitous path from the memory of gratification taken as an end-presentation to the identical occupation of the same memory, which is again to be attained on the track of the motor experiences. The state of thinking must take an interest in the connecting paths between the presentations without allowing itself to be misled by their intensities. But it is obvious that condensations and intermediate or compromise formations occurring in the presentations impede the attainment of this end-identity; by substituting one idea for the other they deviate from the path which otherwise would have been continued from the original idea. Such processes are therefore carefully avoided in the secondary thinking. Nor is it difficult to understand that the principle of pain also impedes the progress of the mental stream in its pursuit of the thought identity, though, indeed, it offers to the mental stream the most important points of departure. Hence the tendency of the thinking process must be to free itself more and more from exclusive adjustment by the principle of pain, and through the working of the mind to restrict the affective development to that minimum which is necessary as a signal. This refinement of the activity must have been attained through a recent over-occupation of energy brought about by consciousness. But we are aware that this refinement is seldom completely successful even in the most normal psychic life and that our thoughts ever remain accessible to falsification through the interference of the principle of pain.
This, however, is not the breach in the functional efficiency of our psychic apparatus through which the thoughts forming the material of the secondary mental work are enabled to make their way into the primary psychic process—with which formula we may now describe the work leading to the dream and to the hysterical symptoms. This case of insufficiency results from the union of the two factors from the history of our evolution; one of which belongs solely to the psychic apparatus and has exerted a determining influence on the relation of the two systems, while the other operates fluctuatingly and introduces motive forces of organic origin into the psychic life. Both originate in the infantile life and result from the transformation which our psychic and somatic organism has undergone since the infantile period.