The return of the libido to phantasy is an intermediate step on the road to symptom development and well deserves a special designation. C. G. Jung coined for it the very appropriate name of _introversion_, but inappropriately he also lets it stand for other things. Let us therefore retain the idea that introversion signifies the turning aside of the libido from the possibilities of actual satisfaction and the excessive accumulation of the phantasies hitherto tolerated as harmless. An introvert is not yet a neurotic, but he finds himself in a labile situation; he must develop symptoms at the next dislocation of forces, if he does not find other outlets for his pent-up libido. The intangible nature of neurotic satisfaction and the neglect of the difference between imagination and reality are already determined by arrest in the phase of introversion. You have certainly noticed that in the last discussions I have introduced a new factor into the structure of the etiological chain, namely, the quantity, the amount of energy that comes under consideration. We must always take this factor into account. Purely qualitative analysis of the etiological conditions is not sufficient. Or, to put it in another way, a _dynamic_ conception alone of these psychic processes is not enough; there is need of an _economic_ viewpoint. We must say to ourselves that the conflict between two impulses is not released before certain occupation-intensities have been reached, even though the qualitative conditions have long been potent. Similarly, the pathogenic significance of the constitutional factors is guided by how much _more_ of a given component impulse is present in the predisposition over and above that of another; one can even conceive the predispositions of all men to be qualitatively the same and to be differentiated only by these quantitative conditions. The quantitative factor is no less important for the power of resistance against neurotic ailments. It depends upon _what amount_ of unused libido a person can hold freely suspended, and upon _how large a fraction_ of the libido he is able to direct from the sexual path to the goal of sublimation. The final goal of psychological activity, which may be described qualitatively as striving towards pleasure-acquisition and avoidance of unpleasantness, presents itself in the light of economic considerations as the task of overcoming the gigantic stimuli at work in the psychological apparatus, and to prevent those obstructions which cause unpleasantness.