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In the course of evolution, described here as a process of progressive “internalization,” there arises a central regulating factor, usually called the “inner world,” which is interpolated between the receptors and the effectors. We are familiar with it in human adults as one of the ego's regulating factors. The breadth of the subjective world, the degree of sensitivity to experiences, etc., reflect individual differences in this factor. Here, however, we are not concerned with the inner world as such, but rather with its role in objective functional relationships.
Freud (1920) impressed on us the importance of the “stimulus barrier,” which permits the transmission of “only a fragment of the original [stimulus] intensity” (p. 27) into the interior of the organism. But there is at first no such stimulus barrier against instinctual drives, and therefore displacements of drive stimulation are more significant in mental economy than are displacements of external stimulations. For this reason the biological functions of the inner world (and its relations to the environment) are easily disturbed by the instinctual drives. This closeness of the inner world to the instinctual drives has, however, a positive significance