In this paper Weiss discusses the sources and mechanisms of the various methods of defense and the factors which are effective in their analysis. He stresses the difficulty of drawing a definite line between ego and id and he uses Federn's conception of 'ego boundaries' and their displaceability as a theoretical explanation of observable facts. Weiss's basic theoretical conceptions, however, are not always the same as those generally accepted. He talks, for example, about qualitatively different ego and id cathexes of mental strivings, and of 'destrudo' as the moving force of the death instinct. He values so highly the so-called 'functional phenomenon' (Silberer) in his dream analyses that his conceptions become very similar to those of Jung, a similarity which he himself recognizes and acknowledges.